PREFACE.
The writer has long been impressed with the fact that the mode of resistance commonly ascribed to face hardened armor is incorrect. It seems that many have applied to it the theory upon which the development of compound armor was based. That is, the hard face was intended to smash the projectile without allowing penetration; the body and back was to assist the face under impact, and to hold it together even after it had cracked and failed.
Modern improved projectiles are seldom crushed from the point. The point may be fused and abraded or chipped off in breaking up the hard face, but actual disintegration of the projectile only occurs when the resistance that the plate is able to bring on the area of the shell in contact with it is sufficiently great to suddenly check the shell and cause it to break up over its weakest lines through its own inertia. Failure at the point may, however, arise even with low velocities, when the resistance of the plate is less local, provided the energy of the shot is incapable of effecting penetration, or in the case of inferior projectiles.
The usual action of the hard face, however, is that through its inability to bend or flow, it prevents the displacement of the more plastic metal beneath it towards the front, and thus brings the resistance of the whole thickness of the plate to bear before the projectile can advance.
The important retarding influence of the fragments of the hard face carried in by the projectile, is seen in the easier perforation obtained by projectiles whose ogivals are protected by soft steel caps. The cap appears to act as a lubricant or sleeve, covering the asperities of the hardened metal. It is necessary for the cap to be driven into the plate to derive any advantage from it. Doubtless, when thus confined this soft metal transmits pressure as rigidly as the projectile itself, but being capable of flowing, the steel slips through it comparatively unharmed. It is believed that a thicker hardened surface, undulated to prevent or limit flaking, will cause the projectile to carry in sufficient of the hardened face to render the cap incapable of performing the work required of it without increasing its size to a prohibitory extent.
The writer has much for which to thank the officers with whom he has been associated in the Bureau of Ordnance in the way of information. Mr. Millard Hunsiker, now in charge of the manufacture of armor at the Carnegie Steel Company's works, at Homestead, has also kindly placed at his disposal valuable information. He also owes considerable to the Inspectors of Ordnance and officials at the works of the armor makers. These gentlemen are in no way responsible, however, for the conclusions reached. It has been the intention of the writer throughout to avoid discussing those technical details which have been developed by and are the property of the manufacturer rather than the patentee, and by means of which alone the process of face hardening can be made a commercial success.
SECTION I.
HISTORY AND MANUFACTURE.
Face hardened armor is the direct outcome of efforts to avoid the failures resulting from attempts to temper homogeneous steel plates of sufficiently high carbon to give a very hard face. It is theoretically the perfect armor plate, and doubtless would have been developed long ago had that theory only been enunciated; for the various steps followed in its manufacture, except in certain details, have long been known to the metallurgical world.
Lieutenant Jacques makes the following statement in a recent discussion of the armor problem: "We will not enter here into a discussion of the merits of those who have succeeded in getting their names attached to the various patented methods of surface hardening, but hope that those who deserve it will get the pecuniary benefit. Ellis treated the first thick plate many years ago; Harvey revived this method, and with the assistance of the Navy Department secured patents which received attention from abroad because of the prominence our Navy Department gave them."
The writer does not believe that Mr. Jacques intends to imply that the Navy Department has the power to obtain or assist to obtain an illegal patent. The assumption that Mr. Harvey revived the Ellis patent is not correct. The old cementation process was carried on usually in cast iron or fire-clay pots at a much lower temperature than that now employed. Had Mr. Harvey proposed merely to cement or convert steel at a temperature above that of molten cast iron, a temperature which would soon have destroyed the old cementation pots, there would still have been considerable novelty in the claim. But Mr. Harvey proposed to do something more by using this high temperature : he proposed to improve the steel, to impart to ingots or other objects of low steel, such as Bessemer steel, the qualities of refined crucible steel! That he succeeded in this, and that his process is in this respect one of a number somewhat akin to it by which inferior steel is improved, must be known to every steel maker. Whether this particular process is essential to the cementation of such high grade material as that of which armor is manufactured, is a different question. It is certain, however, that the Harvey patents cover the process when carried on at that high temperature.
There is an error of minor importance in the article of Lieutenant Jacques above mentioned. The title of Figure "F," Bethlehem 17-in. N-Steel Carbonized, Indiana's Barbettes," is incorrect. Later, in describing Figure "Y," the attack of the same plate by Johnson capped shot, he again speaks of it as carbonized. This was not the case, and its perforation should not be charged against face hardened armor; had the Indiana's 17-inch Barbette plate been face hardened, the premium velocity shot would have smashed on it, as it did on the "Massachusetts" Barbette, instead of perforating it with ease.
The writer has the greatest respect for the energy and ability of Mr. Ellis, but if credit is to be given for the cementation of armor, he must share the honor with others.
We learn in Lieutenant Very's "Development of Armor for Naval Use" that early in 1863 a Mr. Cotchette submitted the following armor proposal to the English Iron Committee: "Upon an armor plate, say 3 inches thick, weld a surface of blistered steel ¾ of an inch thick; or 'convert,' to a depth of ¼ of an inch, the face of an armor plate 31/2 inches thick, the plates being subsequently passed through a pair of rolls for consolidation and to reduce the blisters. The face of the plates could then be hardened."
As early as 1867, Jacob Reese, of Pittsburg, Penn., in patenting a cementation compound, proposed cementing and hardening the surface of armor plates. No attempt appears to have been made, however, to carry out his proposition. Ten years later, John D. Ellis patented an armor face hardening process in which a plate wholly of soft iron or having a steely part on either one or both faces had one or both of these surfaces cemented with charcoal in an ordinary converting furnace. This cementation might be effected either before or after the plate was reduced to its finished size.
In the same year, 1877, the Cammell-Wilson patent was allowed, in which two-fifths of the back of a hard steel plate was decarburized and softened, leaving the face hard and strong. The same firm at this time tempered low steel plates by plunging them in water, which rendered them tougher and more tenacious than when cooled in the air.
In August, 1877, the first Wilson compound plate was tested; this plate had a steel face and a four-inch wroughl iron back.
In 1878, Wilson proposed soldering the steel face to the iron back by means of tin, zinc, spelter or bronze (a perfectly feasible method, by the way, which may yet be employed to secure thin armor to the ship). This steel face was to be formed of a number of hexagonal or other shaped pieces, in order to localize fractures.
Whitworth, too, proposed, for the same purpose, securing hexagonal plates of very hard steel to a softer back by means of screws.
At the Portsmouth trial of 1888, the "Jessop" plate consisted of a three-inch front cast steel plate, composed of twelve separate pieces of very hard cast steel fastened in a special manner to the seven and one-half inch rear piece of soft cast steel. The theory was that the laminations of the outside plate would localize destruction and prevent the extension of cracks through the plate. This theory was found to be correct.
Thus far there seemed to be no consideration given to a mean between hard steel, containing about one per cent, of carbon, and which cracked and peeled from the backing, and soft wrought iron, which allowed perforation. The fact that steel could be made sufficiently tough to resist equally as well as compound armor without cracking, although claimed by Schneider, was generally denied by armor makers. No wonder that, under these circumstances, certain authorities made the assertion that physical characteristics had nothing to do with ballistic resistance.
In 1889, the attention of Commander W. M. Folger, U. S. N., the Inspector of Ordnance at the Naval Gun Factory, was attracted by a description of the Harvey process as applied to engraver's plates. At that time it was difficult to obtain steel suitable for gas check disks on account of its cracking in tempering, and Commander Folger concluded to try the Harvey process in obtaining the desired steel. A number of sets of rough steel disks were accordingly Harveyed, machined, and tempered, in a most satisfactory manner. Efforts were then made to Harvey a number of small caliber armor-piercing shell, the manufacture of which, in this country, was meeting with very slight success at that time. This attempt was made by grading the carbon from point to base in a foundation billet of mild steel afterwards forged into a shell. The carbon shell were, however, unable to compete with those containing chromium, and no great success was attained until long afterwards.
Commander Folger then decided to apply the process to armor by bringing the carbon in the face of a 28 per cent, carbon plate up to a point that would take a chill; the low carbon center and back retaining their softness. In this way he believed that the plate could be uniformly heated and cooled throughout, leaving it free from structural strains and with the minimum amount of distortion, defects which had been found to be very serious in certain tempered French plates of homogeneous steel. It will be seen that this was really an application of the Ellis process at a temperature higher than that usually employed in cementation.
The first experiment was not a success; the carbon penetrated three inches in depth, far beyond the reach of a true chill; but while attaining a percentage of 1.05 C at the surface, it fell immediately to 0.68C. In consequence, the plate, which was only six inches thick, through being hard rather than strong over its front half, lost much of its extensibility and toughness, while the comparatively low percentage of carbon below the surface film prevented anything more than a very superficial chill.
This plate was tested in June, 1890. Excellent 6-pdr. armor-piercing shell made no impression upon it, but a 4" A. P. shell weighing 36 lbs., and moving with a velocity of 1890 ft. s., broke it into four nearly equal fragments. The shell failed to penetrate; it merely crushed the face in 1.25" and then broke up.
Two problems connected with the tempering of armor plates appeared prominent at this time: First, the avoidance of distortion; second, the rapid extraction of heat in order to carry the chill farther into the metal. An effort had been made to overcome the first difficulty in the autumn of 1889 by a spray apparatus designed by the writer to cool the plate uniformly. This consisted of pipes radiating from a central feeder ; on the upper sides of these were the spray holes gradually diminishing in size from the center to the ends. In this manner it was hoped to so regulate the volume of spray as to cool the center of the plate, surrounded as it was on all sides by hot metal, at the same rate of speed as the edges. This apparatus, however, being applied to only one side of the plate, did not overcome the difficulty. The spray pressure employed was over fifty lbs. per square inch. In tempering shell it had been noticed that the metal originally smoothly finished became ridged and wiry, the tool marks being brought out and strongly accentuated. This was easily explained: the fibers or drawn-out particles of the metal, sheared off and bruised down by the cutting tool, were expanded and raised in heating, but the subsequent contraction in hardening being principally in the direction of their length, left the ends bristling up, marking the ridges.
It was suggested that the presence of these ridges must expedite the chilling of the metal, for, while heat was abstracted from the surface of a ridge or sides of a gash by the spray, it could only flow into the ridge from the body of the shell over the much smaller chordal area; hence its more rapid flow from the interior owing to a greater head, as it were, being maintained.
The idea of covering the surface of an armor plate, either in casting, forging, or machining, with gashes or corrugations so as to increase the area exposed to the cooling medium followed immediately; and after that came the possibility of varying the proportions and spacing of these enlargements so as to avoid distortions by obtaining a uniform cooling effect from the center to the edges of the plate. No efforts were made, however, at this time to determine the value of these ideas, and in the next three years the Harvey process was developed into practicability by the two armor making firms in this country under the immediate direction of Commander Folger, then Chief of the Bureau of Ordnance. The distortion feared was controlled by giving the plate an initial set in the opposite direction, by spraying both sides of the plate instead of one, so that by modifying the pressure on one side or the other as required, the contractions were made uniform from the faces inward and the plate retained in shape, and occasionally by rectifying under the press. The resistance of these plates so far exceeded that formerly obtained from simple steel and compound plates that the process at the start was regarded as satisfactory, no effort being made to carry the chill into a specified depth for each thickness of plate.
In 1891, Mr. Harvey patented the application of his process as a method of obtaining a decrementally hardened armor plate. It was his contention that, when the cementation was carried on at a temperature above that of molten cast iron, the effect produced was not confined to the carburization of the metal, but was entirely different from the ordinary cementation process, in that the metal was improved in quality, rendered fine grained, soft and susceptible of hardening to a degree corresponding to the height of the temperature employed in cementation. His process was therefore distinguished from the old ones employed by the manufacturers of blister steel and improvers of steel for over two hundred years, as the "high temperature cementation process." Doubtless such an improvement occurs through the volatilization of deleterious components and change of structure in inferior metal, but analyses show that this is not the case in the superior quality of metal now employed for armor in this country.
In January, 1891, a 10.5" Schneider steel plate was Harveyed at Newark, tempered at the Washington Navy Yard, and tested at Indian Head. Its behavior was so excellent that a cementation furnace was at once erected at the Gun Factory and a number of experimental all steel and nickel steel plates of varying composition furnished by the Carnegie Steel Company were treated and compared ballistically with similar oil tempered plates. Upon the experience thus gathered, the first directions for finish Harveyed armor from regular makers was based.
In September, 1891, three 10.5" plates were Harveyed at the Carnegie Steel Works and two at Bethlehem; these plates were employed in comparison with oil tempered plates to determine the relative resistance of high, .40 to . 53 per cent, and low, .22 to . 24 . per cent, carbon, all steel, nickel steel, Harveyed steel, and Harveyed nickel steel plates. The Bethlehem high carbon Harveyed nickel steel plate was by far the best. The Armor Board, however, expressed some scepticism as to the accuracy of the carbon analyses.
In July, 1892, Bethlehem made two 10.5-inch Harveyed nickel steel plates, one forged to 12.5 inches, Harveyed, and then forged to 10.5 inches; and the other Harveyed at its final thickness of 10.5 inches. The resistance of these plates was phenomenal for that time, the latter being the better of the two, through defective tempering of the first.
In October, 1892, Bethlehem manufactured a 10.5-inch curved Harveyed nickel steel plate; and in January, 1893, a 14-inch plate was made by the same firm. Upon the tests of these plates, ranging in thickness from 3 to 14 inches, the requirements of service Harvey plates were based. It is only fair to the Naval Bureau of Ordnance to note that the entire responsibility in case of failure, as well as the expenses and program for even the last details of treatment rested at this time upon it. Early in 1893, the first contract for service Harveyed armor was made with the two great armor making firms of the United States.
The temperatures thus far employed in cementation were stated to exceed 2500° F.; it is now believed that they were as a rule under 2010° F. Since that time many difficulties have arisen and been overcome; certain of these have been referred to the Bureau as unforeseen and concomitant to the process, and therefore lying within the sphere of its responsibility, so that the process as it exists at present, while giving full credit to the armor makers, is a result to which the Bureau has continually given its assistance.
So many misleading statements have been made and so many doubtful claims advanced as to the advantage of great depth of carbon penetration, while the entire process seems susceptible of so many modifications and improvements, it would be best to describe briefly the various steps of the manufacture, and more at length the theories built on those steps. For it is a fact that the theories both of cementation and hardening follow the practice but lamely and are open to numerous criticisms.
It must be understood that in describing a manufacturing process, it is necessary to draw the line sharply between the general plan or theory, which is doubtless fully protected by the patent, and the various practical details born of the manufacturer's experience and by which he makes the patent a commercial success, and the process economical and profitable.
The information which is accessible to the public, however, in patent specifications, text-books, and the well-known methods of practical men in overcoming the difficulties to be encountered, is considerable.
The ingot from which the armor plate to be face hardened is forged must be even more carefully selected than in the case of oil tempered plates. The carbon gases in cementation seize upon and reduce the oxides found where defective welds exist, as in pipes, cold flaps and snakes, leaving thin fissures which are apt to extend and even fracture the plate in water tempering, owing to the lack of continuity of metal, breaking up and rendering the stresses irregular and unbalanced. In large ingots, especially when cast at too high a temperature and slowly cooled, the segregation of the metalloids, that accumulation of the specifically lighter and more fluid combinations of phosphorus, sulphur, carbon and silicon with iron, in the upper and central portions of the ingot where the metal last congeals, destroys its homogeneity and causes its thermal and physical characteristics to vary so much in a short distance as to practically amount to lack of continuity, especially under the extremely sudden and violent stresses of tempering. The segregation is composed of hard, brittle metal, and though its coefficient of heat expansion or contraction may not differ sufficiently from that of the body of the plate to cause important stresses, still it is surrounded and braced by that metal and must expand and contract with it This would not be serious in a slowly cooled or heated plate. In the case of a wide, thick plate quickly chilled on both faces, the exterior particles set over an expanded interior and are fixed while in a state of tension. Later successive layers of the interior cool and contract, placing the exterior in a state of compression, but these inner layers being then the more mobile of the two, instead of curving the exterior so as to wholly satisfy their contraction, are themselves placed in tension, gradually released in part by the yielding of the metal. This yielding must nearly correspond, unit for unit, with the remaining extension of the surface, otherwise there would be a sliding movement of one strata on the other. The tension is undoubtedly greater toward the center of the plate, however, as the edges of the strata cool earlier, and hence in a somewhat more expanded condition. It thus appears that very nearly the same amount of combined flow of particles and elastic extension is required over each unit of each strata of a uniformly cooled plate, increasing, however, toward the middle line, where in a segregated plate the metal is most brittle and least able to respond. Segregation in the ingot may therefore cause the internal rupture of a plate in tempering, no matter how uniformly and skillfully the plate be cooled. The recent failure of an 18-inch face hardened plate representing the Indiana's side armor was entirely due to this cause, the flaw being at right angles to the line of greatest contraction.
The armor plate is forged nearly to finished dimensions and rough machined, an allowance being made in its thickness to compensate for oxidizing of the back in the cementation furnace, as well as later scaling due to the bending and tempering heats. Allowances are also made in the length and width and the angles of the sides with each other and the faces, it being generally found that all faces will become convex, opening the bounding angles. The amount and method of application of these allowances are, of course, manufacturing secrets, varying for every thickness, shape, composition and treatment of plate.
The face of the plate is then carefully scaled. Abroad many of the experimental plates have been carefully planed off. The sand blast, as employed in cleaning castings, does not seem as yet to have been utilized, although it is undoubtedly just as efficient and more economical than any of the other methods.
This scaling is very important, for not only does the film of oxide act as a non-conductor, retarding the heating of the metal, but it is necessary for the carbon to reduce this oxide before it can act in further carburizing the plate. This was well understood long before this process was applied to armor, for most of the early patentees of cementation processes and compounds indicate methods of cleansing the castings or forgings to be treated. When the scale is not removed the plate will have many soft spots.
Following the scaling, the "soft strips" or parts of the plate which are to remain untreated, in order that bolt-holes for deck or other fastenings may be made, are laid off, and the plate placed in the furnace with these parts opposed to a non-carburizing or sand bed instead of the carbon. All other parts of the face except a narrow strip about the edge, left, by direction of the Bureau, not only for finishing, but to diminish as much as possible the tendency to form edge-cracks in tempering, are cemented. This process of limiting the operation of cementation has also been known and practiced for many years.
Plates of slight curvature are bent before carburizing; those of large curvature are bent afterwards before tempering. In this later bending operation superficial cracks occasionally occur in considerable number. These will be discussed later. Whatever the shape of the plate, a bed to correspond is swept in sand on the floor of the furnace, and upon this is placed a six-inch layer of the cementation compound, be it Harvey's or Pettino's mixture, or, for that matter, any one of a number of patented combinations which, upon being heated, produce carbon or hydrocarbon gases, and claim to obtain practically the same results. The Harvey mixture is probably nearly as cheap a mixture approaching a standard of uniformity as is at present manufactured; it is not peculiar in its action, however, and abroad other carbonaceous compounds are generally employed. It is composed of equal parts of animal and wood charcoal, the former being the expended "char" from sugar refineries. The combination is patented, the claim being that the heavier animal charcoal gives the light, powdered wood charcoal a body, enabling it to be handled without waste or the danger from explosions which result from heating a mixture of air and such combustible dust. The wood charcoal probably acts more as a dilutent and cheapener of the "char" than anything else; the mixture, however, renders up more slowly and uniformly its gases than would otherwise be the case, although it is stated that the approved method at present is to place a half-inch layer of pure animal charcoal next the plate, A too rapid introduction of the carbon destroys the grain and strength of the metal, making it resemble pig iron. The great thickness of the layer is rendered necessary by the volume of gases required to be generated in the long process, which exhausts the material, as well as the importance of gradually obtaining and maintaining a high and equable temperature. The great thickness of non-conducting material between the flues and the plate makes it a long operation to obtain the desired temperature, but once reached, sudden fluctuations are impossible.
The plate being placed in the furnace, its sides and back are covered with sand or loam, rammed down, then a layer of firebrick, the furnace cover is placed in position and the ends walled up. At the Carnegie Steel Works, where natural gas is used, a platform car forms the bottom and sides of the furnace, and this being charged is run into its housing and the fires started.
There are many varieties of cementation furnaces, each claiming special advantages, the important feature of all being the obtaining of a uniform and high temperature by surrounding the brick or fire-clay flask with furnaces and flues conveying the products of combustion towards the stack.
The furnaces using natural gas and oil are probably the most costly, on account of the numerous valves required, but once erected they are the most convenient and permit a nice adjustment and variation of the temperature. They are doubtless also more rapid in their action if desired.
The original furnaces employed by Mr. Harvey were very slightly different from the old-fashioned cementation furnaces; they used coal; afterward oil burners were introduced. That erected at the Naval Gun Factory in the winter of 1890-91 used soft coal, and those built at Bethlehem in 1891 are similar. Two regenerative furnaces at Homestead were temporarily converted into cementation furnaces in the summer of 1891, but their present splendid plant of twelve gas furnaces was built in 1893, and are entirely different from and regarded as superior to the old type of furnaces.
The difficulties to be guarded against and overcome in working these furnaces, no matter what the fuel, are very great. The high temperature and the great weight and size of the charge, as well as the long time required, prevent the use of iron flasks such as are used in cementing small articles, for even should the white-hot iron fail to break down under the weight, it would quickly oxidize and become unserviceable. Fire-brick or other forms of intractible material must, therefore, be used for the lining of flues and flasks. These expand largely and become extremely tender at a high temperature, introducing other dangers from settling or collapsing, by means of which air may gain access to the carbonaceous material, which is then quickly consumed and the metal fused. To raise an armor plate weighing between thirty and forty tons to the temperature of from 2230° F. to 2500° F., required by the Harvey process, and keep it there for weeks, supported on brittle fire-brick, is to run a serious risk.
These considerations, and the great expense of the process as at present applied, led the writer to investigate the practicability of reducing the temperature and time of cementation at one and the same time. The problem seemed difficult, especially as it was claimed by some that the process, even when carried out with the utmost skill and patience, was hardly a controllable one, and that a variety of results from apparently identical conditions might always be expected. The project of increasing the cooling surface of an armor plate in tempering, by corrugating or gashing its surface, was recalled, and the feasibility of employing these enlargements of the surface at an earlier time to expedite the cementation seemed unquestionable. This germ of an idea quickly expanded and the proposition became—to cover the face of an armor plate with a series of gashes, or pockets, or corrugations, by means of which the area, over which the carbon or other cementing material could penetrate, might be multiplied, and an equal percentage of carbon introduced into the superficial layer in a proportionately less time than in the case of a plane surface. These enlargements to be so proportioned as to permit the maximum percentage of carbon to extend in to a greater or less depth according to the thickness of the plate, thence shading off into the body of the plate so as to prevent a too sudden transition of grain and extensibility along a single plane—that of the inner limit of the chilled surface—in order to avoid the flaking off of this surface under impact.
The impracticability of rolling or forging such a surface down after cementation without leaving it covered with defective welds, cracks, or seams, was apparent from the first. The writer had, however, been much impressed by the numerous severe tests of Gruson chilled iron armor, from 1882 to 1890, in which it was shown that cracks confined to the chilled surface in no way reduced the resistance, and never originated, extended, or gave direction to the cracks due solely to impact. The success of Sir Joseph Whitworth's experimental plate, in which his proposition "to prevent cracks by manufacturing them" was exemplified, seemed also to indicate that such superficial defects, far from weakening the plate would prevent flaking and the extension of cracks. This conclusion was at this time supported by the excellent behavior of a face hardened plate, the surface of which was covered by a network of cracks formed in rectifying, and soon after by that of three other plates tested to destruction. In fact, on one of these plates the cracked surface was more resisting than the sound, which was to be expected, as the chill was able to enter more deeply where the edges of the cracks permitted deeper contact of the cooling agent.
Another feature of the proposition is that as the various ridges and corrugations would attain the desired temperature far earlier than the flat surface of the plate; this would reduce the time of the process materially, especially as it would not be attempted to obtain the peculiar advantages claimed from the use of a high temperature, as in the Harvey process, at all. The intention being merely to introduce a certain percentage of carbon into the superficial layers of the plate in the same manner, and at the same temperature that cementation has been carried on for many years past.
Such a process would be extremely useful when applied to rectangular blocks for the sea faces of forts, turrets and floating defenses having no room for a slope. These blocks would not necessarily be forged and shaped to the extent and accuracy of dimension required for ship armor. There are many existing fortifications of little defensive strength on account of the exposure of their masonry faces to attack, these could be protected at comparatively little expense by deeply cemented face hardened armor. There are also locations such as Roma shoals, Race rocks, etc., where turrets will be required protected by armor of this description.
It is evident that any new process which at one and the same time permits practically the same results to be obtained at a much lower temperature and in less than one-half of the time now required will be of great value, as not only will the output of a furnace be more than doubled and the cost of fuel and labor per ton of output less than one-half, but the life of the furnace will be much longer and the time, labor, and materials expended in repairs largely reduced.
The advantages claimed for the proposed process are, however, by no means confined to the reduction of the cost of manufacture. It is firmly believed that a more resisting armor can be made in this manner than is at present possible by any other process.
It is also proposed to expedite, in a measure, the heating of the charge by placing numerous pins of good conducting material in the bed of the furnace. Fluctuations in temperature will be hardly more possible than at present, while time will be saved by heating the non-conducting mass as a whole rather than from one surface.
When thin plates are cemented, a number are placed in the furnace face to face, with a layer of the carburizing material between. (This plan was first employed at the Bethlehem Iron Works early in 1893.) Embedded in this are wrought iron tubes, extending from end to end of the furnace, and in these are placed the test rods, which can be withdrawn from time to time for inspection, without admitting air to the interior. Their color, from end to end, indicates the uniformity as well as degree of temperature of different parts of the charge, and the various flues are dampened or burners and fires regulated accordingly.
When the desired temperature is reached, determined either by the color of the bars or by means of a pyrometer employed at one of the tube openings, the fires are regulated to maintain that temperature for a greater or less time, according to the thickness of the plates.
Considerable time is lost in gradually cooling the charge, as it cannot be exposed to the air at a high temperature, as oxidation and scaling would result, the plate might also be chilled or air hardened. Especially is this the case with nickel steel, the nickel seeming to render more sensitive and to increase the hardening capacity of the carbon, so that the difference of the effect of cooling in the air of a cold, moist climate as that of Sheffield, England, and the dryer, warmer atmosphere of Pittsburg or Bethlehem might cause the difficulty said to be found abroad in machining unhardened carburized nickel steel, and which our armor makers have been able in a great measure to avoid. The plates are then air annealed from a cherry red in order to break up the large crystals in the cemented face.
After carburizing, analyses are made at various depths from each end of the plate, to determine the percentage and depth of carburization. The plate is then machined and bent to such shape and dimensions as experience indicates will most nearly result in those desired after hardening. This machining, as a rule, includes all work done on the hard face, although an electric annealing apparatus has lately been devised, by means of which the hardened metal may be softened locally when desired.
The plate is then carefully heated to the temperature required for hardening, the hardness and depth of chill increasing, within a certain limit, with the height of temperature to which the plate is heated. It will be seen at once, the higher the temperature the more plastic the metal and the greater the distortion. The lower the temperature can be kept and still produce a hard surface the better for the manufacturer, as the subsequent rectification will be less difficult.
The plate being heated to the desired temperature is placed upon the tempering stand, and a powerful spray from a large number of small, evenly spaced jets is forced upon both sides of it. This spray is modified from time to time as required to keep the plate in shape. After the plate's exterior has been thoroughly chilled, it is lifted into the oil bath and left there until cold. Occasionally a plate will be found that will not respond to treatment, becoming distorted, in which case the process is repeated. In so doing, of course, there may be a loss from scale as well as the oxidation of the superficial carbon. If the plate is but slightly out of shape it may be bent cold under the press; this, however, frequently causes the hard face to crack in a most alarming manner. The effect of these cracks, however, on the ballistic resistance is, as has been stated, of no consequence.
Finally, the plate has its bolt holes tapped in the back and is finish-machined, it sometimes being- necessary to employ an emery wheel, or electric annealing, at the hardened edges to obtain the desired perfection of joints and butts.
A great difficulty in the treatment at first was the oxidation on the back of the plate; there is no loss from the carburized face unless it is overheated. In England a clayey cement has been used with good results. At St. Chamond it has been the custom to decarburize the back of the plate in order to render it more ductile while carburizing the face. It is doubtful whether any benefit is derived from this, however, unless the plate later receives some forging. It is also said that plates are very much improved after cementation by a careful annealing in carbon. Doubtless, if armor plates are machined after cementation, a thorough annealing before hardening would remove any stresses liberated by the removal of the surface metal, and which would tend to complicate and render those introduced in tempering unmanageable, otherwise the annealing seems expensive and unnecessary except, of course, when the plates are removed from the cementation furnace at a temperature sufficiently high to be air hardened.
The effect of water hardening a face hardened plate varies not only with the depth of the strata of metal but with its composition. Careful analyses show that in good plates the normal carbon may be found at a depth of 1.25", but that the effect of water tempering will be found to reach to the heart, increasing the strength and toughness to a remarkable degree even though the normal carbon may not exceed 25 per cent. Vickers' claim to obtain this effect at a depth of seven inches in a ten and one-half inch plate is fully justified by facts.
SECTION II.
CEMENTATION.
The practical features of the manufacturing of modern face hardened armor having been discussed, it will be interesting to note the present state of the theories of the two important steps, cementation and hardening.
The art of cementation was practiced by the ancients. Tubal- Cain made steel by surrounding iron with charcoal and exposing it to the long continued action of a comparatively low and slowly penetrating heat. Later, furnaces were constructed which could be sealed so as to exclude the air to avoid melting or oxidizing the charge, while the higher temperature employed permitted the carburization to be carried on rapidly.
In Landrin's Treatise on Steel is given an interesting account of experiments made by Reaumur in 1722 to determine the best compound with which to convert iron into steel. For this purpose, Reaumur used not only the materials commonly employed for case hardening in France, but specially prepared and exploited mixtures, as well as certain formulas said to have been obtained in Germany. Iron bars were heated in crucibles with various inert substances, such as sand, potter's clay, ashes, glass and lime, "the only apparent change being a loss of fibre in fibrous iron and a diminution of thickness of lamellae in laminated iron."
Various salts and alkalies were also tried, without difference of effect, in combination with these inert substances. Oils, when mixed with sand or clay, burned off and were lost before the metal was sufficiently heated. A combination of oils and alkaline salts, as soap, or soot with charred horn and leather, however, was found to transform the iron into steel.
"Charcoal, soot, or old burned leather, of themselves, produced a fine, hard steel, difficult to work, and even after forging full of flaws and cracks. Pit coal, powdered and sifted, had a very rapid effect, diminishing the volume and corroding the metal which became hard and fine, but harsh steel."
His conclusion was that powerful alkalies helped the conversion, but the resultant steel was difficult to work, full of flaws and incapable of welding or drawing out. A peculiar effect occasionally produced by certain salts, as sal ammoniac, green vitriol, etc., was that the steel was not lasting, for when forged and hardened once, it had a fine grain, but forged and hardened a second time, it had scarcely any grain. Finally, common sea salt was regarded as the most suitable for the conversion of iron into a fine, hard steel, easily worked and lasting.
"The composition which answered the best for converting iron into a very fine and hard steel is 2 parts soot, 1 part powdered charcoal, 1 part ashes, and ¾ part of common salt.
"The formula should be varied to suit the iron. The greater the percentage of oily matter, found mostly in the soot and charcoal, the more rapid the process, though the steel is apt to be flawy and hard to work. Increasing the percentage of ashes slows the operation and diminishes the deleterious effects; the minute proportion of alkali in the ashes acting as a carrier, the earthy matter as a moderator. The salt is not absolutely essential, but it hastens the operation, adding to the fineness and hardness of the steel as well as largely reducing the amount of composition required. Increasing the amount of salt increased the flaws. In converting irons tending to become harsh, 1 part of lime or calcined bones might be added with the result that a steel otherwise impossible to forge could be easily worked." One-eighth part of lime added diminished the blisters which later gave a name to this kind of steel.
So much for the published knowledge of the process of cementing steel 175 years ago. Since that time a large number of processes have been advocated, and it will be interesting to note the more or less reliable claims of some of these.
In 1859, Mr. Johnson patented "a cementation compound of equal parts of quick-lime, bone dust, and wood charcoal, which, after an intimate mixture, was exposed to dry weather for several days." He explains "that this enabled the lime to absorb carbonic acid from the atmosphere, by which means he obtained the necessary carbon in the purest and most convenient form." He found that Swedish or Danemora iron "containing so much P as to give an odor of it when twisted at a red heat " is cemented much more rapidly than other irons. "A bar of such iron 3/16" thick being converted completely in two hours, while a similar bar of English iron was converted but 1/16” deep in the same time." Believing that P was essential, he added bone dust on account of the basic phosphate contained, which either enters into combination with the iron or, by more or less doubtful catalytic action, aids in accomplishing the same result. This mixture could be used again and again after each operation by exposing it to the atmosphere so as to take up CO2, and adding a small amount of lime.
The fact is, of course, the highly oxidized lime probably took up moisture by selection of H from the air as well as C from the carbon present in the charcoal.
The presence of P was regarded as important by the late Mr. Harvey, who found that the operation was rendered much slower and less effective without P, and that the bone charcoal was therefore the most important ingredient in his mixture. As, however, the basic phosphate is not reduced at the usual temperature of cementation, its effect can hardly be due to the absorption of P by the metal.
In 1861, Mr. Weston explained the action of a mixture of cyanide of potash and charcoal as follows: "The cyanide is decomposed by heat into cyanogen gas and potassium, and the first upon contact with the metal is broken up into C and N, the C uniting with the iron while the N unites with the charcoal, and in the presence of the potassium forms another portion of potassium cyanide. Any compound of cyanogen with an alkaline metal may be used."
It has been asserted by M. Fremy that cementation cannot take place without nitrogen. This is not so, cold iron buried in powdered charcoal absorbs carbon even without the aid of heat. Nitrogen is merely a convenient gaseous carrier. Under the effect of heat it combines with carbon present, forming the easily decomposed cyanogen gas which penetrates the expanded pores of the metal and weakly gives up its carbon to the iron. If hydrogen is present, a small proportion of H4N is formed, which accounts for the ammonia gas found accumulated in the shrinkage cavities of castings or often perceptible when metal is fractured.
In 1868, a Mr. Sheehan places in the bottom of his retort fragments of limestone covered with a perforated plate, and above that a carbonaceous mixture of 200 parts of charcoal saturated with water, 30 parts of muriate of soda, 12 parts of sal soda, 5 parts each of black rosin and black oxide of manganese. "On heating, the carbon is expelled from the limestone and unites with the oxygen and carbonaceeus ingredients above to convert the iron."
In 1875, M. Eyqueur employed peat with 1 ½ per cent, ammoniacal salt, preferably hydrochlorate. He states that "the carburization takes place under the simultaneous action of ammoniacal and carbureted hydrogen gases, the iron passing to a state of zoto carburet and the rapidity depending upon the nascent condition of the gases.''
In 1891, Brown's compound was patented; it is composed of 87 per cent, pure carbon, 8 per cent, calcined lime, 4 per cent, soda ash, and ½ per cent, each of tungstic acid and sal ammoniac.
The action is said to be that "the soda ash frees the metal of oxygen, opening the pores or intermolecular spaces, while the calcined lime eliminates the oxygen set free or remaining in the retort. The bone carbon then commences to throw off its carbon, and with the help of the ammoniacal gas generated from the sal ammoniac a pure cyanogen gas is generated which permeates the metal, carrying with it the free tungstic acid which tends to give the metal greater hardness."
In 1893, a Mr. Hunter patented a compound of 25 parts muriatic acid, 16 of salt, 32 of chloride of lime, 32 of carbon. The action claimed is that "hypochlorous acid, HClO, generated in contact with the heated carbon and metal by which it is decomposed into Cl, O, and H, the oxygen and hydrogen taking up carbon and with it penetrating the iron, the operation being facilitated by the presence of the Cl. The salt may be omitted but is important when the metal contains considerable silicon."
In Macintosh's process, wrought iron bars were suspended in a furnace, the walls of which are highly heated; de-sulphurized coal gas is then passed through. The process was very efficacious, though expensive.
There are also many gaseous methods of cementing steel, that of Schneider, for example, in which a highly heated retort contains two armor plates placed face to face and separated by a frame work at the edges, thus forming a chamber into which hydrocarbon gas at a constant pressure is introduced at a high temperature.
There are also numerous other processes of combining the cementation and improvement of iron and steel, by heating in the presence of carbonaceous mixtures. The explanations of very few, however, are satisfactory, and the claims of many others are based on assumptions which are far from being generally accepted as scientific facts.
The processes may be divided into groups, as they employ gaseous and solid or liquid compounds. Probably all are really gaseous in action, for it is difficult otherwise to explain the transfer of solid carbon from the distant and large sized lumps of charcoal employed in the old cementation process. It has also been found in cementing armor that the carbon gases at times penetrate considerable thicknesses of sand and impregnate the steel beneath.
The movement and commingling of atoms of many different substances when closely associated is known to occur under the influence of heat. The well-known experiment of Sir Lowthian Bell in which smoothed discs of cast iron of 3.25 per cent, and wrought iron of .04 per cent, carbon, were tightly bolted together and heated in a furnace for one month with the result that the cast iron lost 1.07 per cent, and the wrought iron gained .348 per cent, carbon, would seem to indicate at first that the carbon gained, under the influence of heat and pressure, something of the freedom of a gas. Still the enormous volume of elemental gases contained by the cast metal, renders such a conclusion unnecessary. This is also the case with the old method of cementation, in which a heated bar was stirred in molten highly carburized cast iron and then quenched; or in the comparatively late process, in which iron is cast directly upon the face of an iron or low steel plate and then exposed to severe heat for a long period of time with the result that the carbon gradually becomes so distributed as to destroy the plane of demarkation.
This interchange of atoms is not confined to carbon alone, for it is noted in the Journal of the Iron and Steel Institute, Vol. I., 1889, p. 368, that the welding of iron to nickel brought about the transposition of atoms. For after dissolving off the iron back it was found that the percentage of iron in the nickel had increased from 0.9 per cent, in the original to 3 per cent., the normal percentage of iron being found only at a depth of .45 of the thickness of the nickel. Iron is stated to be volatile at a medium red heat, for when alternate sheets of nickel and iron are heated to redness for some time, the former increases in weight through absorption of iron, a true alloy of iron and nickel forming at the surface of the latter. Whether the nickel has a special influence, or whether this volatilization is going on at all times and in all directions from heated iron is not stated. It would therefore seem that the iron has the power to meet the carbon half way, both being volatilized and possessing affinities, when, of course, the formation of a definite carbide would naturally follow.
In Sir Lowthian Bell's Principles of the Manufacture of Iron and Steel, he demonstrates the readiness with which carbon is deposited at and up to a red heat in iron sponge from carbonic oxide. In the Journal of the Iron and Steel Institute, Vol. II., 1891, he also notes that nodules of iron oxide in the bricks lining the flue along which the gases for a blast furnace were conducted caused the deposition of carbon from carbonic oxide which penetrated the bricks, thus cracking them apart. Mr. Snelus also found that red brick in flues took up enormous quantities of carbon from the gases, one brick containing as much as 45 per cent, of carbon.
Sir Frederick Abel also stated before the Iron and Steel Institute in 1892: "The carbon impregnation of an iron ore takes place at as low a temperature as de-oxidation, which in Cleveland ore occurs between 392° F. and 410° F. At that temperature freshly reduced spongy iron reduces carbon from carbonic oxide to an extent corresponding to 20 to 24 per cent, of its weight, but as the temperature approaches a red heat, the deposition of carbon diminishes considerably in amount. The increased effect of a nascent condition of both iron and carbon is here apparent. Experiments also showed that nickel, and to a smaller extent cobalt, suffer reduction from their oxides (below red heat) with deposition of carbon."
In the Encyclopaedia Brittannica is found "The process of cementation is that of the occlusion in the iron of CO formed by the combination of C with the air in the charcoal. This is then decomposed by the iron into C and an iron oxide, which is then reduced by a second portion of CO.
The CO2 penetrates less readily through the metal than the CO2 and in doing so forms blisters. This seems hardly probable. It is much more probable that the CO2 having satisfied its affinity is the only gas left in evidence in the presence of the oxidized scale or blister.
Judging from the statements made by Sir Lowthian Bell and Sir Fred. Abel, the formulas given in the Encyclopaedia are correct for low temperatures; Sir Fred. Abel states, however, that as the temperature approaches a red heat, the deposition of carbon largely diminishes.
As the temperature at which carbon combines with iron in the carbide FE3C lies at about 1200° F., it would appear that above that temperature the carbon must be associated with the iron in another form, perhaps merely deposited, while the carrying gas, be it N in cyanogen, O in the oxides, or H in the hydrocarbons, joins some more volatile substance present, or escapes in the elemental condition.
From this it would appear that when the O present is exhausted the process must stop, a conclusion which agrees with the practice of renewing exhausted compounds by aeration.
The iron exposed in the pores has a capacity as FE3C, for one fourteenth its weight of C. In the entire mass only 63 per cent. of that has been found of combined carbon.
The changes in volume of the metal in solidifying and cooling are very irregular; thus, steel on congealing expands, on cooling contracts, but in the latter case neither regularly nor continuously. There are three points of recalescence or evolution of heat accompanied by an increase of volume in cooling from a temperature slightly over 1800° F. These, according to Mr. Osmond (see Journal of the Iron and Steel Institute, 1890, Vol. I.), are: ist. A slight evolution of heat at about 1562° F. 2d. A very faint evolution of heat at about 1382° F. 3d. A point, absent in mild steels but strongly marked in high carbon steels, at 1200° F. These "critical points" and their meaning are discussed in a valuable paper on the Physical Influence of Elements on Iron, by Prof. J. O. Arnold, in the Journal of the Iron and Steel Institute, 1894, Vol. I. The highest critical point is considered to be due to chemical changes accompanying the evolution of hydrogen. That at 1380° F. is regarded as physical, due to the passage from a plastic to a crystalline condition; and the lowest is due to the combination of iron and carbon into the definite carbide FE3C.
Undoubtedly the process of cementation is affected by the various conditions of expansion and structure of the metal found between the normal temperature and the melting point. There are reasons for believing, however, that no advantage will be obtained by exceeding 1832° F., all of these phenomena being manifested below that temperature. Above it, the metal is regarded as contracting to the melting point, the pores are diminished in size and the carbonizing gas has become extremely attenuated. Doubtless the greater molecular activity at this high temperature will permit a considerably greater absorption of carbon in the surface layer with an abrupt reduction to the normal percentage. The time required to reach and cool down from this temperature, the cost of fuel, together with the greater risk and uncertainty, render the high temperature less satisfactory than another method which may be proposed. That is, by corrugating the surface or covering it with shallow pockets, the walls of which being thin will be affected much earlier and at a lower temperature than the massive body of the plate. Later these irregularities may be forged down if desired.
Percy says that pure carbon will not cement, and that it will only continue to combine while gaseous matters are given off. It is evident, however, judging from the most successful and rapid-carburizing agents, that the process of cementation is best carried on by a compound which under heat influence liberates an easily decomposed carbon gas. This, penetrating the pores of the metal in a nascent condition, combines readily with it, especially if the iron be freshly reduced. This is known to be the case with hydrocarbon gases, and cyanogen, and is believed to be the case with carbonic oxide and dioxide. Any material which when heated would continuously render up these gases would enable cementation to be carried on. Above the temperature of 1200° F. this process is probably effected, in the case of carbon gases, by the deposition of carbon and the formation of an iron oxide. The latter may be reduced by CO with the formation of CO2 which passes on into the metal repeating the operation, the smaller volume of C, and the less degree of completeness of each succeeding operation, causing a gradual diminution of the carbon deposited.
It follows, therefore, that if a larger surface of metal is exposed to the action of the gases, as by gashing or scarifying the surface of the plate, the process will be greatly expedited, the volume of gas directly in contact with the plate being not only much greater but at work at the same time over the entire depth of metal to be treated.
Hammered iron is said to be more rapidly cemented than rolled iron perhaps on account of the rough scale left in the latter case. It is also said that the porosity of steel is generally in an inverse ratio to its tensile strength; this is not always so, still it is very certain that the low tensile requirement before treatment makes possible the employment of a steel which takes up the carbon very readily.
The process of cementation also reduces the amount of sulphur present; according to Boussingault, one-half at least is volatilized in the form of a carbon sulphide. This doubtless refers to inferior grades. It would seem that by the disengagement of volatile matters associated with the iron, the latter would be left in a receptive condition, favorable to rapid cementation.
The heavier the charge the longer must it be maintained at the temperature of cementation to obtain the same depth and percentage of carbonization; in fact, the amount of carburization in equal times varies approximately inversely as the volumes of the charges.
The temperatures at which the cementation is carried on varies greatly; that employed in the manufacture of blister steel is about 2100° F., that for the Harvey process, "above the temperature of molten cast iron," must exceed 2228° F. The desired effects, however, have been produced in about the same time at as low a temperature as 1750° F., and many excellent armor plates have been cemented at a temperature never reaching 2000° F. In fact, little if any of the excellent face hardened armor manufactured at Bethlehem has been cemented at a temperature above that of molten cast iron. At Carnegie's the temperature has also been reduced, it is believed with benefit to the metal. That found most satisfactory abroad is said to be 2000°-2050° F., which is below the melting point of even white cast iron. It is probable that with greater experience European armor makers will reduce the temperature.
The time required to reach the required temperature varies; a plate 6.4" thick required 4 days, and the temperature was maintained 6 ½ days, after which it was allowed to fall. In the case of a 9. 5" plate, these times were respectively 6 and 8 days. These plates were of excellent quality.
The percentage of carbon required on the surface by the Bureau of Ordnance is that which will not through its regular diminution reduce the depth of chill given by the hardening process, limited and regulated as it is to prevent distortion.
SECTION III.
HARDENING.
Henry Marion Howe explains the effect of hardening in improving the strength and toughness of steel as follows:
"Dissimilar rates of contraction produce a kneading effect. There must be between the various layers considerable kneading pressure and rubbing; and as this, in kneading dough or putty, through compression; in forging, through compression and tension; in molasses candy, through tension, appears to increase the intermolecular cohesion, so we may ascribe to this feature of hardening the increased strength and toughness." . . , "The hardness proper is due to the maintenance of the chemical condition existing at a red heat, but the changes in tensile strength and ductility to a joint effect of chemical and physical origin. Annealing may obliterate all the effects of sudden cooling except that due to the kneading of the metal, which resembles forging and increases the strength, so that by proper management we may increase both tensile strength and ductility by tempering and annealing."
The Societe de Chatillon et Commentry claim to have established the fact that soft steels simply cast, after being lead tempered, have the properties of forged steel. The process, however, requires great knowledge of the conditions for making the steel, the temperatures to maintain varying very sensibly with the nature of the metal.
M. Pourcel, of the Terre Noire Works, has also long contended that all the benefit of forging can be obtained on large castings by a proper course of treatment.
It is the idea of Frederick Siemens in tempering glass that by uniformly cooling every part of the body of the glass, no matter how rapidly this is done, the contraction of the mass—the cessation of molecular movement—is uniform and there are no strains introduced. Granted that no irregular strains would be introduced, such as snap a tempered steel shell, still there must be minute balanced couples of stresses which hold the molecules together with greater tenacity than in the case of ordinary glass.
If these couples making, perhaps, an enormous aggregate, are irregularly disseminated among the weaker couples of ordinary glass molecules, then tho.se stresses tend to develop which so frequently cause the spontaneous fracture of tempered articles. Such a condition must result when steel is heated too rapidly or irregularly. The carbide is not uniformly disseminated, the metal is not uniformly expanded, and the result of rapid cooling cannot help but accentuate these irregularities until perhaps the surface is covered with cracks or the metal actually flies to pieces.
It is difficult to conceive a rearrangement of molecules causing a body to occupy a larger volume according to the degree of hardness when chilled than when slowly cooled, without the introduction of stresses. It seems probable that the unsatisfied affinities of the carbon for the iron, existing, however, only during a very brief period of the cooling, places the whole body in a state of constraint. There is strong doubt that at any subsequent period of cooling these stresses are released by a conformity of the molecules, as each instant their movements become more difficult; besides a shrinkage of the mass would hardly satisfy a chemical affinity.
Professor S. P. Langley has demonstrated that it is possible to somewhat harden steel from a temperature no higher than that of boiling water, which would indicate the continuance of a very powerful affinity, as the movement of the atoms must then be extremely difficult. Similarly it is known that severely hardened tools have been found with the lapse of time to have become tougher and less brittle hard.
The hardening tendency is said to be proportional: (1) to the amount of carbon, hence the volume of chemical attraction ; (2) to the rapidity of cooling, hence the proportion of that attraction unsatisfied; (3) the height and duration of the temperature of hardening, hence the amount of carbon dissociated from the carbide and disseminated among the grains of iron.
At a temperature above 1200° F. the carbon existing in the form of a carbide of iron, irregularly coagulated in the steel, commences to disintegrate and dissolve itself through the mass of metal. This movement is undoubtedly the more difficult the less the metal is expanded and heated above the temperature of chemical combination. The more slowly the metal is heated to the higher temperature, the tougher it becomes without loss of hardness ; this toughness increasing with the length of time of exposure to that temperature. The higher the temperature of hardening, within limits, the deeper and stronger the effect of the chill. Upon cooling the metal slowly, however, the carbon again seizes the iron and with the aid of the forces of chemical affinity and incipient crystallization again becomes segregated.
The range of maximum hardening effect has, therefore, for its upper limit that temperature at which the heat dilation first overcomes the chemical attraction, and for its lower limit that point where the chemical attraction is no longer able to move the sluggish atoms.
"If the steel be long exposed to a high temperature, say a light red or orange, it assumes a coarsely crystalline structure which it retains in cooling, and its toughness and strength are greatly impaired. The more slowly the steel is cooled within reasonable limits the softer and tougher it becomes, the major part of the effect being produced in cooling from a cherry red to scarcely visible red.
"The best general results in hardening are produced by quenching from the lowest temperature which will produce the desired result. The more rapid the cooling the harder the steel and, up to a certain point, the greater the tensile strength, but if very violent the strength may be diminished. The lower the C the more rapid should be the cooling to give the greatest advantage."—Howe.
The rapidity of cooling, in turn, depends upon the cooling medium. "In general, the greater the specific gravity, specific heat, mobility, latent heat of gasification, coefficient of expansion, and thermal conductivity, and the lower the boiling point and the initial temperature of the cooling media, the more suddenly will the immersed metal cool."—Greenwood.
The thicker the piece the greater the chill required to produce the same hardness.
Experiments show clearly that the transmission per degree of difference between hot gases on one side and water on the other side of a plate was directly proportional to that difference, the total transmission therefore being proportional to the square of the difference. The more smoothly machined the faces were the less efficient in transmitting heat. This might be expected, as the rougher they are the larger the heat transmitting area.
Caron concludes, after experimenting with mercury, water containing different salts, covered with oil, or containing syrupy or mucilagious matters, that the degree of hardness and other effects appear to be inversely proportional to the square of the time of cooling the metal.
Chernoff says, with regard to cooling in water, the conductivity of hot metals is very small, and that although the external visible parts soon show the desired fall of temperature, yet the central portions remam very much hotter. In fact, while the specific heat increases with the temperature, the conductivity decreases, iron losing nearly 25 per cent, between 0° C. and 100° C. This evidently limits the depth of "chill" obtained even with the most violent treatment, no matter what the composition of the plate.
The fact that in a thin plate there is a smaller accumulation of heat to be dissipated in chilling than in a thick one, indicates at once that, as the hardening effect is due to the rapidity with which the temperature is caused to fall from redness to 450° F., and that as the specific conductivity of the metal diminishes as it becomes hotter, the heat transmitted between two points, while varying nearly directly as the difference of temperature, is less and less for that difference as the interior of the plate is approached. At the same time the outer layer parts with its heat with a rapidity nearly due to a flow from a mean point between redness and 450° F. to water of 40° F. This is, of course, hardly the truth, the heat of the surface being dissipated mainly in the specific heat of the spray and the latent heat of its vaporization, rather than by conduction. Nevertheless, the exterior layer being reduced thereby, almost instantly, to a certain temperature t, the second layer parts with its heat with a decreasing rate as its temperature falls and approximates that of the first. The result is a rapidly decreasing rate of fall of temperature as the surface is receded from. This rate decreases almost as rapidly for another reason, that is, that the specific heat of the hot metal is considerably greater than that at the surface, so that even if the flow of heat was equal at the two points, the interior would require a proportionally longer time to fall a degree than the exterior. So long, therefore, as the claim is upheld that hardening is due entirely to the carbon in the steel, and the rapidity of the fall of its temperature, there will be a point quickly reached beyond which the metal cannot be chilled; and so long as certain elements, as chromium and nickel, are believed to have no power in themselves of hardening or increasing the conductivity of the metal, their effect in delaying the change from hardening to cement carbon cannot be very important, so far as increasing the depth of the chill is concerned.
The advantages to be gained by breaking up the surface to be hardened by gashes and ridges are manifest from the above. The comparatively small volume of metal in the ridges may be given the same hardness with a less severe chill than the unbroken surface of the plate. This means that the quenching temperature may be lowered and the distortion diminished. At the same time, the fissures will permit the body of the plate to be more rapidly cooled, thus increasing its toughness.
The phrase "decremental hardness," as applied to face hardened armor is very misleading; so far as hardness pure and simple is an advantage to such armor it is usually confined to a comparatively thin and uniform layer, below which the metal exists in a decrementally toughened, rather than hardened, state. At the same time, hardness with its consequent brittleness is to be avoided at a great depth, as the plate will tend to split.
It is known that there is no difference whatever, under the elastic limit, between the extension, for equal stress in equal lengths, of soft and tempered steel. Mr. Edmonds, of the Woolwich Gun Factory, stated in 1891 before the Iron and Steel Institute that the modulus of elasticity is scarcely altered by oil-hardening. That is, for example, a nickel steel armor plate whose elastic limit in the untempered condition is 46,000 lbs., and when tempered 66,000 lbs., would stretch equal amounts for equal stress in each condition. In the first case, however, permanent deformation would begin with a stress of 46,000 lbs., and in the second with 66,000 lbs.
Mr. J. G. Dagron has also found by a series of experiments that the permissible compression load on iron and steel columns varies, not as the strength of the material, but as its modulus of elasticity.
It is in fact the modulus of elasticity which chiefly concerns us, as the superficial hardness given highly carbonized metal by waterquenching is very different from that obtained in oil-tempering, and largely increases the modulus of elasticity. Steel has been obtained having a tensile strength of 400,000 lbs, per square inch with practically no flexibility and very little elongation. The enormously increased modulus of this metal over that which it possesses in the annealed condition, indicates in a fair degree its increased resistance to compression, abrasion, and puncturing, in fret its hardness.
At a certain point beneath the surface of a face-hardened plate, depending upon the severity of the chill and the percentage of carbon, the original modulus may be found. The elastic limit and the tensile strength below this point have, of course, been raised, the former proportionately more than the latter, but the extension per unit of stress under the elastic limit remains practically uniform to the back of the plate. Towards the face, however, the modulus increases, at what rate or to what extent it is impossible to say. It is merely known that the hardness does not in every instance correspond with the depth of the chill. By the "chill" being understood the fine, bright, and uniformly grained surface layer, sharply divided in appearance from the heterogeneous interior of the plate. The thickness of this layer varies with the composition and treatment of the steel from a barely perceptible film to about 0.6". Although the metal below the chill may be very hard, its uniformity in appearance leads to the supposition that the change of modulus does not occur within it, but rather with the change in character of the metal at the border of the chill, thence increasing towards the face. This assumption is supported by the fact that flaking, due to the unequal elastic extensions of adjacent layers, occurs principally at this depth, seldom or never below it, and often outside of it. Doubtless the difference in the structure of the metal may account for the flaking, as well as the change of the modulus; that they are coincident, however, seems unmistakeable.
The existence of flaking, in that it indicates the sudden release of stresses exerted in resistance to shot penetration over a considerable area is a serious defect. The chill, however, is not always so sharply divided from the metal below; in fact, certain Harveyed plates have not flaked at all, the metal around the impact chipping out in wedge-shaped pieces, showing a more gradual diminution of hardness; such plates crack. The sharp line of demarcation of the chill is perhaps due in some cases to checking the spray from time to time to permit rectification in hardening. Such a procedure would tend to cause laminations of varying hardness, more or less distinct as the rapidity of cooling is greater and the temperature higher, from which the plate is cooled.
The action of the hardened face under impact is to bind together the tougher elastic particles beneath, opposing the extension produced by a depression of the surface. If the under metal at the surface of the chill extends, it must either crack the face or shear away from it. It seems, therefore, that if the chilled surface occupied the faces and sides of a large number of narrow and shallow gashes in the face of the plate, so as to be sharply broken up, flaking would be prevented. At the same time the surface would be more rigid and braced against, as well as preventing the extension of the metal below. Such gashes need not weaken the plate, as they would be confined to the hardened surface, which is otherwise bound to crack and flake before the interior is extended.
Theoretically the depth of surface chill should vary with the caliber of the projectile to be resisted; for while the zone of its resistance to the advancing ogival increases with that caliber, and the surface already crushed down with its square, the energy of the shot varies with the cube. Efforts to resist greater energies by making the body and back stronger by an increased percentage of carbon have usually led to the plate cracking under impact.
Attention must be paid, however, to the limitations of carbon steel in hardening and toughening. Also to the fact that the change in tensile strength due to tempering follows a different law than the hardness.
Both the tensile strength and ductility of the mildest steel are greatly increased by quenching, though the hardness may be scarcely changed. The same result will be produced on higher carbon steels quenched from a temperature below that affecting the carbon, although, as noted previously. Professor Langley has to a small degree hardened steel by sudden cooling from the temperature of boiling water.
It is evident, therefore, that the more sudden and complete the chill, the greater the increase of toughness in the body and back of the plate. Should the metal be highly carbonized for any considerable depth, however, it would not only lack strength and toughness, but the contraction strains might cause it to crack and flake off spontaneously or at least under impact. If the chilling should be made less severe, or from a lower initial temperature, in order to avoid these external defects, the interior might hardly be toughened at all. It appears, therefore, that as the depth of chill now obtained must in all probability be increased if projectiles of the latest type are to be resisted, the present system of carbonizing and hardening would not be satisfactory. To get the chill in deeper, the carbonization must extend deeper, and the percentage on the surface be higher; this would require greater time and expense in the cementation. In hardening, the higher carbon in the face would prohibit quenching from as high a temperature as now employed, not only on account of the danger to the plate, but on account of the much greater difficulty in controlling or preventing distortion. Should the surface of the plate, however, be covered with fine shallow cracks or serrations spaced from seven-eighths of an inch to an inch and a quarter apart, it would be possible in cementation to give the metal a practically uniform and moderate percentage of carbon to the bottom of these grooves, from which depth it would shade off quickly to the normal. Upon hardening, the surface through which the heat is abstracted having been thus enlarged, it would be possible to chill much deeper, considering the intentionally low content of carbon, than is at present possible.
"Changes in hardness are almost entirely due to changes in the carbon, apparently closely following the changes from cement to hardening carbon. The increase of hardness is practically proportional to the amount of carbon; it is not due to the stresses set up, because both interior and exterior are hardened, though under opposite stresses; also thin bars are hardened more than thick, through cooling more suddenly, though their stresses are less severe. At the same time, while pure iron is placed under violent stress when quenched, still it is not hardened. This is in opposition to Ackerman's theory that the changes in hardness, ductility, structure, and much of that in tensile strength is due entirely to compression which forces the carbon into the hardening state; this theory is plainly incompetent."—Howe.
Still, Caron found that blister steel after forging contained more hardening carbon than before, and that pressure favored the absorption of charcoal carbon. On the other hand, the effect of compression, as an aid in hardening steel, has been long known. At Moutlucon pressure was applied to steel in hardening when at a cherry red. Liquid steel containing more than o. 50 per cent, carbon is sensibly hardened if cooled under a pressure of from 7 to 10 tons per square inch. The proportion of combined carbon is always greater under pressure than when the metal is uncompressed. M. Clemendeau, in hardening steel for tools, places the cherry red metal in a receptacle it completely fills, and then subjects it to enormous pressure ; the greater the pressure the harder the steel.
The fact undoubtedly is that hardness is primarily due to the carbon. Professor J. O. Arnold in a recent essay on the Physical Influence of the Elements on Iron, read before the Iron and Steel Institute, makes the following statement: "That no element except carbon has ( per se) the power of conferring upon quenched iron the power of abrasion hardness to any extent worthy of consideration. Whether the adamantine hardness of quenched high carbon steel is due to the individual properties of an extremely attenuated carbide of iron or to an allotropic change produced in the iron itself, by the presence of dissolved carbon, there is no evidence to show, nor is the matter of much practical importance since such hardening power is possessed by carbon alone." These are strong statements and yet correspond to the general opinion of metallurgists, although there is still a considerable diversity of opinion among them.
When, however, the method and degree of hardening effect produced by a certain percentage of carbon is considered, it will be found that the chemical composition, rate of cooling, pressure, and temperature, all exert important influences ; and to these in consequence have frequently been ascribed the results which, however modified, really pertain to the carbon alone.
Thus high carbon steel cooling past the critical point at 1200° F. undergoes a molecular change made manifest by the evolution of considerable heat, sufficient in amount to retard the cooling. It is said that from this point the metal becomes more and more dense as heated until fluid. The fluid density of steel, the composition of which is not stated, has been given as 8.05; in the solid state it was only 7.8. This evolution of heat at 1200° F. is accompanied by a contraction of .004 (Barrette) in a steel containing 0.9 per cent, carbon, which, occurring wholly in the small percentage of carbide formed, must be much greater there. However, by compression the tendency of the metal to expand below the point of recalescence in the formation of carbide is opposed, and the effect of contraction by cooling produced, and hence the cooling is hastened by the further evolution of heat. Doubtless the great depth of chill in cast iron projectiles can be explained by the magnitude of these forces, as that operation increases the specific gravity of the metal fully 3.5 per cent.
A substance expanding in congealing or liquefaction may within limits be compelled to retain its denser state against the influence of heat by sufficient pressure. Conversely, when a body upon being heated expands, it may be led to part with its heat more readily in cooling if subjected to pressure. The same effect is produced on the solution of a salt by pressure as if it was a solid melting.
Professor J. Thomson considers the following to be a physical axiom: "If any substance or system of substances be in a condition in which it is free to change its state [as ice, for example, in contact with water at 0°C. is free to melt], and if mechanical work be applied to it as potential energy in such a way that the occurrence of the change of state will make it lose that mechanical work from the condition of potential energy without receiving other potential energy as an equivalent, then the substance or system will pass into the changed state. Thus the lowering of the melting point by stress is the cause to which is attributed the plasticity of glaciers."
Steel test pieces often show a fracture, the center of which is grey, becoming brighter towards the edge, when, if broken without tension, the fracture is homogeneous. W. Hempel ascribes this to the combination of the carbon under pressure. The increased strength obtained in wire drawing and cold hammering is explained in the same way.
The application of pressure is undoubtedly therefore an important assistance in tempering. It may be said that the molecular attraction, constantly in opposition to the dispersive action of heat, is assisted by pressure, although the latter may only be felt in the exterior layers of molecules.
It is believed that the pressure brought to bear in hardening on the surface layer of a carbonized plate by the initial contractions of the walls of the gashes or cracks, above spoken of, would correspond in its effects to an increased percentage of carbon or a more violent local quenching, thus forming an additional reason why a certain depth of chill could be obtained with a lower percentage of carbon and less danger of distortion.
COMPOSITION OF ORIGINAL PLATE.
Before continuing it is important to consider the influence of the composition of the original plate; bearing in mind the fact that the effect of water tempering should not be confined to hardening the surface, but include a marked toughening of the metal throughout.
On this subject there will be found a great diversity of opinion. Generally speaking, however, this arises from the confusion of the influences exerted by a component on an existing steel alloy with the characteristics per se of that component. Liberal quotations are made from the valuable discussions by Professor Arnold, Osmond, Hadfield, Brustlein, and others, before the Iron and Steel Institute.
The presence of chromium in the original plate is advantageous for several reasons. Ordinary cemented cast steel has large crystals, while those of cemented chrome steel are small. M. Brustlein, of the Holtzer Works, at Unieux, France, says: "In chrome steel the temper penetrates deeper than in plain steel, having an equal amount of carbon. This is attributed to the great affinity of chromium for carbon, favoring the dissolution of the latter in the metal, and thus maintaining it with greater readiness in the combined state. In manganese steel the same thing occurs; in fact, a small amount of manganese hastens the chilling effect. Chrome steel, however, scales badly, like nickel steel, and the difficulties of its manufacture are very great. First, it requires an intensely high heat for reduction; second, incomparably more rapid solidification than mild steel occurring in the change from white to yellowish white heat ; third, the formation of an oxide when exposed to the air which cannot be reduced or entirely separated from the mass of the steel; these difficulties increase with the size of the ingot; fourth, very great shrinkage; fifth, highly carbonized chrome steel burns very easily."
Generally, chromium has a less hardening tendency than manganese or highly carburized steel, but it imparts more tenacity, and the tendency to crystallize by excess of heat is not so great. Manganese steel works better hot under the hammer than chrome steel, but the former works particularly well anyway. Again, manganese steel welds with great facility, while chrome steel welds badly or not at all. "Steel containing a high percentage of carbon and chromium, especially the former, as .77 per cent. C, 5.19 per cent. Cr, will harden when cooled in the air. It is, consequently, self-hardening. Heretofore it has been believed that tungsten compounds alone had this property."
M. Brustlein also says "chrome steel is especially valuable on account of increasing both tensility and elastic limit without diminishing elongation, as would occur in carbon steel, where an increase of tensility invariably means a decrease in ductility." M. Brustlein is wrong there, as Howe has shown how proper treatment of carbon steel increases both elongation and strength. He, Brustlein, says chromium steel also seems to harden more readily. Chromium plays the part of a hardener, even without the intervention of a cooling medium; therefore when such a medium is employed the hardness is intensified.
Hadfield declares: "It has not been proven that a piece of chrome steel of a given diameter would harden more deeply when quenched than a similar piece of carbon steel.
"Probably if the right hardening temperature were obtained for each class of steel, it would be found that the chromium steel was tougher after hardening than the carbon steel, and it is also probable that it would harden at a somewhat lower heat, but that the effect of hardening would penetrate further is not proved.
"Theories have been advanced that chromium holds carbon in the combined state and that therefore chrome steels harden more readily. Seeing that the carbon present in all steel is in the combined state, whether chromium is present or not, this explanation does not offer much satisfaction. Chrome steel gives great resistance to compression, but in the absence or lowness of carbon it has in this little or no superiority to similar aluminium or silicon steels. So long as the carbon is about ,30 per cent, or under, the effect of chromium crystallization is small, but when the carbon is greater the action is more vigorous, or the carbon is enabled to act more vigorously."
F. Osmond, in the Journal of the Iron and Steel Institute, 1887, Vol. II., says: "Manganese retards both the molecular change of iron and recalescence during cooling from a high temperature, or in other words, maintains the carbon in solution and the iron in the condition ?, the effect being greater in proportion to the amount of manganese. The same effect is produced by the rapid cooling of steel containing no manganese, so that the presence of manganese exerts much the same influence as the process of tempering, a conclusion which agrees with the known mechanical properties of steel containing manganese. Tungsten has the same property in a still more marked degree; but chromium appears to produce no similar effect. Silicon has no influence on the effect produced by manganese (it hardens of itself, however). Sulphur seems to neutralize part of the manganese, diminishing its action. Phosphorus has no appreciable effect on the modification of the iron nor on recalescence."
From the remarks of Mr. Hadfield it would appear that, in order to obtain any characteristic effect of chromium in the heart of an armor plate, it would be necessary to run the carbon there up to a prohibitory point, although the cemented surface would obtain the full benefit of it. In France this difficulty has been to a certain extent overcome by combining the chromium with nickel, which seems to act as a sensitizer, emphasizing the hardening effect of the carbon in the chromium alloy, while retaining the necessary ductility; in fact, toughening the metal rather than rendering it hard and brittle. The celebrated acier speciale of St. Chamond is also a chrome-nickel alloy, said to contain .40 C, 1.0 Cr, and 2.0 per cent. Ni.
All of Vickers' Harveyed plates are said to contain some chromium; it is also found in some of Brown's and most of Schneider’s. It is extensively used by the French in their protective deck plates, the principal feature of which are their extreme ductility and toughness.
In the United States no extensive experiments have been made with chrome steel armor, the difficulties of manufacture of which are indicated above by M. Brustlein, It is believed that as our armor at present compares very favorably with the Harveyed nickel chrome of St. Chamond, which has been developing for some years, that more is to be expected from an alloy of nickel manganese. In this connection, it is interesting to note the effect of manganese on carbon as indicated in Professor J. W, Langley's equations of annealing and chilling. These were published in the Transactions of the American Society of Civil Engineers,
A recent process, in which a mild steel body and back are cast upon a bed of ferro chrome and then water hardened, deserves attention. After experiencing the usual difficulties accompanying the development of a new process, most satisfactory results are claimed to have been obtained, the percentages of chromium and carbon averaging about 5.5 percent, and 1 percent, at the face respectively, and running out to the normal in 1.5". If the resistance of the face hardened cemented plate was entirely due to the hard face, rather than a tough body bound together by that face, it might find in this cast plate a dangerous competitor on account of its cheapness. It is feared, however, that the result cannot but resemble somewhat a cast, homogeneous, chrome steel plate, the body and back of which have been weakened.
In this connection, it is worth while to note that M. Montgolfier, Directeur de St. Chamond, declares that although their celebrated chrome-nickel armor increased the velocity to perforate armor of a thickness equal to the caliber of projectile 100 meters, the application of the Harvey process raised that velocity fully 100 meters more.
SECTION IV.
THEORY OF ITS RESISTANCE.
The empirical formulas for the perforation of wrought iron plates differ so widely in their results, as the thickness of the plates or the calibers of the guns vary, that they have been called a disgrace to the science of mathematics. This seems like blaming a good servant for not accomplishing the impossible.
When it is considered that these formulas represent forced generalizations from a comparatively small number of experiments in which the actual qualities of projectiles and armor, often merely guessed at, were assumed to be identical, the diversity of opinions is explained.
Steel often varies considerably in its composition from heat to heat as well as being strongly influenced by many conditions of temperature and treatment in the course of manufacture; it is therefore much more liable to variations in quality than wrought iron, and fewer general laws as to its behavior can be made. The problem was attempted, however, and the DeMarre formulas, believed to be fairly accurate for Creusot steel plates, resulted. In these the projectile is not supposed to experience any change of form while passing into and through the target. As this condition is rarely fulfilled under the conditions of test, there is usually a certain amount of the projectile's energy expended on its own deformation, with a consequent relief to the plate. Especially is this the case when the deformation is of the nature of an expansion of the shell, thus increasing the area directly opposed to its advance. When the plate is hardened, tending to check the projectile suddenly and to crush or break it up, the energy thus diverted is still greater, while that remaining, being distributed among fragments acting in detail, produces proportionately less effect. Evidently the calculation of the resistance of a single plate of this description, even though the method and extent of its actual resistance had been determined by experiment, would be complex enough ; and if it is sought to generalize, the irregularities in quality are so great as to render the deductions of little value.
They become even still more unreliable in considering hard faced armor where the chemical composition is irregular, and the physical characteristics vary from the hardest chill on the surface to wrought iron-like softness at the back.
Variations in quality of armor-piercing shell, even of standard make, may occasionally be expected ; that still greater ones are contained in the larger and cruder armor plates is equally true, so that when the two are about evenly matched, it is difficult to determine within narrow limits what is a normal result. The resistance of such plates cannot, even with the present comparatively simple condition of the art of face-hardening, be made a subject of calculation ; and different applications of the processes of cementation and hardening, so little understood at present, may render the subject much more complex in the future.
The popular impression is that face-hardened armor resists the projectile by crushing it from the point; this rarely occurs, except in the case of very inferior projectiles, which go to pieces like a Prince Rupert's drop upon the point being crushed; or, in the case of soft shells, the point upsets and the head expands into a mushroom (see photos 315 and 326a). Many good projectiles retain sound and nearly perfect ogives, two-thirds way from point to bourrelet, after having forced the head into a plate (see photos 53, 91 and 311a). Others have the point rubbed off and the sides of the ogival abraded, scored, and twisted (see photos 310, 312(2, 314a and 325a), but in none of these cases does the destruction of the shell start from the point. In nearly every instance, the failure of the projectile is along a conical shearing plane inclined about 45° to the axis of the shell with its apex at the center from which the head of the chamber is described. This is due to the unsupported walls of the shell splitting longitudinally and sliding over the head which has been arrested by the plate. An instance of this is shown in the excellent Texas side armor plate from Bethlehem; a fragment of the wall of the shell is embedded in the plate by the side of the ogival. (Fig. A.) It will also be seen in the same figure that the crater surrounding the shot, usually ascribed to the impact of fragments of the body and base, is often nothing more than the flaking or prying off of wedge-shaped fragments of the brittle surface.
Certain of the concentric cracks seen around impacts are due to the blows of the walls of the shell, the upset and fused ends of which bear witness to their behavior. Others again are due to the cracking of the brittle surface upon the plate being "dished.'" These cracks have been noticed in many compound plates ; even Gruson armor has shown them. There is the greatest difference, however, between the behavior of thick and thin plates in this respect, the former, as a rule of hammered or pressed steel, attacked by calibers but slightly exceeding two-thirds of their thickness, but which nevertheless averaged over one-eighth of the width of the plates ; the latter, of rolled steel, attacked by calibers equal or even one-third greater than their thickness, and which rarely exceeded one-twelfth the width of the plate. In thick plates, the impact is usually surrounded by a shallow crater formed by displaced wedge-shaped chips of the brittle surface, and from that short radial cracks extend. The backs would seem less equal to the task of holding the mass together than in the case of thin plates, for cracking is far more frequent.
In the thin plates, there is usually considerable flaking of a nearly uniform depth about the impact. The surface of this flaking, and especially the fracture at its edges, is concoidal, appearing to follow a wave form originating at the impact. A remarkable peculiarity of every impact noted on good, thin, rolled, face hardened plates is the similarity in shape and proportional dimensions of the fragment broken from the plate.
In the sketch showing a section of a 6" plate attacked by a 6" projectile, it will be seen that the face has scaled over a diameter of 15.5"; the frustum of a cone starts from this edge of the sound surface, curving down quickly into the slope of a 90° cone. There may be several concentric cracks around the impact, and each one will be found to be the origin of a similar conical surface. These conical back bulges can be initiated with comparatively light blows, yet possessing the same general dimensions and shape as with the severest impacts, only in the latter case the frustum may be completely detached. Perforation, however, would occur by the projectile's head breaking out a small fragment directly opposed to it. A somewhat similar behavior is seen in the case of the third 4" impact on the 3" plate, No. 4.
Similar bulb-like foliations or sheathes may be seen in the cases of plates 935, 3"; 883, 6"; 874, 6", and in the accompanying sketch of Krupp's gas hardened 10.25" nickel steel plate. The impact shown is that of a 112-lb. projectile moving at 2160 ft. s. velocity. The plate was nearly matched, the perforation being 12.2".
In plates where the body and back is hard and rather brittle, as in many of the earlier all steel face hardened plates, this peculiar effect is not produced. Projectiles inferior to the plate crush and chip away a crater in its face in which the ogival is welded. Projectiles superior to the plate either crack it apart or break a rough-shaped cylinder out of it, much as in the case of hard and very tough homogeneous steel alloy plates. (See sketch of Hadfield's 5-inch manganese steel plate.) Thin face hardened plates usually fail in this manner when perforated by undeformed projectiles.
The question may be asked, If the plate is going to fracture under the blow, why must it give way over an area of from 16" to 24" in diameter, when one of from 6" to 14" would have sufficed to allow the projectile to pass.? The reason must be that the projectile's head, forced in as it is, acts as the keystone of an arch exerting pressure in every direction normal to its ogival. At some points, located on a circle about 16" in diameter, the opposition to that pressure by the rigidity of the surrounding plate, supported and stiffened by its hardened face, is sufficient to shear the two apart. It is evident, however, that the resistance to the advance of the projectile is far greater than the energy required to punch out a hole about three times its diameter ; that is, a peculiar resistance is experienced by the shell. In some instances the shattered ogival appears welded to the plate ; more commonly, however, an extremely hard shell head is squeezed, scored and ridged by the fragments of the hard surface carried in by it. Hence it is that so much assistance is lent the shell by a soft cap forced into the plate with and around it, and to a certain extent sheathing the hole and protecting the shell. This seems at least very plausible, as when capped shell perforate the plate there usually appears to be no cone formed. The plate, however, is dished in the vicinity of the hole perhaps 0.5", at any rate far more than in the case of a non-perforating shell, which still may have badly racked a considerable area. The advantage of a rigid backing in the cases illustrated by this figure are apparent. Some doubt may be felt of the value of the ogival head, however, in such cases. The plate having broken along the sides of the cone, the energy expended in forcing the ogival into the plate must, in a measure, have been wasted. It would seem that a similar but much smaller cone would have been broken out by a flat-headed projectile, in which case all the energy would be concentrated on the area opposing the projectile. This, in fact, is probably one of the effects of the cap. It is interesting to note in this connection that in a trial of Gruson chilled iron armor with flat-headed and pointed projectiles in 1885 the mean penetration of the former was 1.935", and of the latter only 0.459".
The total energy of a rapidly moving projectile at the instant of impact has been expressed in two ways, as the effect expected is that of penetration or racking. In the first case, the tons of energy per inch of circumference gives the relative punching effect very accurately in soft, thin plates, provided the projectile is not deformed. In the second, the tons of energy per ton of plate gives relatively the amount of energy a hard plate is required to absorb. It is evident that the important questions of location of impact, projectile and plate deformation, and transformation of energy into heat, are here out of consideration. Actually beyond a certain point for each combination of energy, thickness and quality Of plate, and rigidity of structure, the mere increase in weight has nothing whatever to do with the plate's resistance.
The limited dimensions of armor plates and the occasional location of impacts near the edges qualify the results, whatever theory is advanced; still, the principles developed in the attack of the central region of a plate of indefinite size are fundamental and something may be gained from their consideration.
The comparison of tons of energy delivered per ton of plate is obviously always unfair to the plate, unless it be in each case absolutely homogeneous and symmetrically disposed and supported around a normal impact. In addition, to obtain the comparative racking effect pure and simple, the expenditures of energy in penetration and deformation must be the same.
On the other hand, the energy per inch of cross-section of shot, especially with different calibers, gives no reliable comparison, unless the shots strike normally, escape deformation, and each inch of their sections do equal amounts of work. This is evidently impossible except in a disk-shaped projectile of molecular thinness. If, however, we regard the impact of projectile upon the center of a face hardened plate as the arrival of a succession of thin disks held in elastic relation to each other by intermolecular forces, perhaps the mutual reactions of such a plate and shell may be better understood.
A soft plate, the behavior of which is better known, if struck by a punch, stretches before it and tears away circumferentially, a disk being forced ahead while the elongated fibres forming the walls of the cavity are pushed aside and compressed into the body of the plate until the displaced metal crowds the surfaces adjacent to the impact up into front and back bulges. The diameter of these is chiefly influenced by the elastic strength of the metal and their height by its softness or ductility. That is, in the case of metal having a high elastic limit and hence lying near the failing point, a flow of metal started by pressure at the impact would be accompanied by a comparatively slight movement of the metal over a considerable area. In a metal having a low elastic limit the volume displaced is equal, but the movement under the same circumstances would be local. Probably a very fair comparison of the toughness of two plates would be the cotangents of the angles of their bulges under similar penetrations. If the metal lacks ductility, the great difference in displacement of adjacent particles must be accompanied by its rupture, the fringe flakes off or even a crater may be formed.
Let the behavior of a homogeneous oil tempered plate be considered, when subjected to pressures under the elastic limit. The punch depresses the hard and elastic surface immediately in front of it a distance y, the radius of the base of the depression thus formed being- x. The extension
of the metal will, however, be extremely small even when it has a
high elastic limit, in which case x will have considerable length.
This subject has been touched upon by a number of physicists
in the investigation of the absolute measurement of hardness; with this difference, however, that, in the determinations of hardness, uniformly slowly applied pressures were alone considered, so that no wave movement due to the inertia of the material resisting a rapidly extending stress, as shown in the exaggerated sketch above, could occur. A most interesting memoir on the subject, by F. Auerbach, translated from the German by Carl Barus, may be found in the Smithsonian Report for 1891. Auerbach's experiments were founded on a theory by Hertz "that, if a sphere be pressed upon a plane so as to produce a surface instead of a mere point of contact, the impressed area would increase, within the elastic limit, as the two-thirds power of the total pressure, it will also increase as the two-thirds power of the radius of the sphere."
It was found that the hardness increased with the curvature of the sphere or lens; that is, although both lens and plate be cut from the same material, rupture will always occur in the latter the former remaining intact. He calls attention to the analogy between the increased resistance of the convex lens and the surface tension of liquids. With that significance, the behavior of the lens and plate would resemble that of a truss and simple beam respectively. The metal of which a projectile is composed may be made at least as hard and certainly of superior quality to that of the surface of a face hardened plate. There is no reason, apparently, why the superficial film of the latter should not be ruptured under pressure, therefore, before the point of the former. The support of the underlying layers, though they add to the resistance of the outermost one, do not increase its hardness. Upon impact the point of the projectile receives a concentrated support, as it were, from the better placed layers behind it crowding forward. There is no reason at all, therefore, why a sound and properly made projectile of better material than the plate should break up from the point. To do so, as Captain Tresidder has said in a recent article entitled "Notes on Armor Plates and their Behavior under Fire" (Occasional Paper No. 7 of the Royal Engineers), the disintegration of the projectile must commence the instant the point strikes the plate, but that will hardly occur even with the most inferior projectiles, unless the superficial resistance of the plate far exceeds their energy of impact. The deductions of Captain Tresidder appear to be drawn from data which he has seen fit to withhold, they are so widely different, however, from experience obtained in this country, that advantage is taken of this opportunity to comment upon them ; not for criticism's sake, but to draw out views, if existent, not apparent to the writer.
To be just, Captain Tresidder's paper was written in 1893, it was published, however, with a few modifications, in October, 1894. His theory of the smashing of the projectile upon a face hardened plate is that of pulverization on impact progressively from the point; this is in keeping with his statement that the action of the soft iron cap employed on certain experimental projectiles is to support the point laterally, so that the initial splitting cannot be done without bursting the cap, as "pulverization must be initiated before penetration or it will not occur at all."
A multitude of parallel, conical, shearing planes are supposed to form in the projectile from the point (which becomes in consequence a spindle, towards the base) by the arrest first of the point, then successively the sheared conoidal segments which split over the point and the other segments gone before. This breaks the shell into a myriad of fragments, which are then mashed together in the indent. Though how there can be an indent if the shell is pulverized and has its energy divided among minute fragments is inexplicable. The whole theory is, in fact, inexplicable even though confining its application to chilled iron projectiles.
If an inferior projectile, or for that matter a fairly good one, be fired against a very superior face hardened plate at too low a velocity to penetrate, its energy may be sufficient to cause it to fly to fragments, hardly leaving a mark on the plate. This seems to bear out Captain Tresidder's theory and contradict that advanced with regard to the relative hardness of a sphere and a plate. The latter did not contemplate impact, however, or for that matter still less, impact with insufficient energy to penetrate. This stoppage of the projectile on the surface of the plate would correspond in its effect to a much greater energy distributed over a greater distance, only in the latter case the head of the projectile, having penetrated, would be supported by the surrounding metal.
With higher velocities, however, good projectiles will penetrate until the resistance has increased sufficiently to counterbalance or nearly counterbalance their remaining energy, when the shell, being stopped, is much in the same condition as the projectile fired at an impenetrable plate with that remaining energy. The result is that, as a rule, the unsupported part of the projectile splits and shears over the head gripped in the plate.
What the face hardened plate does is merely to present so great and concentrated a resistance to the projectile's advance at some one instant in the impact as to stop it ; disintegration follows as a matter of course.
The loosely worded explanation that face hardened plates stop the projectile by breaking it up and distributing its energy, puts the cart before the horse, the effect is confused with the cause. A broken projectile or some of its fragments may get through a plate, but if it does, these fragments have acted together as a unit; the projectile cannot have been wholly crushed though the entire resistance of the plate has been expended in breaking it up.
In connection with the above, the following tests may be interesting. The plate was the Brooklyn's 3-inch Carnegie face hardened nickel steel ballistic plate. The projectiles were of service 4-inch Wheeler-Sterling type, with and without caps. They were of good and uniform quality and composition.
There is much to be learned from these tests, even allowing for a certain variation in the plate from point to point, although the impacts were closely grouped, and allowing for a still greater variation in the separately cast, forged, and treated projectiles; the latter differences being perhaps further accentuated by variations in velocity, angle of impact, etc.
It appears from the above that an energy incapable of affecting the plate may still destroy the projectile; also that the plate may exert sufficient resistance to break up the shell without destroying its point, and finally that the cap does not support the point against splitting, but rather eases both plate and projectile under impact.
It may now be asked, Of what use is hardness on the surface if it does not serve to crush the point of the projectile? The answer is that the more rigid surface, the more certain that the energy of impact will be widely distributed while the resistance of the entire thickness of the plate is brought to bear against the advance of the projectile, as the displaced metal can only flow to the rear. At the same time, when penetration is effected the projectile carries with it a mass of jagged, untractable fragments which greatly impede its advance.
A very important accompaniment of hardness is elasticity, which increases, however, much more rapidly than the hardness does. It is, of course, of special value under impact, permitting a greater depth of plate to aid the surface.
Now, the velocity with which the pressure increment at any instant is transmitted over the stiffly elastic surface is far greater than that of the advance of the projectile, so much so in fact that the assumption is not difficult, that the depression is due to the position of the advancing shell at the same instant rather than its position an instant before.
Let AB represent the original plane of the surface of a thin plate, and C, P', the position of points and P when the surface is depressed to its elastic limit by a force acting along OO'. The correct shape of the depression is represented by the dotted line. The radius of the circle on which any point P is located has been increased from OP to KP', and this circle is therefore in a state of tension. Had the element OB, in being pressed down into the position O'B extended equally over equal distances, the depression would be a true cone, every point of which would be subjected to equal circumferential and elemental tensions, while the base AB would be indefinite in extent. Such a condition would only result when the elastic limit was zero. It is apparent that the stress transmitted through any unit length of a concentric zone or circle, such as that passing through P', must be inversely proportional to the radius KP'. In the same manner the extension of that unit, under the elastic limit, will be inversely proportional to KP' In fact, the depression will be of the character of a semi-cubic parabola.
Now, if the pressure is increased above the elastic limit, the metal, if ductile, near the center O' would elongate rapidly, allowing the shell to advance, carrying the metal down before it; this action would result in the plate being left dished and without a fringe, as in fact is the case when a comparatively thin plate is attacked by a large caliber projectile.
In the case of a thicker plate or a smaller caliber for the same plate, however, the case is different. The initial tears, before the point, release both radial and circumferential tensions, the points of the sectors thus formed draw back and the maximum tension is carried to a zone outside that which has just failed, extending the radial cracks and permitting the 'point to advance and attack a second layer in a similar manner. The free points of the sectors are then forced out of the way in the direction of least resistance, curling up about the point forming the "fringe." There appears to be a great regularity in the number and position of radial cracks or tears as shown on the back bulges of wrought iron and simple steel plates, as well as in the front bulge and fringe of steel plates. Doubtless the location of the first crack formed in the highest stretched zone of a fairly uniform metal is at its least ductile and strongest point. To permit this zone to stretch on one side more than the other would require a side movement of the shell, or the metal to flow around it, becoming attenuated on the ductile or cracked side and banking up on the other. The zone, barring such a side movement of the shell or, owing to the very small time limit, improbable flow of metal, must stretch equally in sensibly equal distances; the hardest and least extensible portions would therefore be the locations of the initial tears which would also be spaced quite uniformly about it. This explains why the direction and location of the tears in wrought iron plates seem so little affected by the fibre ; also why on occasion a whole section of fringe or bulge will be broken out through its failure to extend.
The length of the cracks would undoubtedly be a function of the elastic strength of the plate. Thus, the point of a shot having opened a stiffly elastic plate starting radiating cracks, the more the points submit to bending, the more distant zones would be subjected to stretching beyond their elastic limit continuing these cracks. At the same time, the surfaces of the points being stretched radially more than the metal beneath, concentric cracks might also be initiated.
After the point has passed the surface, the latter is held down by its cohesion to the intact stratum of metal added to the friction of the shell on the walls of the opening, but resists with all the elastic strength of a layer of metal increasing in thickness as the shell advances. Should, now, the advancing plane of greatest stress pass through a weak spot, or should the accumulating elastic strength of the front part become equal to the undertaking, it will pull away and a circular lamellation be formed which will practically serve, so far as the point is concerned, as the face of a new plate.
It is evident, that in the passage of a shot through a plate, the formation of every lamellation is accompanied by the liberation of an amount of energy in the projectile necessary to bind down the preceding stratum of the plate, and which is also indicated by the tearing away of the reacting face, or by an apparent accession of energy in less elastic plates by breaking out the back bulge. The toughest plate is undoubtedly the one in which there are numerous incipient and no developed lamellations of this sort, for their entire absence would only occur in an extremely soft and weak plate, or an extremely hard and brittle one.
Consider now a thin, rolled, face hardened plate having a soft but tough back. The hard, thin surface being very stiff and elastic, the cone of depression will be wide spread, shallow, and supported by the back over a considerable area. Should it crack radially, as it must do quickly under stress, it is still held together or bridged by the back, thus permitting still further bending and extension of the supporting area before penetration; this again leads to a further extension of the radial cracks and stretching along them of the back. The stiffer the combination of face and back, the greater the area of the plate supporting the impact. The appearance of concentric cracks in the face indicates that the hard face is too thin or the stiffness of the back is insufficient. Had the back been weak and the hard face thick, the brittle behavior of the latter would have characterized the impact. Such concentric cracks are seen in the photo of the 3" side armor of the Brooklyn. If the hard face had been too thin or soft, a front bulge might be formed as well as a fringe which would flake off. If the hard face was thicker and the back gradually less hard and strong, the best effect would be produced. By the time the surface layer had been bent to the point of cracking, the metal beneath would have communicated the stress though a greater and greater area as it was transmitted to the back of the plate.
Thus far the hardened surface, where it has not been shattered or flaked off, has bound the metal below it together, preventing its extension. And this is the true object of the hardened surface ; not, as seems so commonly supposed, to, of itself, shatter the projectile, the soft back being merely to hold its fragments in position, as in compound armor. The hard surface and the metal immediately below it have for their duty the prevention of all forward and lateral displacement of the metal beneath. This would be difficult enough, anyway, on account of its own rigidity, but the assistance it has demanded from the hard face is frequently seen in the latter's extensive flaking. There being no front bulge nor fringe, the metal before the projectile is carried in, gripped between it and the surrounding walls. It still binds the softer metal together, causing the entire thickness of the plate to resist as a unit, even though being incapable of flow, it is ground to fragments, which in turn score, fuse, and abrade away the surface of the ogival of the projectile.
The enormous increase of resistance in face hardened armor is brought about in this manner, not simply because it is more difficult to force the point of the projectile through the inch of hardened steel face. It is evident that, while a certain thickness of hard face is necessary to secure the best results, if it readily flakes or shears away from the metal beneath, a point is quickly reached beyond which its increased thickness is of no particular advantage, as it would fly to pieces immediately upon being struck. If this hard face is, however, carried into the back a slight distance in a series of small gashes or pockets, its stiffness would not only be greatly increased but shearing or flaking prevented by the hooks or protuberances extending into the softer metal. At the same time the advantage of a thicker hardened layer would be obtained without detracting from the elasticity and toughness of the body of the plate.
When a soft plate brings an undeformable projectile to rest it is done gradually, and if the plate offers no sudden change in its resistance as the projectile penetrates, it must have regained a position of equilibrium when the latter finally stops. The structure may have moved, bolts snapped and the plate been set back, still the reaction is directly opposed and equal to the action so long as it continues. In the case of the rebound or disintegration of the projectile, the case is different. The balance of forces is suddenly destroyed, leaving a large amount of unbalanced energy in the plate which must vibrate as a bell under the blow of a hammer. The bolts and fastenings of face hardened armor should therefore be of the toughest, most elastic quality, though not necessarily, in thin plates at least, more numerous than in the case of softer armor. An important consideration in this connection is the peculiar weakness of non-homogeneous armor to vibration. If the method of resistance contemplates the destruction of the projectile, then vibration must ensue, and the tendency of these vibrations to be uniform in amplitude and velocity at equal distances from the impact requires equal duty of strong and weak parts ; the same play of those elastic and free to move as those bound down by bolts or otherwise stiff and brittle. Now, the amplitude of these vibrations are a function of the elasticity of the plate in the vicinity of the impact, and though it diminishes as it recedes, a less elastic portion of the plate called upon to vibrate to this extent may be unable to respond and will crack. Thus isolated cracks are sometimes formed at distant points, and in one instance (see figure showing 4" Brooklyn barbette) repeated impacts on one end of a plate shook off the other and unsupported end. Similar occurrences have been noted in compound plates.
The amplitude of vibration on the surface exceeds that on the back by nearly the amount of elastic compression and extension of the thickness of the metal. To make it clear, we may suppose the plate made up of thin flexible sheets of steel separated by layers of rubber, when the actual movement of the last or back sheet would be less than that of the face by the compression of the rubber between ; and this would be less and less for each layer, as the force transmitted was distributed and expended in the compression. Now, if one of the steel sheets should be replaced by a thicker and stiffer or less elastic one, the vibrations of the latter through having less amplitude must cause not only a shearing stress along its face, but a considerably greater tension there on the rebound than exists at any other plane. Hence, as would naturally be the case when this unyielding stratum occurs at the junction of the layers of a compound plate of which the face has great elasticity, and the back, lead-like, tends to retain the shape of the hollow of the initial wave, the face is apt to tear itself away when it takes the form of the crest. This is the serious objection to compound armor, and in fact, to all hard faced armor in which the face, while preserving its continuity over large areas, differs in a marked and abrupt manner in elasticity from the metal beneath. (See photos 259 and 265, plate A883, showing flaking.)
The photograph of plate A883 shows how considerable flaking may occur in even a most excellent plate where a superficial, continuous, and very hard chilled face is supported on a tough but extensible back. Many methods have been devised for overcoming flaking of this sort in compound armor as well as preventing the extension of deeper cracks. In 1877, Whitworth set hard steel plugs, intended to break up the projectiles and to prevent the extension of cracks, in a soft steel plate. Later he applied the same principle to other plates by covering the armor with small plates of hard steel, intimating that the best way to limit the cracks in steel would be to "manufacture" them. In 1883, Whitworth is quoted as saying, "that an armor plate of compressed steel, built up in segments in such a manner as to prevent the extension of a crack or split beyond the limits of the segment in which it was produced, would suffice not only to resist but break up any projectile of an ordinary character." In 1878, a Cammell-Wilson patent proposed to localize cracks by soldering plates to the face of the armor. In the same year, Ellis patented the introduction of wrought iron bars in the surface of compound plates for the purpose of reducing the lengths of cracks in the steel when perforated by shot. A far simpler, if not more efficacious, method than any of these would be merely to gash or score the hard film on the face hardened plate. These gashes, pockets or corrugations would also serve not only for the more expeditious and better controlled cementation, but would permit deeper chilling in hardening ; the surface would be more rigid, less inclined to flake, and shearing away from the soft back would be prevented.
That such gashes or openings forming breaks in the continuity of the hard surface do not detract from the plate's resistance is evident, as on account of the rigidity and brittleness of the surface the whole support of the back of the plate cannot be felt until the former is extended, and hence broken. Numerous tests have been made of face hardened plates in which cracks from 0.5" to 0.8" deep, some as wide as 0.12", were formed in bending the plates after cementation and before hardening. The range of temperature in which the highly carburized surface could bend without rupture being far less than in the case of the plate's back. (Several examples of plates showing cracks of this description are given.)
The chill cracks confined to the surface of certain Gruson armor plates have been tested in a most thorough manner with results similar to those obtained in the case of face hardened armor. That is, they seemed in no way whatever to weaken the plates ; no cracks were initiated by them, nor did they give direction or extension to any of the fresh cracks formed by impact.
It may, of course, be asked : If it is an advantage to extend the stress of the blow over as large an area as possible by causing the depression to be wide and shallow, would not that result be prevented by cutting the surface up into small sections which would tend to localize the bending effect by shortening the arm? In reply, it may be stated that in stiff hard faced plates attacked by comparatively small calibers, the depression is so shallow that radial cracks seldom extend beyond what would be the position of the fringe in a soft plate. Also that in more elastic hard faced plates, or stiff plates attacked by large calibers, the angle of the cone is such that the brittle surface is unable to adapt itself to it in the immediate vicinity of the impact, so that both radial and concentric cracks are formed, causing flaking in no way affecting the integrity of the tough and elastic metal beneath.
Thus far, thin plates alone have been considered. In the case of thick plates, the typical failure is by cracking to an edge, then, perhaps, allowing perforation. There are a number of reasons why the thick plate is more liable to crack than the thin one. The metal displaced by the point of the projectile in thin plates crowds before it, causing a back bulge. In a thick plate there may be sufficient stiff body and back before the projectile's point to prevent this at first, forcing the metal aside about the ogival and either causing great fragments of the face to fly or splitting the plate. With the larger caliber the same slight penetration requires a much wider opening ; the effect of the shell, so far as the layers are concerned, is therefore local, being more concentrated and abrupt. The only way for thick plates to avoid cracking seems to be by failing locally ; that is, by possessing a weak body and back, which of course may be carried too far.
In this connection, it would be well to consider the various causes from which plates crack under impact, 1. Initial stresses, flaws, or other defects. 2. Wedging apart of the plate. 3. By bending or breaking through on the giving way of the structure behind the impact.
In certain armor plates the first shot develops important cracks while those following have comparatively little effect. Thus in the 3" face hardened nickel steel plate No. 4 (Fig- 60/91), the first 6-pdr. shot caused a through crack from the impact at the center of the plate to the bolt hole at the left lower corner. At the same test, a 3" all steel plate was cracked through by the first shot, after which repeated impacts produced no other cracks. Evidently these first cracks were entirely due to stresses in the plates caused by faulty tempering. These stresses being evolved by the plates cracking, subsequent impacts produce effects strictly proportional to their energies.
The so-called "wedging effect" of a projectile whose caliber is large compared with the thickness of the plate, is merely the result of a combination of the uniform radiating crack or tear before described, with lines of structural weakness due to existing stresses, defects in the metal, or the proximity of an edge or old impact. In this case one or more of the otherwise incipient radial cracks open out and render the extension of the rest unnecessary. The idea that cracks run to weak points is of course absurd, they follow the weakest lines which may or may not lead towards the nearest edge or weak spot. They may start from weak spots or lines of stress, however, independent of the impact, through vibration. If, however, as in the case of a large caliber projectile, the zone of metal stretched to its elastic limit, preceding that in which cracking has occurred, both being practically concentric and circular, widens as the projectile enters until it reaches an edge or defect, the plate snaps to that point. After this the entering shot acts as a true wedge to pry apart the plate with an effort concentrated perhaps on a part of the circumference of the shot hole as yet showing no weakness whatever. A good example of this is the cracking of the Schneider all steel plate in the armor trial of 1890 under the 8-inch impact.
The 3" plate No. 4 labored under the disadvantage of having been cemented and hardened while containing twelve 2" through bolt holes. Yet, after shattering twenty-one 6-pdr. armor-piercing projectiles, at 1804 ft. s. velocity, it stopped three 4" 36-lb. A. P. shell, moving at velocities of 1700 ft. s. The cracks in this plate do not appear at all due to the wedging effect of the projectiles. Thus the third shot has nearly broken a large conical fragment out of the plate without cracking to or from the impact. Another shot is surrounded by concentric cracks, showing the toughness and elasticity of its backing. It will be noted that in weak backed compound plates the radiating cracks. from successive impacts often cross at all angles, apparently having no effect on each other. Another peculiarity in the case of the 3" plate is that neither the 6-pdr. impacts nor the bolt holes appeared to determine the direction of the cracks, which in fact, in all but two instances, seemed to avoid the former. Two experimental face hardened plates (Fig. 224), on the faces of which were a number of tap bolt holes, behaved in a similar manner. There is reason to believe that these plates were in a state of stress, however, and that the crack near the first impact, caused by the second shot, started from the edge of the plate. Cracks of this sort may generally be detected by their running by the side of the impact and not to it. Repeated impacts, of course, weaken these plates ; large areas are caused to vibrate violently, and are perhaps extended or compressed, while original lines of weakness are accentuated by the natural accumulation of strains about the more sensitive points.
In considering the resistance of plates composed of layers of varying strength and elasticity, the problem is undoubtedly rendered more complicated, from a mathematical point of view, by regarding the plate as if made up of thin sheets separated by springs corresponding in strength and amplitude to the varying elasticities. Some light may be thrown on the subject, however, and the attempt will be made. In this case, the energy of the projectile should also be regarded as divided among a series of disks of molecular thickness separated by weightless springs whose play and. strength correspond to the elastic compression and strength of the metal at the various points.
Should such a projectile strike a rigid, impenetrable target, the first disk delivers up its energy, which is transformed into heat, and the possible vibration or movement of the structure as a whole. It is probable that some energy is also stored in the elastic deformation of the molecules themselves. The second disk arrives with its energy diminished by that required to compress the spring before it ; if there is sufficient left to overcome the molecular forces its particles may be forced over and among those of the first disk, the remainder of the energy going into heat or compressing the molecules themselves. Doubtless this transfer of energy is taking place at the same instant along a considerable length of the projectile, involving numerous disks and springs, and forming a wave of compression longer or shorter according to the strength or weakness of the springs and the velocity of the disks. The velocity of propagation of this wave determines the duration and character of the impact. A sudden stoppage of the leading members of such a system in which, through weakness of the springs, the wave of compression is very short, may cause it to be crushed in detail ; crushing must occur anyway against such a target when the disks contain more energy than the springs can absorb, or may be converted into heat in the crushing together of the molecules ; for it is inconceivable that two particles stored with kinetic energy could exchange that for a state of rest at a higher temperature upon the first instant of an imperfect impact. If the compression of all the springs to the minimum elastic point, plus the heat imparted to plate, projectile and the surrounding atmosphere, as well as that expended in deforming the molecules themselves, did not consume all the energy, that remaining would be employed in forcing the particles in among each other, causing a displacement in the direction of the least resistance at right angles to the line of flight. The resultant force would create a shear plane at 45° to the line of flight. The compression of the springs or molecular forces is doubtless involved with that of the molecules themselves, it has been thought advisable, however, to consider them separately on account of the purely speculative nature of the latter.
The stiffer the springs, or the higher the elastic limit of the metal, the longer the wave of compression and the greater the number of disks acting together. With a low elastic limit, the projectile would be set up or crushed in detail. If its energy be entirely stored in the compression of its springs and dissipated in heat it will rebound, perhaps in fragments, through crushing on impact.
Suppose now that the target is composed of layers held apart by springs similar in character to the projectile. It is apparent that it will require considerable more energy to compress the first plate spring than that of the projectile, on account of the extensive supporting area. The latter will, therefore, where the two are nearly matched, be in a state of full compression some time before the plate ; and it will then act as a unit upon the layers of the plate in detail, more or less marked according to the latter's elastic weakness and play of springs. In this way penetration of a homogeneous plate occurs; its resistance being necessarily expended more or less in detail.
If the exterior springs in the plate are extremely stiff and have very little play, as in face hardened armor, it is apparent that a much greater compression must be exerted upon the projectile with a very slight movement of the surface layers of the plate, and if the latter's exterior is not yet wholly compressed when the projectile is, the remaining- energy must be considerably diminished before penetration can occur.
In a theoretically perfect face hardened plate, the elastic limit of successive layers would be regularly lower, starting from the base of the chill and extending inward. As the extension or compression of each, within the elastic limit, would be proportional to the stress, and as the stress, being distributed over larger and larger areas, is less for each successive one, a point might be reached when every layer opposed to the advancing projectile is stretched to its elastic limit. The slightest further advance is, of course, met by a vastly increased resistance, the front layer of the plate possibly cracking and the others elongating. The projectile being fully compressed, any sudden increase in resistance will cause a more or less abrupt retardation, indicating a considerable loss of energy which, not being expended on the plate, must be done on the projectile.
It may be asked. Why would it not be better to have the entire plate of the high elastic strength of the face. If resistance to an instantaneous non-persistent impact was all that was required this might be the case, but something more is wanted, that is, the ability of the plate to hold together at least for a minute space of time. In metal of high elastic strength, the failing point is usually but little beyond the elastic limit; there is no broad range of permanent elongation before failure, so that a crack initiated at the surface would run through the plate with but a moderate increase of stress.
Recently a 6" face hardened plate was perforated by 6" projectiles of an improved type at a striking velocity of 1700 ft. s. The same plate had withstood service armor-piercing projectiles at a velocity of 2100 ft. s. This has led to the idea that, with still further improvement in projectiles, more resistance will be lost by the comparatively soft body and back of Harvey plates than will be gained by their hard face, so that a return to a homogeneous but a tougher type of plate probably of some nickel, chrome, or manganese steel alloy will be in order. This argument is fallacious. The body of Harvey plates, especially those of moderate thickness, are usually tougher than the old oil tempered plates, having about the same elongation but often considerably greater tensile strength.
In addition, while protesting full faith in a rapid advance and development of the metallurgical' arts, especially now that the capacity and application of the electrical furnace has been so greatly extended, the writer fails to see that the promised alloy has yet appeared above even the experimental horizon. As under the most favorable circumstances it required three years of experimentation before service face hardened armor was sufficiently developed to be contracted for in this country, and nearly a year more before the process had been demonstrated as practicable and a success, it does not appear that the new alloy, as yet unknown even to the laboratory, is a dangerous competitor. Even should it appear, it will doubtless be as susceptible to the various processes of face hardening as its weaker predecessors which have been thus benefited.
The objection to the homogeneous plate, whether water tempered or oil tempered, is that if tough rather than hard it resists in detail and through its own destruction almost as does wrought iron. If it is hard its elastic limit is near its failing point, it can stand one instantaneous impact, perhaps, but it cannot flow ; there is no way of absorbing energy except in a weak fashion by cracking, and if the projectile is not destroyed perforation may ensue. It is therefore better, as the back of the plate is approached and a less and less demand made upon its elastic strength, to lower the elastic limit and increase the ductility. There is a limit to this, however, in thick plates. There is another point in this connection. Some authorities do not regard elongation as of much importance under impact ; they point to the failures of car axles and mishandled boiler plates. These, despite the ductility shown in their test specimens, frequently appear, when fractured in service, sharp and even crystalline with no signs of ductility. Ductility cannot be shown unless the metal draws down. If a stress passes over a line of weakness of less resistance, as at the edge of a cooling plate, the opposing forces working on each particle in that line are balanced to the last. There does not appear to be, as in the case of a test specimen, a severe stress on the exterior and a less one on the interior, whereby the particles are given an opportunity to separate successively and flow in the direction of least resistance. Stress is applied to each individual particle, and the separation of a single pair means the separation of all.
One point more : until penetration occurs the cone of depression is intact ; to resist the first penetrating effect therefore until the base of the cone is widespread is to greatly increase the resistance. The first effect of penetration in homogeneous plates is a distinct flow of metal. If that before the projectile refuses to become viscous under pressure and flow ; if it lies there, for example, like granite, that may be pulverized but the particles of which will not slide over each other, an entirely new character is given the resistance of the surface of the plate. The point of the projectile must carry this intractible film before and around it. Every advance drags in more of the jagged fragments of the hardened surface, cutting and scoring ogival and plate as well, thus largely increasing the volume of metal which must be removed to permit the passage of the shot. There is a different thickness of this hard surface most suitable for every thickness of plate. Its object is to distribute the energy of the projectile; to prevent the flow of metal up and around it ; to force down the plate surface and keep its resistance extended over as large an area as possible, as though bottled up and braced at the point of impact. Thus the absence of a front bulge must indicate that all of the resistance of the plate has radiated from the shot's head, and been widely distributed. The presence of a front bulge indicates a more local resistance, the displaced metal having simply eddied about the shot's head.
The idea has been expressed by some that the merest film of hardened surface is all that is necessary. This is incorrect. Service projectiles which have succeeded in piercing good face hardened plates, frequently have a considerable depth of the ogival surface fused and scraped off. As the head of the projectile enters there is opposed to each zone of the surface of the ogival the resistance of strata of varying hardness and elasticity, the largest at any one instant being that of the hardened surface. Should this be a mere film, its peculiar effect would be hardly appreciable, although the varying characteristics of the strata below would permit the plate to concentrate its resistance and give a good account of itself.
It "is found that when projectiles large in caliber compared with
the thickness of a face hardened plate most nearly match it, the
head enters until its bourrelet is clear of the intact hard surface
when it breaks out a dome-shaped back bulge, to which its head
remains welded, while the body and base are crushed and thrown
to the front. In most cases, a few feet less velocity will make the
difference between complete perforation and the bare clearance of
the hard surface by the shoulder of the projectile. The crushing
of the latter in such a case naturally takes place at the instant
maximum resistance is offered ; that is, when the crumbling, expanding
zone of hard metal about the ogival is most depressed
and strongly and extensively supported by the under layers.
The subject of caliber in the attack of face hardened plates is important. The question may be asked. Would a face hardened plate, barely capable of resisting a certain caliber of projectile, be unable to resist a larger caliber having no greater energy?
The area of zones in the ogivals of the projectiles exposed to like resistance in the two instances vary as the calibers, but the sections of the projectiles along the planes of greatest weakness vary as the squares of the calibers. The larger caliber would, for this reason alone, inflict more damage on the plate before being broken up. Also, as its energy consists in a much larger degree of indestructible, or at least less easily diverted mass, and less of velocity, it would produce an equal amount of damage upon the plate with a proportionately smaller loss of velocity than the smaller caliber. From this it is evident that of the two projectiles the lighter, with less resistance to crushing, has its velocity diminished much more suddenly and violently than the larger caliber with broader planes of resistance. It would therefore appear that when the target is superior to the gun, and as the vast majority of impacts in action will be oblique where this will be the case, the larger the caliber, even with the same energy, the greater the damage inflicted. The introduction of armor of increased resistance is certain therefore to bring about an increase in the calibers of the guns.
On the other hand, when plate and target are about evenly matched, the larger caliber must break away a greater volume of the plate before perforation, this volume varying as the squares of the calibers. The resistance of the plate as a whole is also more certain to be felt against the slow moving projectile. Again, the larger ogival tends to concentrate its attack on each successive strata more than the smaller one ; that is, it breaks away as much of the hard face as the latter with but a fraction of its penetration, so that if there should be a plane of weakness or inferior strata, there would be greater danger of breaking the plate up in detail. This is seen in the attack of laminated or piped plates by large calibers, when the back bulge is broken out in slabs.
ADDITIONAL NOTES AND CORRECTIONS.
With the ultimate object of correctness in the paper, Lieutenant Ackerman submitted the following additional notes, giving credit when due.* They were submitted after the discussion.
At the end of the first paragraph, page 8:—It has been suggested, for example, that the gas check disks referred to, having been cut from round bars were suffering from radial forging strains ; had they been "upset" before being machined and tempered, or subjected to a long annealing, they would have tempered without difficulty.
At the bottom of page 20:—This paragraph is, as Mr. Howe states in the discussion, incomprehensible. It should read "The lime probably took up moisture from the air, becoming slaked, while the charcoal absorbed oxygen."
In regard to the foot note on page 21, CuO should be Cu2O. See criticism by Mr. Howe.
After "constraint " in the middle of page 29 :—This statement needs a generous qualification ; chemical affinity is potential energy, it can exert stress only as a result of its satisfaction as indicated in the following paragraph. See discussion.
After the last complete paragraph on page 29 :—This statement is incorrect ; change of carbon from the " cement " to the "hardening" form is dependent only upon the temperature.
After "hardness," line 4, page 30 :—This refers to hardening for tempering. See Howe, Metallurgy of Steel, page 22, §39.
After "elasticity", line 4, page 33.—This statement should be qualified so far as the words "largely increased" are concerned. It is found that the exterior film of a hardened cylinder is more dense than the interior. Also that the modulus varies nearly with the seven-thirds power of the density. See discussion.
After "plate," line 3, page 35:—That is, the plate would be affected to a greater depth.
"Ackerman," line 2 from bottom of page 35 should be "Akerman."
After "fluid," line 2, page 37 :—This statement should be materially qualified. There is considerable expansion before the melting point is reached, although at that point the metal is more dense than at 1200° F.
After "This evolution", line 4, page 37:—This reference is believed to be incorrect ; it is not essential, however, to the argument.
After last complete paragraph, page 37.—Professor Abel's experiments indicate that the condition of the carbon is hardly changed in wire drawing. This, however, is not essential, it is known that by forging through the critical point some carbon is retained in the "hardening" form; my contention is that pressure exerted at that time has the same effect. See Howe in discussion.
After line 22, page 39:—This apparent contradiction of views between Messrs. Brustlein and Howe is due to an error of my own. They are both correct. The former refers to the effect of varying percentages of carbon. The latter to the effect of treatment in steel containing any particular percentage of carbon.
On page 5, nth line from the bottom:—"A 28 per cent, carbon plate "should read "A .28 per cent, carbon plate." Page 16, third line, "Roma shoals " should read "Romer shoals." Page 18, seven lines from bottom, "25 per cent." should read .25 per cent. Page 45, line 4 from top, "90° cone" should read "45° cone."
DISCUSSION.
Mr. Wm. Allen Smith.—I expected to have sent you before this some remarks on the chronology and details of the negotiations between the late Mr. Harvey and the Naval Ordnance Bureau, but I have been laid up with an attack of grippe, and have been so pressed with other matters that I have not been able to get into shape the criticism of your paper which you asked for. I may, however, remark, that on page 5 it appears to us you give rather more credit to the Navy Department than is quite just, in view of the fact that Mr. Harvey had perfectly well defined views of what he wanted to accomplish before he saw Commodore Folger ; and really by his presentation of his views so interested Commodore Folger as to induce the Commodore to recommend the experiments by the Navy Department which were subsequently made, and which showed the correctness of Mr. Harvey's views as to the best method of making a perfect armor plate.
We would also remark that we do not know and do not believe that what Mr. John D. Ellis did, in respect to applying the process of cementation to armor plate, amounted to anything more than a crude, abortive, and abandoned experiment ; and in view of the fact that Mr. Ellis did nothing more than employ the ordinary cementation process, it is going rather too far to describe what he did as the " Ellis Process."
In the second paragraph on page 8, there is a slight error concerning the Schneider plate, which, as a matter of fact, was both super-carburized and hardened at the Washington Navy Yard, and was tested in February, 1891.
The erection at the Washington Navy Yard of the furnace for treating this Schneider plate was begun in November, 1890. These matters of dates are of course of minor importance. But it seems desirable to mention that Mr. Harvey, early in his negotiations with the Navy Department, and in support of his views, exhibited to Commodore Folger a block of steel, one side of which had been treated by his process, and had been made so hard that it could not be indented by a center punch driven by a sledge hammer. The exhibition of this small block of steel to Commodore Folger preceded the completion of the arrangement by which the original 6-inch plate was sent to Newark to be Harveyed.
I will only add that your contribution to the literature efface hardened armor is both valuable and timely.
Mr. H. M. Howe, Boston, Mass.—We must thank the author for a great deal of very valuable information which he gives as to the actual procedure in Harveying and hardening armor, and for his interesting and suggestive propositions. As a metallurgist I must compliment him on the amount of very recently published information with which he has familiarized himself on the exceedingly difficult and complex question of the hardening of steel. So difficult is this question, however, that there are some points on which I can perhaps set him right, and others as to which I should be glad to learn his authority: and so it is with the question of Harveying. Let me touch on these before taking up his main contention that gashing should increase resistance.
Cementation.—On page 21, line 6 and foot-note, the author, while correctly asserting that the phosphoric acid of bone-charcoal is not reduced in cementation, yet thinks that its phosphorus acts as a deoxidizing agent, as phosphorus does in reducing copper-oxide in making phosphor-bronze (by the way, it is cuprous oxide, Cu2O, that the phosphorus in this case reduces, not cupric oxide, CuO, as the author says). This must be a slip of the pen. ' The unreduced phosphoric acid of the bone-charcoal, a fully oxidized compound, of course cannot take up more oxygen, and hence cannot deoxidize any other substance. In making phosphor bronze we use, not phosphoric acid, but phosphorus in an unoxidized and hence an oxygen-taking form.
On the same page, line 17, what is his authority for the assertion that cold iron buried in charcoal absorbs carbon even without the aid of heat ? This must be an error.
Why quote, on page 20, line 26, "Dannemora iron" (by the way, one of the brands of iron especially noted for its freedom from phosphorus), " gives out an odor of P when twisted at a red heat ? " Such an assertion is incomprehensible.
The same page, the last three lines. "The highly oxidized lime" (I had supposed that lime was a perfectly definite oxide of calcium, and that all lime was equally oxidized), probably took up moisture by selection of H from the air " (but whence is to come the oxygen which, uniting with that hydrogen, could form that moisture ? surely this cannot be serious) "as well as C from the carbon present in the charcoal." What kind of compound was then formed between lime, water and carbon ? A hydrated oxycarbide of calcium ? To form lime-carbonate we need oxygen, not hydrogen.
Page 21, line 31. "On heating, the carbon is expelled from the limestone and unites with the oxygen . . . above." Can it be necessary to explain that, when limestone is heated, not carbon but carbonic acid is liberated?
Page 22, lines 4 to 14. "Soda ash frees the metal (iron) of oxygen." That soda ash, carbonate of soda, an already fully oxidized substance, is further to take up from solid iron with which it is in contact the oxygen which that iron does not contain, does not call for much comment. " The calcined lime eliminates the oxygen ... in the retort." Here again, a fully oxidized stable substance, lime, is to take up oxygen. " Pure cyanogen- gas is generated, which permeates the metal, carrying with it the free tungstic acid which tends to give the metal greater hardness." Without further explanation or evidence, we must regard this as unworthy of serious attention. Tungstic acid (anhydride) is not volatile. When heated as here with charcoal, it is reduced to metallic tungsten and tungsten dioxide. Tungsten volatilizes at about 3400° F. (1900° C). But if tungstic acid were carried into the metal as here asserted, what good would it do ? It is not tungstic acid, but metallic tungsten that hardens steel.
On page 23, the last four lines, the author says that it seems that in Harveying iron and carbon both volatilize, " when of course the formation of a definite carbide would naturally follow." Carbon has the power of migrating through hot plastic iron, and in Dr. Fleitmann's experiments, to which the author refers, it was reported that iron had the power of migrating in hot plastic nickel. Later experiments have shown that lithium can migrate in hot plastic sodium-silicate glass. These migrations in plastic substances are extremely interesting and instructive : they remind us of crystallization in like masses ; but they do not imply, they hardly suggest volatilization. It is in the highest degree improbable that either iron or carbon volatilizes as such in case-hardening or Harveying. Nor, if they did volatilize, would it at all follow as the author asserts that " the formation of a definite carbide would naturally result."
On page 26, line 3, the author asserts that iron contracts in heating from 1832° F. (1000° C.) to the melting-point; on page y] , line i, that it contracts in heating from 1200° F. (649° C.) to the melting-point ; and in line 4, that the recalescence is accompanied by contraction. In each respect he is completely and curiously mistaken. Indeed, the contraction of iron in cooling from bright redness down to the recalescence-point, and its expansion at that point, are notorious, and if I remember aright, are what led to the discovery by Gore of the recalescence. The enormous shrinkage which occurs in the early part of the cooling of steel castings, and which is one of the chief stumbling blocks in their manufacture, the like contraction of steel ingots, of rails, plates and other products in cooling are so well known and such obvious contradictions of the author's assertions, that one fancies that this must be a case of heterophasia, of saying the very opposite of what is intended. See page 109, J. I. S., 1891.
On page 37 , four lines from the bottom, he seems to quote, as an explanation of the increase of strength of steel caused by wire-drawing, that the pressure which accompanies it forces the carbon into combination, by which he must mean, forces it into the hardening state, because most, and usually all, of the carbon in steel is in any case combined. Now, as wiredrawing causes like strengthening effects not only in the softest iron and steel, the freest possible from carbon, but also in all other malleable metals and alloys so far as we know, it would not be reasonable to suppose that the like effect in case of steel is chiefly due to so unlike a cause as forcing into combination the carbon which these other substances lack. But, quite apart from the extreme unreasonableness of such a supposition, we have Abel's experiments showing that these cold-working operations do not transfer the carbon to the hardening state.
While the reactions on pages 24 and 25 by which the author explains the chemistry of cementation may be true, yet I believe that the operation is far more clearly explained by two well-known, simple, fundamental reactions,
2CO=C + CO2
by which carbon is deposited within the metal, where its absorption needs no explanation, and
CO2 + C = 2CO,
by which the resulting carbonic acid renews the supply of carbonic oxide, and thus enables a small quantity of oxygen initially present to transport an indefinite quantity of carbon into the iron. This transportation of the carbon from the charcoal into the iron is all that needs explaining. There may be other intermediate and side reactions, but these which I have just given seem to be the end-reactions which suffice to explain the whole clearly. Oxidation of iron may occur as Lieutenant Ackerman supposes, but if so, it appears to be only an intermediate reaction, which serves rather to confuse than to clarify matters. For example, his deduction that there is an exhaustion of oxygen which must stop the operation seems to spring from this, and is probably misleading.
Hardening.—The third paragraph of page 29 argues that the smaller density of hardened than of unhardened steel in itself implies stress. Stress there no doubt is in hardened steel ; but we infer it from other evidence. The mere lightness as such might be readily explained by difference in chemical constitution. In the following sentences he seems to think that unsatisfied chemical affinity may create stress ; this seems to be a confusion of ideas, or a misuse of words.
Immediately below, and also in the last three lines of page 34, he naturally mistakes the results of Professor Langley's experiments. These showed that quenching steel, even from as low a temperature as 108° C. lightened it somewhat. They have been repeatedly quoted as proving hardening. But they merely showed that this quenching made the metal slightly lighter. No measure of hardness was attempted, nor have I ever seen any reason to believe that the metal was made in the least harder.
Near the end of page 29 he says that the hardening tendency is said to be proportional to the duration of the temperature of hardening. I have never heard this suggested before. So far as my observation goes, all the evidence tends to show that when steel is raised to a given temperature, measurably above the critical point, W, the degree of hardening is practically independent of the length of time that the steel is held there. In other words, the change from cement to hardening carbon seems to take place with very great rapidity. No considerable length of time is needed to complete it.
On page 39, lines 10 and 11, the author falls into the curious error of supposing that manganese steel forges particularly well and "welds with great facility." Would that it did ! Unusual precautions are required in forging it ; even after these have been discovered and adopted, forging it is no easy matter, and as for welding it, why nobody thus far has succeeded in welding it properly so far as I know, even with great experience in electric welding.
From among the assertions which the author makes or quotes, some of which I fear must be wrong, I select the following, to ask him to give us his authority.
Page 27, line 7. "That steel of low tenacity carburizes with greater ease than other steel" (this is as I understand him).
Page 27, line 5. "It is said that the porosity of steel is generally in an inverse ratio to its tensile strength."
What is the porosity of steel, and how does one determine it ? Speaking broadly, the density of steel is inversely as its tenacity. If we were to draw any inference from this as to porosity (and I for one should not dare to), it would be that the porosity, which should if anything be inversely as the density, should be directly as the tenacity, and not inversely as the author has it.
Page 33 lines 1 to 4. He asserts that water-quenching highly-carburized metal greatly increases the modulus of elasticity. Such evidence as I have met agrees with his previous and opposing assertion (page 32, line 26), that water-quenching has little effect on the modulus. The high tenacity, 400,000 pounds per square inch, which he quotes apparently as indicating an increase of the modulus, is no more evidence as to the modulus than the weight of a book is as to the weight of its author. The detailed information as to the modulus implied on this page should be of great value.
Page 30, line 3. "The more slowly the metal is heated to the higher temperature, the tougher it becomes without loss of hardness ; this toughness increasing with the length of time of exposure to that temperature." I can recall no evidence even pointing in this direction.
Page 34, seventh line from the bottom. An unqualified assertion that "Both the tensile strength and ductility of the mildest steels are greatly increased by quenching." My own extensive tests* had showed that quenching from above the critical point greatly lessens the ductility of all the steels which I tested, including one with 0.09 per cent, of carbon, which is certainly among " the mildest steels." There are some things which suggest that quenching from below the critical points, below which no true hardening occurs, may increase the ductility. This does not, however, help the author at all, who from the context evidently refers to quenching from above the critical point. He must have some full data of which I am ignorant, unless, as I fear, he is completely mistaken.
On page 35, line 1, as an inference from the assertion on which I have just commented, he further asserts: " It is evident, therefore, that the more sudden and complete the chill, the greater the increase of toughness in the body and back of the plate." As the context shows that toughness here means ductility, his conclusion seems to me not only far from evident, but in the very highest degree improbable.
To the same apparent error I ascribe his mistake on page 32, lines 19 to 25, that, save at the very surface, the plate is decrementally toughened, not decrementally hardened. Here he appears to be completely in error.
On page 8, line 6, he says that doubtless deleterious components are volatilized in the Harvey process from inferior metal. The volatilization of sulphur has been observed under like conditions ; but has that of any other deleterious component?
On page 37, line 2, he quotes the specific gravity of molten steel as 8.05. This is surprisingly if not improbably high.
Let us now consider the author's main contention that gashing the face of a plate will
(1) Cause it to heat more quickly, and
(2) To carburize more quickly, thus saving time in Harveying;
(3) By increasing the pressure on the face of the plate will increase the degree of hardening for a given temperature, thus permitting the use of a lower quenching temperature, and finally, thus leading to less severe stresses within the plate ; and
(4) Will increase the resistance to penetration. Let us consider these separately.
A plane parallel with the face of the plate, and slightly below its surface so as to lie just tangent to the bottoms or troughs of these gashes, divides the plate into two parts, a solid unbroken part, quite like the whole of any ungashed plate, and a lot of ridges or other elevations rising from this solid part, consisting of the steel left between the gashes. According to the direction of the gashes, these elevations may be true ridges or simply peaks ; if the gashes are far apart, the ridges become table-lands. But, whatever their shape, let us for simplicity call this part of the plate above the bottom of the gashes the " ridge" part, and that below their bottom the "body" part.
I. Heat Absorption.—We may concede that the existence of a properly proportioned ridge part between the body part and the source of heat might hasten the heating of the body part, because the greater surface of the ridges would absorb heat more quickly, and because the high conductivity of the metal would quickly transfer into the plate any heat taken up by the ridges.
But it seems to me that the saving of time and fuel, while it might exist, would be inconsiderable,—so slight as to be wholly unimportant.
2. While the increase of surface due to gashing would cause more carbon to be taken up per hour by the plate as a whole, yet this excess would, I believe, be found in the ridge part. The ridge part would retard the carburizing of the body part, because all the carbon which reaches the body must now pass through the ridge part, save what little enters at the bottoms of the troughs between the ridges. The case is wholly different from that of the mere introduction of heat. In the latter case, the fact that the ridge part separates the body part from the source of heat may well be outweighed by the high thermal conductivity of the metal, which would transfer into the plate with extreme rapidity the increased quantity of heat which its increased surface led it to absorb. Not so with the carburizing gases, however. The ridge part would, in effect, stand as an irregular barrier to the carburizing action, which would have to reach the body through the ridge part, and therefore much more slowly. We must remember that, while heat passes by conduction through iron quicker than through air or through the atmosphere which bathes the plate, yet the reverse is probably true of the carburizing action.
3. While there is much to suggest that pressure increases the degree of hardening caused by any given quenching, I see no reason to believe that it plays an important part. But, if it did, then it seems to me that the author's gashes should have exactly the opposite effect from what he supposes, and should lessen rather than increase the pressure.
When a plate is quenched, its skin presses on its interior because the skin cools faster, and hence contracts faster than the interior. This of course at first throws the skin into tension, the interior into compression. The pressure of the skin on the inside may be likened to the tightening of a strap. A gashed skin is like a crimped strap, with corrugations transverse to its length. Clearly, for given shortening an initially crimped strap will press less tightly than one initially straight. So, it seems to me, that, for given contraction, a gashed skin must press less strongly on the underlying parts than an initially straight, smooth skin.
Gashing might oppose flaking. The surfaces of least cohesion should, in a gashed plate, be roughly parallel with the gashed surface, because the variation both in carburizing and in hardening would run roughly parallel with it ; and such irregular surfaces would be less likely than plane ones to coincide with the surfaces of high stress.
4. Whether because of, or in spite of my ignorance of resistance to penetration, the argument that it should be increased by gashing seems to me utterly fallacious.
First, let us recognize clearly that, besides increasing the hardness proper, Harveying and quenching also increase enormously the compressive strength and tenacity, and their resultant, transverse strength, all beneficial, and also brittleness, a most injurious property. Hardness proper and compressive strength, both composite quantities, with many elements in common, are yet distinct. Chilled cast iron and quartz, each at least as hard as hardened steel, have little compressive strength and little tenacity under impact. I believe that the author makes a capital mistake in confining his attention unduly to hardness proper, which I believe aids penetrative resistance but little, and neglecting the transverse, compressive, and tensile strength, which I believe are the main components of resistance. The brittleness which accompanies these beneficial effects of hardening, of course, as such, lessens resistance.
Let us resolve the resistance which a hard face offers to penetration into what I suppose must be two of its chief components, (1) normal resistance parallel with the axis of the projectile, and (2) radial resistance which the face offers to being driven parallel with itself, radially away from the point of impact, as the ogive progressively forces its way farther and farther into the plate, and stretches the hole out wider and wider.
The normal resistance is like the support of a snowshoe, or of thin ice on a pond ; it arises from transverse strength. The gashes are equivalent to corrugating snow and snowshoe, or ice ; in either of these cases we may admit that corrugation might increase the transverse strength, and that the normal resistance of a gashed, i. e., corrugated half-inch thickness of face, reckoning from the crest of the ridges to half-inch below the troughs of the gashes, might be greater than that of a plane half-inch. This means that the normal resistance of a lo-inch plate with ;1/2-inch gashes might be greater than that of a 9.5-inch ungashed plate ; but I do not see clearly that it should be as great as that of a lo-inch ungashed plate.
But when we come to radial resistance the case is far worse. The isolation and the consequent exaggerated rate of cooling of the ridges would, indeed, give them increased compressive strength, but also increased brittleness. This isolation, the presence of these gashes between the ridges, must, however, deprive them of lateral support, and must thus affect most unfavorably both these effects of the exaggerated hardening. For on the one hand it makes their compressive strength useless for radial resistance, just as a weak foundation makes the strength of a strong column useless ; while on the other hand it must, I fear, make their brittleness all the more damaging. Thus I fear that, as regards radial resistance, the ridges approach the condition of grains of sand or emery, hard but useless, attached to the body of the plate.
The hardening, and hence the tensile, compressive and transverse strength of the body part, are of course greatly lessened by the retarded cooling which the interposition of the ridge part between it and the water causes. Thus the gashing in effect concentrates both the beneficial and the harmful effects of hardening, the strength and the brittleness, into the ridges, where the beneficial ones are inoperative and the harmful one is more noxious. This, it seems to me, is likely to outweigh enormously any supposed gain in normal resistance ; and I fear that a 10-inch plate with half-inch gashes would be much weaker than a 9.5-inch ungashed plate, as well as far more costly. One may not prophecy in such complex questions ; but these considerations seem to me to deprive the proposition of all promise.
Lieutenant W. Irving Chambers, U. S. N :—I have found Lieutenant Ackerman's valuable contribution to the literature of armor very interesting and instructive, and I can only hope to add to its interest by a few remarks concerning the process briefly alluded to in the second paragraph on page 41. This is known as the Chase-Gantt process and was discovered at the Midvale Steel Works, Philadelphia.
The fact that the best, if not all, of the modern armor-piercing projectiles are made of chrome steel is good evidence concerning the shock resisting properties of that metal and the capacity of that metal for being worked and treated under the hammer or in the oil bath and annealing furnace. Although chromium has given the best results in their armor plates, so far, the patents of these gentlemen cover the use of manganese, tungsten or any other element or alloy they may choose to employ in their special way. However, the metal of their entire plate is not strictly " chrome steel " and it is well to note that this is chrome faced steel only. The discovery of a comparatively cheap and simple method of introducing chromium and car_ bon together into the face of a plate by absorption, during the process of casting, marks a new and distinct era in the treatment of steel by alloys.
The bed of the mould is simply prepared with the desired alloy and baked to sufficient hardness and then the molten metal, nickel steel or steel of any desired grade, is quickly poured, as in a simple casting, leaving one or more heavy risers to feed the shrinkage as it slowly cools.
In this easy "method of cementation" the molten metal fuses the alloy, the chromium rapidly penetrates the lower face and, on account of its great affinity for carbon, carries the latter with it to a depth which is readily regulated by certain details in the process. It is supposed that the best results will obtain from a depth of hardness equal to about one-fifth the thickness of the plate. After casting, the plate is oil tempered and annealed as often as desirable and the face is then water hardened as with the Harveyed plates. The interior "pockets," which Lieutenant Ackerman shows to be so important, are always formed in this case by the hooks or projections of the hardened part which penetrate the softer part during the process of casting, or while the carbonized chromium alloy is penetrating the molten mass.
These desirable hooks thus formed are, of course, unevenly distributed. They may, however, be readily accentuated, regulated, or produced in any desired form, square or otherwise, m the interior, by suitably preparing and placing the alloy in the mould with that end in view, and the face of the plate after casting will be perfectly smooth.
Such a plate, with a homogeneous soft back of mild steel evenly graded in toughness to a hardened surface that cannot be touched by a specially hard tool-steel center punch, seems to approach the theoretical condition mentioned on page 64. However, as Lieutenant Ackerman says, it is possible to have- the back too weak to satisfy the desired conditions, but if the simple cast steel of the Chase-Gantt plate be found too weak, it can be forged or rolled down to any desired extent. For example, one of these plates, cast 11 inches thick with a hardened face about 2 inches deep, was heated and forged down to 6 ½ inches thickness ; a piece of this was then heated and rolled down to one-half inch thickness, in which condition the hardened face was found to have been reduced almost proportionately, i. e., the final thickness of the hard face was about one-eight of an inch.
The patentees of this process think that they will be able to produce results equivalent to the best service armor in plates simply cast, of the same thickness, without forging ; i. e., through the quality of the cast and the invisible kneading that occurs during the oil tempering and annealing processes. Of course we hope they may, but I am not so sanguine about such a marvelous achievement. It seems certain, however, that they are able to produce a greatly increased depth of hard carbonized surface in conjunction with a wonderfully tough and homogeneous back. There is, apparently, no difficulty in casting a plate thirty or more inches thick with a hardened face from three to six inches thick, and in forging or rolling this plate down to ten inches thick with a hardened face from one to two inches thick. It is also, apparently, easier to produce uniformity of results in thick plates than in thin ones.
In consideration of the valuable experience gained during the development of our present service armor and of the relative simplicity and cheapness of this rival, I anticipate a speedy development of chrome-faced armor and a considerable increase in its resisting power over that of the present service armor.
Lieutenant C. A. Stone, U. S. N:—I have read, with much interest. Lieutenant A. A, Ackerman's paper on face hardened armor, and I consider it a most valuable contribution, and one which brings together, in an able manner, the recent results on this subject with most interesting explanations, I would like to see Lieutenant Ackerman's proposed armor plate made and tried, and I hope this will soon be done. Lieutenant Ackerman's explanation of the resistance offered by the Harveyed plate by being elastically dished under the impact of the shell, is very interesting. The resistance which a plate would thus offer would appear to be similar to that offered by a circular disc when loaded at the center and supported around its circumference ; a mathematical expression for this I have never seen.
On page 52 are given the results of the trial of 4” shell against a 3'' Harvey plate with a velocity of 600 f. s ; no penetration was attained and the shell were smashed. The result was caused by the resistance of the plate to being dished elastically. From this it would appear that, for a certain velocity, that resistance is sufficient to demolish the shell. I remember seeing at the Washington Navy Yard, some years ago, a plate which had spiral springs behind it, which had been fired at. Several large holes through the plate indicated the result. I do not know who was the author of this experiment. In this case the springs would offer but little resistance, and the inertia of the plate would allow it to be penetrated before the springs could do much work, even were they stiffer. In the case of the thin Harvey armor plate, the resistance it offers under impact to being elastically dished is very great ; the plate, in the neighborhood of point of impact, being a most powerful spring. Thick Harvey plates will dish elastically less than the thin ones, of course, as the carbonized and tempered part is a less portion of their thickness and mass ; for this, among other reasons, thick Harvey plates will be relatively more easily penetrated than the thinner ones.
Lieutenant Ackerman has not said much about the effect of the long continued high temperature of carbonization on the body of the thick plates. This tends to undo, to a great extent, the good effect of the previous forging, tending to render the body of the plate crystalline. Experiments are being made for the purpose of determining the effect of additional forging after carbonization and before tempering. A 14'' plate in which this was tried is now awaiting ballistic test.
On pages 39 and 41 of Mr. Ackerman's paper mention is made of nickel chrome and of nickel manganese Harvey plates.
If the additional forging after carbonization should not prove sufficiently successful in the case of the thick nickel steel Harvey plates, it appears to be probable that better results may be attained by the addition of chromium or manganese.
On page 33 of Mr. Ackerman's paper, it is stated that at a certain point beneath the surface of a face hardened plate, etc., the original modulus may be found.
I think that experience shows that ordinarily the carbonization extends to a greater depth into the plate than is of any use with our present means of tempering. This leaves behind the tempered face a stratum of metal higher in carbon than is desired. With our present methods of carbonization, to obtain the required amount of carbon near the surface, this unnecessary and objectionable depth of carbonization appears to be unavoidable. Experiments upon this point are much needed, but the ballistic test is the only means of determining with any certainty what results have been obtained. This is, of course, expensive and also very often not decisive, the result being due to other causes than those the effect of which we seek to determine.
The percentage of carbon, at different depths from the face of a carbonized plate, is determined by chemical analysis at each one-sixteenth of an inch of depth, until the normal amount of carbon is reached ; this is at a depth of an inch and a quarter or an inch and a half, usually. These percentages of carbon may, of course, be laid off as ordinates on any convenient scale, the abscissas being the distances from the face of the plate at which these percentages occur. A curve drawn through the ends of these ordinates will give, for any case, what we may call the curve of carbon for that plate.
With our present method of tempering, the chill only reaches a depth of about five-eights of an inch ; all parts of the plate to this depth are more or less tempered ; the amount of tempering at any point would evidently depend, the tempering means being the same, on the amount of tempering carbon at that point, and the effectiveness of the chill, which depends probably on some function of the depth.
If we had some recognized method of determining hardness, we could, together with carbon determinations in a plate, make hardness determinations at the corresponding depths. These degrees of hardness could be laid off as ordinates on the same figure as that showing the curve of carbon, and if a curve were drawn through the ends of those new ordinates, it could be called the curve of hardness. This latter curve would become parallel to the axis of abscissas at a point about five-eighths of an inch from the origin, the curve of carbons becoming parallel to this axis at a distance of an inch and a quarter or an inch and a half from the origin.
A comparison of these two curves would show the relation which exists between the amount of carbon and the degree of hardness obtained by the tempering, at the different depths, and, if this comparison were made for a number of plates, it is probable that information would be obtained therefrom which would be of much use in the manufacture of Harveyed armor plates. A sketch is enclosed to illustrate these curves, the full line being the curve of carbon, and the dotted line the imaginary curve of hardness.
Lieutenant Ackerman, U. S. N.:—I have much for which to thank the gentlemen who have discussed this paper. Few practical or business men care to correct or elucidate views which may benefit their possible competitors; others will not venture into the realm of conjecture where the disclosures of a few months to come may confute the expressed opinions of to-day.
I appreciate all the more then the honor which Mr. Howe with his great wealth of experience has done me in criticizing the paper at such length. He has pointed out errors in it which shall be frankly admitted and called attention to statements which require substantiation. He has also made certain other strictures in which I can follow him neither in logic nor in fact. However, I am assured of this, the careful reader will be able to find no error or doubtful statement in this essay which is any more necessary to the line of argument in favor of the proposed new armor plate, than a fungus is essential to the existence of the tree in which it lives.
Mr. Howe's disgust at the peculiar assortment of patent claims for various processes of cementation would be amusing were it not for the fact that he appears to hold me responsible for the errors which they contain. These claims were quoted as an interesting exhibition of the peculiar views held of the process of cementation by certain practical minds. It is expressly stated at the foot of page 22 with regard to them, that few explanations are satisfactory, while many claims are based on unsustainable assumptions.
Erroneous as they are, however, in theory, they will doubtless convey suggestions to some who it is to be hoped will discern the soul of good that lies mingled in their evil.
Exception must also be taken to the frequency with which Mr. Howe declares express quotations which occur in the essay to be assertions, thus apparently attempting to force me to defend a position where I would be at a disadvantage, and in which in every instance noted I have no real interest at stake, especially when the matter is correctly stated at length elsewhere in the essay. I refer, for example, to his criticism of the quotation on page 37 of W. Hempel's explanation of the increased strength obtained in wire drawing and cold hammering; also the remarks ascribed to Professor Barrett on the same page, that recalescence is accompanied by a contraction ; also the first sentence on page 37 that metal becomes more and more dense when heated above 1200° F., until fluid. No assertion was made in these cases, in fact the words "it is said"—were used as disavowing any assumption of responsibility. The behavior of the metal at the critical points is fully explained on page 25.
Should his valuable work on the Metallurgy of Steel be read in this spirit, it would lead to endless contradictions.
For instance, on page 18 of that work this statement appears with regard to the effects of hardening, "It is said to raise the modulus of elasticity." This view of the effect of hardening on the modulus he contests in his criticism of my paper, if it was an assertion in his book, he is now in contradiction; if it was not an assertion, then many of his criticisms of this article cannot have been seriously intended.
The metallurgical premises of the essay which are criticized in the discussion may be grouped under the following heads :
1. That at the extremely high temperature employed in the Harvey process, i. e., above that of molten cast iron, the gases of cementation are not only extremely attenuated, but the capacity of the metal for them is less than at a somewhat lower temperature when the metal is less dense. [No denial is made of the statement that at this high temperature much of the carbon introduced is graphitic or uncombined, unable to assist in increasing the hardness in the subsequent quenching, and to the presence of which in fact the softness and weakness of the metal after cementation is in a great measure due. This intrusion of graphite occurs to a certain but far less extent even at the ordinary temperature of cementation, for it is well known that an analysis of cement steel taken after forging will give a greater percentage of combined carbon than if taken immediately after cementation. Lieutenant Stone has called attention to this fact in his remarks. It may be added that the Carnegie Steel Company had experienced some difficulty in obtaining the requisite hardness on the face of some very thick plates cemented at a high temperature ; analyses showed a fair amount of carburization, but in addition, considerable graphite, which undoubtedly weakened the surface. One of these plates was forged slightly thinner, when the analysis exhibited a quite marked increase of combined carbon. Upon ballistic test, the plate showed far greater resistance than any other plate of equal thickness that has yet been tested.]
Under this head also come the objections made to the "volatilization" of iron at the ordinary temperature of cementation ; the connection of cementation with porosity and porosity with tensile strength ; and the increase of the density of steel when heated to a temperature approximating to the melting-point. Every one of these objections might be admitted, however, and it would still remain apparent that the plate would be in a better condition both as regards crystallization and carburization if not heated above 2000° F. in cementation. In addition it is found that the edges and corners of plates are occasionally burnt, while unavoidable fluctuations of temperature sometimes cause hollows to be melted and scored into the highly carburized surface, which also shows large crystals.
2. Under the second head come the criticisms of the theory that pressure assists in retaining the carbon in the hardening form in quenching, and that therefore an arrangement of the surface by means of which the initial chill will immediately place an external layer of the metal of considerable thickness in a state of tension, and hence compression, thereby not only hastening and increasing its own tendency to harden, but bringing a more direct and powerful influence on the body of the plate.
Although Mr. Howe contends this theory in detail, it is quite satisfactory to note his final position. "While there is much to suggest that pressure increases the degree of hardening caused by any given quenching, I see no reason to believe that it plays an important part." Still it is well known that by forging through the critical point some carbon is retained in the hardening form. My object throughout has been to make use of every possible means of assisting the hardening ; nothing should be considered trivial when it works in the right direction.
Mr. Howe's experience and distinguished reputation should, however, command the most careful attention to every objection he has made. For that reason I will go more at length into the various details than would otherwise be necessary. There is this great advantage in expert criticism, it is certain to help a good thing.
CEMENTATION.
With regard to page 20, last three lines. This paragraph as it stands is indeed incomprehensible. It should read " Powdered charcoal takes up O from the air, while the quick-lime absorbs moisture and slakes."
Page 21, line six and foot note. Mr. Howe makes the following statement: " The author, while correctly asserting that the phosphoric acid of bone-charcoal is not reduced in cementation, yet thinks that its phosphorus acts as a deoxidizing agent." A contradiction is here implied though none exists. In the text it is said, "the basic phosphate is not reduced ;" in the foot note it is said: "It is the writer's opinion that P acts as a reducer of CO," there is no reference in the foot note to the basic phosphate of the bone-charcoal.
The use of CuO for Cu2O is due to an omission unimportant so far as the idea conveyed is concerned ; it is not a slip of the pen.
Page 8, line 6. A strong doubt has been expressed by certain metallurgists, who combine sound theoretical knowledge with much practical experience, as to whether the Harvey and similar processes actually improve inferior metal more than would be accomplished by careful annealing. The improvement in the gas check discs for example, is laid to the fact that the round bar irom which they were cut contained radial forging strains which were released in the long annealing. Had the discs been upset before being machined and tempered, no trouble would have been encountered. It is asked, "Has the volatilization of any other component than that of sulphur been noted in the Harvey process?" In fact, no. Mr. Harvey's claim however is to carry on the treatment between the temperature of molten cast iron and that of about 3000° F. For some distance below the superior limit, various reductions will take place in the presence of the carbon gases. Chief among these is that of the oxide of iron found in overheated or burnt metal, rendering it brittle and of little value. It is well known that metal of this character can be made forgeable and capable of taking a temper when heated in contact with carbon. At the higher temperatures demanded in this process, the basic phosphate previously alluded to would also be reduced, and, as Mr. Harvey stated, would render the operation more rapid and effective, doubtless through assisting in reducing the oxide in the iron.
Page 21, line 17. With regard to cold iron absorbing carbon: In Landrin's Treatise on Steel, the following occurs: "Iron and carbon have a great tendency to unite even when cold. Iron left for some time in a mass of charcoal dust will become hardened, and by and by, may be transformed into steely iron." This work was translated by M. Fesquet, in 1868, so that it cannot be regarded as an authority on modern steel making ; however, it would be absurd to suppose that M. Landrin, who was a practical man, did not know the difference between iron and steely iron.
Referring to the bottom of page 23, Mr. Howe says: "In Dr. Fleitmann's experiments, to which the author refers, it was reported that iron had the power of migrating in hot plastic nickel ... It is in the highest degree improbable that either iron or carbon volatilizes as such in casehardening or Harveying."
Dr. Fleitmann published the results of his experiments with various observations in Stahl und Risen, voV. IX., pp. 9-12. I have been unable to gain access to the original memoir, but the following is from the Physical Property notes in the Journal of the Iron and Steel Institute, vol. I., 1889, p. 368:
"Some remarkable and regularly recurring phenomena . . . led the author to conclude that iron is volatile at a jnedium red heat, and experiments proved this to be the case . . . Further experiments showed that, although the iron is very volatile at this temperature, the nickel does not volatilize at all."
The dissimilarity of the words Verfliichtigung, volatilization, and Wanderung, migration, is so great that I am unwilling to accuse the staff of the Iron and Steel Institute of an error in translation. The statement that a definite carbide is formed to which exception is taken might be qualified. Doubtless several carbides and multiples of carbides are formed, while much of the volatile or migrating material does not combine at all.
Page 27, lines 5 and 7. It is evident that the more porous the steel the more easily the gases of cementation penetrate it ; the greater the volume of carbon depositing gas present, and the larger the surface of metal on which the carbon may be deposited or with which it may combine. It would seem, therefore, that the rate of cementation is a function of the porosity of the metal.
As to the connection between tensile strength and porosity. Dr. Thomer in Stahl und Eisen, vol. VI., pp. 166-168, gives the results of a number of interesting experiments stating: "As a general rule, it appears that the porosity is in inverse ratio to the tensile strength." He accounts for any exceptions to this rule, however, by the irregular disposition ot the pores in an ingot, and the improbability of a single test piece representing the average tensile strength of the larger body of metal.
From this it would follow that steel of low tenacity, that is, porous steel, would carburize with greater ease than stronger steel.
The method of determination of the porosity of steel, in general terms, is as follows: Filings of the metal are placed in the receiver of an air-pump, which is then maintained in an exhausted condition for a considerable period of time. They are then brought into contact with a mobile fluid, such as 90 per cent, alcohol, capable of wetting the surfaces of the powdered metal. The diminution in volume of the liquid and the increase in weight of the filings give means for determining the volume of liquid absorbed; or the porosity. Dr. Thoraer's observations on the subject were first published in Stahl und Eisen in 1884. Two years before, however, while attached to the Smithsonian Institution, I assisted the curator of metallurgy in determining the porosity of coke by a somewhat similar method.
Mr. Howe inquires. What is "porosity"? Dr. Thomer states that by "porosity" he understands only those pores which are of microscopic size, and not the larger blow holes or hollows.
Professor Tait's definition is as follows: "By the term pores we do not refer to visible channels, such as those which run in directions through a sponge, but to microscopic channels which pervade even the most seemingly homogeneous and continuous substances, such as solid lead, silver, gold, etc. . . The rapid passage of gases through unglazed pottery, iron and (hot) steel, etc., shows the porosity of these bodies in a very remarkable manner."
Deville and Troost are said to have obtained very curious results with reference to the rapid passage of various gases through heated cast iron. Carbonic oxide was one of these gases.
Helmholtz and Root have proved that platinum is pervious to hydrogen even at ordinary temperature (Idem).
The ability of hot metals to absorb many times their own volume of gas in cooling is also an indication of their porosity. Methods of determining the quantities of these gases contained in the metal by drilling it under water might indicate their relative porosity.
Page 26, line 3 and page 37, line 1. " It is said that from this point (1200° F.) the metal becomes more and more dense until fluid "; page y] line I, is undoubtedly erroneous except it be taken in the most general sense. I am fully aware, as Mr. Howe explains in his Metallurgy that neither contraction nor expansion in cooling and heating are continuous and regular. What I should have said is that from this point, 1200° F. the metal expands rapidly but neither regularly nor continuously to a temperature in the vicinity of 1832°, after which, at some higher temperature, it contracts, becoming when fluid even more dense than it was at 1200° F. This matter is, however, correctly treated on page 25.
The fluid density of steel given in the paper was obtained from the Physical Property Notes in the Journal of the Iron and Steel Institute for 1881. Mr. Howe's statement that "the enormous shrinkage which occurs in the early part of the cooling of steel' castings " refutes my assertions as to the density of fluid steel is hardly correct. Neither is it a wise one for his argument. The thinnest sections of the casting undoubtedly solidify first, and the skin of solid metal covering them at anytime in cooling is stronger than that covering the thicker parts, as the cooling of the latter is retarded by the larger volume of plastic or fluid metal. The external shrinkage of the latter in solidifying over this expanded mass is small, hence the cause of shrinkage cavities in these enlarged parts. It would appear, if Mr. Howe is correct, that if the founder waited before releasing the casting until the heavier parts were sufficiently firm to support themselves, that the thin and stronger parts, in which the maximum linear and superficial contraction takes place would already have fully contracted and torn themselves apart over the core. On the other hand, it is found possible, provided the design is not such as to brace the casting against its own contraction, to make sound and yet quite complicated castings.
The successful development of steel casting as applied to complicated designs of gun carriages and engine frames may be said to have begun in this country in 1887. About this time I was fortunate enough to gain considerable experience while attached to the Washington Gun Factory as Assistant Inspector of Ordnance in charge of the construction of 6 and 8-inch gun carriages.
This is the practice at present : an allowance for shrinkage, graded according to the section and location of the part, is made in the pattern, which is therefore considerably larger than the casting required. At the earliest possible moment after the metallic surface has solidified in the mould water is turned into the hollow core which is thus weakened or dissolved while the mould is loosened up and stripped, to permit the casting to freely contract. Now, the experiments of Mr. Howard at the Watertown Arsenal* have shown that at a temperature of about 1600° F., metal very similar to that of which steel castings are made has but 14 per cent, of its tensile strength when cold ; the elongation is also much less on account of the weakness of the metal ; there is also every reason to believe that the metal becomes rapidly weaker as it approaches the melting point. Now, if the metal is weaker, as we know, and contracts more, as Mr. Howe states, in the early part of the cooling, it must be apparent to all that the casting would tear itself apart before it would be possible to remove the core and mould which oppose its contraction. The fact is that were it not that the density of the casting when its skin is first solidified is less if it differs at allixom. that of the fluid metal, it would be impossible to make cored castings. The metal in the first setting would be unable to contract over the core without tearing apart and it would be impossible to strip off the mould until it had set. If the major contraction occurred in the change from ihefluid to the plastic state, shrinkage cavities in large ingots would not be tear-shaped, as the viscous metal would close the points up. In support of my opinion that the major part of the contraction occurs in the change from a plastic to a rigid condition and hence comparatively late rather than early in the cooling, I quote the following from an experiment made in cast iron by Mr. Thomas Wrightson.f Strange to say, Mr. Howe refers to this same experiment to prove that the enormous shrinkage occurs " in the early part of the cooling." I see in this experiment nothing but a confirmation of my expressed opinion thatthe major part of the contraction occurs in cooling from a temperature of 1832° F.
Four-inch cast iron balls were suspended upon a spring and submerged in a bath of liquid cast iron. By attaching a recording apparatus to the spring it was possible to form a curve which showed the change in volume of the balls as they heated. On the diagram the vertical ordinates show the positive and negative buoyancy of the ball due to its varying expansion, while the abscissas represent the times of submersion. The sharp oscillation x occurred in loading the spring. It will be seen that the cold iron had a negative buoyancy of 2 oz ; in 23 seconds it had expanded until it had the same density as the fluid metal ; in 2.5 minutes its expansion had produced an upward pressure or buoyancy of 10 oz.; in 4 minutes its buoyancy was at a maximum, 11 oz. At this point it was found that the ball was in a plastic state, as fine wires could be passed through it. At E the ball apparently began to dissolve, an action which had, it is supposed, commenced at D. Sir Lowthian Bell states with regard to similar trials that he had made with cubes of iron, " they sank to the bottom and remained there about 30 to 40 seconds, and then rose to the top, where they remained." Mr, Wrightson further states, " no doubt, from what had been observed as to the floating of steel in a bath of steel, the diagram of a steel ball would have similar, though probably not identical, characteristics." As a result of this experiment it was found that the specific gravity of fluid cast iron was 6.84 while that of the plastic metal was 6.32, a difference of 8.2 per cent. I am unwilling to go as far as Mr. Alfred E. Watkins, who states : " It is useful to know that all shrinkage takes place while the casting is changing from a red to a black heat." But it would appear that I have just grounds for believing that the maximum contraction occurs much later than Mr. Howe appears to intimate.
Page 24 and 25. Although but a matter of conjecture, I prefer my own formulas of cementation to those presented by Mr. Howe.
In the first place, his formula 2CO = C + C02 is hardly complete, and there seems doubt as to its accuracy at high temperatures. The combination here indicated is dependent upon the presence of a peculiarly receptive, perhaps nascent is the word, surface as in the case of iron sponge, or upon some other material as oxide of iron, which will serve as an intermediary, as shown in my formula in the reaction. This matter is touched upon in the first paragraph of page 24. Mr. Howe's formula is also far less simple than it looks. It is well known that CO passed over heated oxide of iron, or spongy iron, is split up into CO2 and deposited carbon. In the case of the spongy iron, however, some of the iron is always oxydized, indicating that the reaction is more complicated than appears from this formula. Also at a bright red heat the deposition of C is practically suppressed ; in fact, Sir Lowthian Bellf states that at a bright red heat it would appear that the oxygen absorbed by the iron reacts on the deposited carbon. Speaking, on page 187, of the splitting up of CO in this manner, he states : " In the description of this curious rearrangement of the elements I suppose that the reducing agent must be a lower oxide of iron, and that the action was probably of the nature expressed' in the formula.
It would hardly seem, therefore, that the phenomenon of cementation was "more clearly explained" by the first at least of Mr. Howe's formulas, since that formula of itself requires expansion and modification in explanation until it strongly resembles one of those it is intended to supplant. It is simple only in this respect, that it does not pretend to tell anything.
The objections to the second formula CO2 + C =2C0, representing an actual reaction, are based upon the conditions under which it must take place. It is a question as to the manner in which the cementation gases penetrate the heated metal, the pores of which in its cold state contained a condensed mixture of gases many times its own volume. A large part of these are of course expelled in bringing the metal up to the temperature of cementation, so that it would then appear that the entrance of the carbon gases is probably of a selective character, possibly through chemical reaction. It seems, however, more probable that they enter through diffusion or the establishment of convection currents. If the gases are forced in through the pores by pressure from the exterior, whether through heat expansion or as in Schneider's gas process, the action is one which Graham has called transpiration. He found that a volume of CO2 passed through capillary tubes under pressure in 83 per cent, of the time required by an equal volume of CO. One would be inclined to believe therefore that, after the reaction indicated by Mr. Howe's first formula, the resultant CO2 which is, of course, formed with the evolution of considerable heat, would pass on into the metal, instead of returning to the surface to pick up additional carbon. Especially is this the case as the entering CO2 is warmer and more expanded than the gas existing in the pores which it displaces. From this it would seem not altogether improbable that the oxygen in the carburizing mixture is gradually exhausted.
Page 29, sixth line from bottom. The criticism of the statement that the height 2ind duration of the hardening temperature affects the amount of carbon dissociated is just.
"Change of carbon from the ' hardening ' to the ' cement ' form requires not only a correct temperature, but a certain amount of time, while on the contrary, the change from the 'cement' to the 'hardening' form depends only upon the temperature and quenching seems consequently to prevent any change from 'hardening' to 'cement carbon.'"
This also covers the criticisms of the statement made on page 30, line 3, that the toughness of the metal after quenching increases with the length of time of exposure to the higher temperature, and that near the bottom of page 29, that the hardening tendency is proportional to the duration of the temperature of hardening. The errors arose from a confusion of heating for hardening with heating for tempering.
In Howe's Metallurgy, page 23, is found " Heating for tempering . . , the hardened steel must be reheated uniformly. If slowly heated the steel is said to become tougher than if rapidly heated, without corresponding loss of strength and hardness. "
Page 29, third paragraph. It is true that the stress of hardened bodies may be inferred from other evidence than their decreased density. The well known experiments of J. H. Howard, at the Watertown Arsenal, indicate clearly, however, that although a part of the increased volume may be due to a chemical change, still it is largely due to the exterior having become fixed in cooling over an expanded interior.
Professor Langley's experiment is said to have merely made the metal slightly lighter and that no measure of hardness was attempted. Mr. Howe appears with great nimbleness now on one side of this argument and then on the other. In the preceding paragraph he takes exception to a change of density indicating stress, as it may be explained by a change of chemical constitution ; he now denies the chemical change, though admitting a change in density. Hardness and elasticity are intimately associated, so that, whether the difficult task of measuring so slight a change in hardness was attempted or not, the steel in Professor Langley's experiment, being less dense, was in a state of stress and its surface actually, though probably very slightly hardened.
The statement that the unsatisfied affinities of the carbon for the iron probably places the whole body in a state of constraint is evidently incomplete. Chemical affinity is potential energy, it cannot exert stress without being transformed in the act of combination. In the following paragraph the matter is made clear, however, when it is stated that certain of these chemical affinities are appeased in time, as in the case of tools, or even through slight fluctuations of temperature, as in Professor Langley's experiment in hardening steel by quenching from a temperature of boiling water.
Mr. Howe's statement, "Speaking broadly, the density of steel is inversely as its tenacity," is a most remarkable one. No wonder that he does not dare make use of it to infer " that the porosity . . . should be directly as the tenacity." If that was true, then ingot metal would be stronger than forged steel. Such a statement, however, naturally follows the taking of a very narrow, rather than a broad, view of the idea criticized. Mr. Howe doubtless refers to the fact that untreated steel has less tenacity than when tempered or hardened ; also, that the density of untreated steel is greater than that of hardened steel, and perhaps that high carbon steels which are stronger than mild steels are also less dense. But here he falls into an error which has prevented him from grasping my meaning, and led to baseless criticisms in a number of instances. He makes no distinction whatever between the density of a body and that of the layers of which it is composed. His views appear to be gathered from the behavior of test specimens, mine from specimens forming a part of an armor plate. These layers, especially in a face hardened armor plate vary in density from surface to center, and this fact cannot be omitted from consideration in the discussion of the porosity of steel. The lighter density of hardened steel is easily explained. Barus and Strouhal state that, " in the case of steel (cylindrical rods) hardened by tempering, we observe a dense external shell surrounding an abnormally rare core in such a way that greatest intensity of stress is exerted in the radial direction ; i. e., at right angles to the axis of figure." In fact the less density of hardened steel is due almost, if not entirely to the enlarged figure arising from the exterior of the body becoming fixed in cooling over the heated and expanded interior.
The result is that the exterior, contrary to the accepted idea, is always in compression ; first, a radial and then, as the central body contracts, a circumferential compression. This, without any aid from chemical change would go far towards making the exterior more dense than the core.
The failure to recognize this fact may explain Mr. Howe's opinion that water quenching has little effect on the modulus of the surface of Harveyed armor plates.
In passing, however, I desire to call attention to his comment in comparing line 26, page 32, with lines 1 to 4, page 33. It is, "He asserts that water-quenching highly carburized metal greatly increases the modulus of elasticity. Such evidence as I have met agrees with his previous and opposing assertion (page 32, line 26), that water-quenching has little effect on the modulus."
The statement on page 32 referred to is; "It is known there is no difference whatever, under the elastic limit, between the extension, for equal stress in equal lengths, of soft and tempered steel." Mr. Howe has no right to say that te7npered steel is water-quenched. Perhaps, however, this is an instance of heterophasia, for on page 17 of his Metallurgy I find —"Definitions.—1. Steel is hardened (in the specific sense of the word) by sudden cooling from a high temperature, usually at or above redness, e. g., by plunging it in water, oil, etc.
"2. To temper (to qualify, to soften) in its specific sense means to mitigate, to partly remove, to moderate the effects of previous hardening. . . . I shall use the word exclusively in this sense, though it is often and not incorrectly employed generically to designate any sudden cooling, whether from an excessively high or a moderate temperature."
The statements in the essay do not require clarifying. On the one hand, steel, say of .35 per cent, carbon, is moderately hardened by dipping in oil, the density of successive layers from surface to heart are practically uniform, not greatly diminished, and the modulus is not perceptibly changed. In the other case, steel originally containing but .22 per cent, of carbon, has been cemented so that the carbon in the exterior layers is brought up to perhaps over 1 per cent. The plate is then chilled in the most rapid and violent manner possible by a spray of ice cold water containing salt. The interior of the plate is toughened, but its modulus is no more affected than in the case of oil tempering. It is different with the thin highly carburized exterior, the density of which is made greater than that of the interior by the intense compression over the latter when expanded, resulting from the treatment, a compression which exists from the beginning to the end of the quenching.
Now, to connect the density with the modulus. Dr. M. G. Wertheim in discussing the constancy or the variability of the coefficient of elasticity in the same substance under different circumstances, and the changes that mechanical treatment, annealing, and elevation of temperature can produce in it, states:
"In each condition the density of the metal was noted; then I determined its coefficient of elasticity and the rapidity of sound, by means of three different methods : by transverse vibrations, by longitudinal vibrations and by elongation. These then are the conclusions that may be drawn from these experiments:
1. The coefficient of elasticity is not constant for the same metal ; whatever augments the density increases it, and reciprocally. . ."
The experiments of Dr. Wertheim, which were of exquisite delicacy, were also carried on with alloys, in the case of which the same conclusion was reached.
Now, on the other hand, the numerous experiments of Mr. Howard and others at the Watertown Arsenal, in fact the results of almost every experimenter, seem to indicate that hardening does not increase the modulus. This is contrary to the conclusion to which I have been led, for the apparent contradiction may be explained by the change of modulus in the successive layers of different density in the hardened specimen. Thus the modulus of the rarefied interior would be less than in the case of the untreated specimen, while that of the dense external layer would be greater, the result being that, so far as the whole body was concerned, no appreciable change in the modulus occurs. In fact this would seem to agree with Dr. Wertheim's experiments on the modulus of alloys, his conclusion being, "The coefficient of elasticity of the alloys agrees sufficiently well with the mean of the coefficient of elasticity of the constituent metals, some alloys of zinc and copper being the only exceptions.
It thus appears that both Mr. Howard's and Dr. Wertheim's conclusions may be correct, and if they are, the modulus of the external layers of metal, the only ones which have been hardened to any extent, must have been increased, perhaps not greatly, as stated in the essay, but certainly to a considerable extent.
But we may get at this in another manner. Attention is called to the intimate relation borne to the resistance to compression, and the rigidity by the modulus of elasticity, and the considerable effect which must be produced on these coefficients by the rapid quenching accompanied by great pressure contemplated in the proposed armor plate. This modulus is expressed in terms of the compression coefficient K, and that of rigidity. Now Mr. Howe admits that the compressive strength of the hardened layer on the armor plate is greatly increased by quenching, and we know that its rigidity is also vastly greater than it was before hardening ; hence, unless the expression here given is incorrect, the modulus must have been considerably increased.
Page 39, lines 10 and 11. The statement is made with regard to manganese steel, " the author falls into the curious error of supposing that manganese steel forges particularly well, and 'welds with facility.'"
I am perfectly aware of the intractable nature of steel containing a high percentage of manganese. I am informed, however, that steel containing from 12 to 14 per cent, of manganese may be easily forged if brought up to heat very slowly, and that a temperature of 1900° F, be not exceeded.
My authority for the statement regarded as erroneous is the following paragraph from a paper of M. Brustlein, of the Holtzer Steel Works, France, which was read before the Iron and Steel Institute in October, 1886 : " Manganese steel works better hot under the hammer than chrome steel, but manganese steel works particularly well. Again, manganese steel welds with great facility, while chrome steel welds badly or not at all."
I also refer to Howe's Metallurgy of Steel, page 42, "So, too, the presence of manganese in solidified steel appears to hinder its oxygenation in heating and forging." Also on page 44 of the same work I find : " As sulphurous irons are malleable at a high, but brittle at a low (red) heat, while phosphoric irons are malleable at a red, but brittle at a high heat, and as manganese counteracts the effects of both, and as it moreover counteracts hot shortness, no matter at what temperature, and from what cause it may arise, whether from phosphorus, sulphur, copper, silicon, iron oxide, suspended silicate of iron or blowholes, we may ascribe its effects to its directly increasing the plasticity of the steel at all temperatures at and above redness, to its even increasing the range of temperature through which plasticity prevails, on the one hand raising the melting-point, on the other lowering the point at which plasticity gives way to rigidity."
Hadfield asserts that the most manganese that can appear in low manganese steel without rendering it wholly brittle and rotten is 21/2 per cent. ] certainly had no idea of approximating to that amount.
It must be apparent to all that neither M. Brustlein in making the original statement concerning manganese steel, nor I in quoting from his remarks, referred to the high per cent, manganese steel which, in fact, is claimed by Mushet, and admitted by Hadfield to be an iron alloy rather than a steel.
Page 34, seventh line from the bottom. Exception is taken to the statement "Both the tensile strength and ductility of the mildest steels are greatly increased by quenching." A similar statement, however, is found in the first report of the committee to the Council of the Institution of Mechanical Engineers, 1883, thus : "There are abundant illustrations of his (Chernoffs) theory to be found in the many writers on steel who have been consulted. Thus Hackney states that quenching mild steel improves its tenacity and ductility." Quenching refers in this case to quenching from above the critical point. Possibly the statement should be qualified, if used generally, as my experience has been of a special rather than a general character.
A few typical cases of improvement by quenching are submitted from a large number at hand. The tests are all longitudinal and taken from the middle thickness of face hardened armor plates ; those marked B are from the lower end of the plate referred to the ingot, while those marked M are from the upper end. The most marked improvement, as a rule, occurs in the former. It may be said that metal at the heart of the plate cannot be quenched, still the improvement is almost invariable in plates under six inches in thickness, becoming less and less frequent as the thickness increases, until in plates over twelve inches thick it is so rare as to lead to the belief that it is in these cases probably due to a variation in the quality of the metal. This fact of course disposes of Mr. Howe's remark that it is improbable that the more sudden and complete the chill, the greater the increase of toughness in the body and back of the plate, as I state on page 35, line 1, for it is evident that the bodies of the thick plates are less affected by the chill than those of the thin ones. It must be again apparent that the results obtained from the treatment of small test specimens give only a general guide as to the behavior of large masses when similarly treated.
In further support of the stated improvement produced by quenching, attention is called to the fact that the French armor makers of the Loire district no longer employ oil tempering on armor as they get better results from water quenching.
The charge that I confound ductility with toughness seems hardly just. The body and back of the plate are already ductile as it comes from the rolls; longitudinal specimens are made even more ductile by the removal of the longitudinal forging strains in heating. There is excellent authority in Mr. Hadfield and Dr. Ball for believing that the increase of tensile strength is probably due to the strains (transverse) produced in the metal by quenching.
Mr. Howe states with regard to this charge of confusing toughness with ductility: "To the same apparent error I ascribe his mistake . . . that, save at the very surface, the plate is decrementally toughened, not decrementally hardened. Here he appears to be completely in error."
It is difficult to conceive how the language used in the essay can be forced to convey any such meaning. The statement is very clear, and is : "So far as hardness pure and simple is an advantage to such armor it is usually confined to a comparatively thin and uniform layer, below which the metal exists in a decrementally toughened rather than hardened state."
A great range of hardness is certainly passed between the surface and heart of the plate. But below the surface layer the valuable characteristic is the toughness which gradually diminishes towards the back of the plate. The decremental hardness is in fact a disadvantage just as soon as it requires less work to crack and displace the metal than it would to cause it to yield by deformation if slightly softer.
THE PROPOSED ARMOR PLATE.
Mr. Howe has omitted the following from my contentions as to the effect of gashing the face of the plate:
1. It will limit cracks.
2. By increasing the degree of hardening for a given temperature on the corrugated or gashed surface, it will permit the use of a stronger foundation plate without rendering it brittle through excessive hardening.
3. That the various gashes or depressions constitute hooks and ridges of hardened metal possessing great transverse strength, and which through their depth produce the effect of an interlocked arrangement of girders.
4. That the penetration of the carbon may be controlled, being extended or limited according to the thickness and original composition of the plate, and that practically the same percentage of carbon may be carried into any desired depth.
5. That no unnecessary carbon is introduced, in order to secure a chilled surface of the desired thickness, by means of which excess of carbon the toughness of a considerable part of the plate is sacrificed through its being made brittle hard.
6. The chill or maximum hardness may be carried to any desired depth.
7. The bodies of thick plates are made more accessible to heat treatment, and that without increasing their carbon percentage.
As many of Mr. Howe's objections would, in all probability, never have been made if he had more clearly understood the arrangement and dimensions of these gashes a few explanatory sketches are added.
With regard to Mr. Howe's remarkable statement that by ridging the plate the "saving of time and fuel, while it might exist, would be inconsiderable;— so slight as to be wholly unimportant."
If the ridge part, as Mr. Howe admits, would absorb heat more quickly, it would commence taking carbon more quickly, which is the end in view, rather than that of heating the body of the plate. Any one who has ever built a fire and appreciates the use of kindling wood, or a crumpled newspaper rather than a tightly folded one, or still nearer to the point, projecting ridges or splinters from heavy wood, will recognize the fact that the ridges of metal, although possessing greater conductivity than the wood or paper, will attain the desired temperature far more quickly than the body of the plate. Now the surface immission, the conducibility per unit of area of surface as Fourier calls it, is the same over the greater part of the ridges as it is in the case of the flat plate. Doubtless it is somewhat less at the bottom of the gashes. We have, however, in one case, heat flowing into the top and sides of a ridge, while it can only flow out at the bottom. In the other case, heat flows in through a surface equal in area to the summation of the areas of the bottoms of all the ridges ; that is, the flat surface of the plate.
As the entrance of heat per unit of surface in each instance is equal, being dependent on the temperature of the source and the conducibility of the metal, a peculiar condition exists in the case of a projecting ridge of square section : it is receiving three titnes as much heat as would be the case if it was flush with the surface in a smooth-faced plate. Now, the rate of flow per unit of area from the ridge across its base into the body of the plate is proportional to the difference of temperatures. But that rate cannot be greater at the start than the rate of flow per unit of area into the ridge, for if it were, the base receiving more heat per unit of time than an equal area of surface would become hotter, and a flow would be established from the base towards the surface. It therefore becomes apparent that no more heat can pass out of the ridge through its base than is received through an equal area of its surface, in fact not as much; in other words the ridge must be getting hotter at a rapidly increasing rate. There are certain other conditions which emphasize this effect. The hotter the ridge becomes the poorer its conductivity ; there is less heat transmitted to the body of the plate for the same difference of temperatures. This decrease of conduction is enhanced by the increase of the hot metal's specific heat, it requires considerably more heat now to raise its temperature one degree ; besides the rate of admission falls as it rises in temperature. It should be noted that Mr. Howe seems to have regarded the conduction as proportional to the amoutitoi heat absorbed by the ridge rather than the difference of temperature and the rate due to the time. In no other way can we explain his statement, "that the heating of the body part might be hastened because the greater surface of the ridges would absorb heat more quickly, and because the high conductivity of the metal would quickly transfer into the plate any heat taken up by the ridges." This problem is far more complex than appears from my incomplete statement. The temperature of the surface increases at a decreasing rate; that of the base increases at an increasing- rate; ultimately the base will approximate the surface temperature; even then, however, no more heat can pass through it than enters an equal area of the surface, or it would become hotter than the source from which its heat is obtained.
Now, no part of a ridge can become hotter than its surface, for if it were, there would be a flow of heat towards the surface. It therefore follows that a point in the ridge will become hotter than one at an equal depth in a smooth plate in equal times, so that the rate of transmission of heat from that point into the body ot the plate will be greater, and this rate will increase as the ridges become more and more hot than the body. It does not follow from this, however, as Mr. Howe states : "The high thermal conductivity of the metal which would transfer into the plate with extreme rapidity the increased quantity of heat which its increased surface led it to absorb." This assertion is plainly an impossibility, and could only have arisen from a failure to comprehend the fundamental law of the transmission of heat; that is, that the rate is a function of the time, distance, and difference of temperatures.
This conclusion is independent of the rate at which heat reaches the plate, it is just as true of the direct blast of the furnace as it is of heat slowly filtering through an asbestos protection or carried by convection currents in the atmosphere.
I will regard it as demonstrated that carburization will begin much earlier in the ridged than in the smooth plate.
Mr. Howe is correct in his belief that the excess of carbon taken up by a gashed plate would be found in the ridges alone. That is where it is wanted. One of the principal defects of the present process is that in order to get a fair percentage of carbon on the surface, it is necessary to run the carbon in deeper than desired, deeper than will be affected by the true chill, and even if it were all retained in the hardening condition by quenching, toughness would have been sacrificed over a considerable thickness of the plate ; the result obtained being something more nearly akin to a weldless compound than the theoretical face hardened plate described in the first paragraph on page 64. I am aware that certain foreign armor makers have boasted of being able to Harvey a lo-inch plate to a depth of 2.5 inches. If they did, the plate as a whole was brittle and less resisting than it would have been if Harveyed to a depth of 1.25 inches ; they merely lost the toughness that should be found, in metal with an elastic limit well below its failing point, over a thickness of 1.25 inches.
This matter is not discussed at great length in the essay, but it is in fact one of the most important reasons in favor of the adoption of the process advocated. A reference to Lieutenant Sione's remarks and diagram illustrating this subject is of advantage at this point. Without doubt the metal is much strengthened to the inner line of carbon penetration ; but its elastic limit is near its failing point ; but little work can be done upon it; it cannot absorb much of the projectile s energy. It lacks therefore the prime qualification of toughness. It is proposed by isolated gashes to carry this hardness with its great compressive and transverse strength into at least an equal depth over lines forming a net-work of girders, and yet separating and partially underlying them will be a matrix of metal untouched by the carbon, and retaining all of its proper toughness. This arrangement will permit an enormous amount of work to be done on the face under impact, without destroying the plate.
Mr. Howe compares the gashed surface with a crimped strap. As the sides and edges of the gashes are the first to chill and set, they are quite different from extensible crimps ; in fact they may be regarded as local increases in thickness and rigidity ; further on, however, he states: "We may admit that corrugations might increase the transverse strength." If the skin is in tension over the body, it compresses it, and is compressed in turn, action and reaction being equal and opposite ; this of course assists the action of the cooling medium. Again, the ridges having a much greater area of emission than the part of the body they cover, will cool more rapidly. Not only that, but the rate of emission is dependent upon the size of the body, and may be represented by a constant plus a term inversely proportional to the radius, for example, of a cylinder The heat being emitted more rapidly from unit area of the ridges, therefore, than from unit area of the flat surface, the instant the spray strikes the plate, each gash is boxed in by hardened steel of great strength—very different from a crimped strap—so that the combined cooling and compressing effect is felt on the body of the plate much earlier and more powerfully than when it is flat and smooth.
Let us regard the effect of gashing on the loss of heat by the body of the plate in quenching. Heat flows directly from the bottom of the gashes which are, according to their depth, that much nearer the heart of the plate. Also, by the far more rapid chilling of the ridges than an equal depth of metal in the case of the flat plate, the temperature of the plane separating the ridge part from the body of the plate will at any time after the commencement of spraying be considerably lower in the case of the ridged than in the case of the flat plate. Heat will therefore be more rapidly extracted from the body in the former case than in the latter. This will make the hearts of thick plates more susceptible to heat treatment, and something of the improvement noted in the case of thin plates may be expected.
It will thus be noted that Mr. Howe's contention that the interposition of the ridges between the body and the water greatly lessens the tensile, compressive and transverse strength of the body part is incorrect. It is also apparent that thick plates can be affected by quenching to a greater depth ; or, if desired, the same results may be obtained by quenching at a lower temperature, thus decreasing distortion.
I am not aware that I have confined my attention too much to hardness proper, the object of the hard face and the necessity of transverse strength is noted at the top of page 56. Emphasis was laid upon the remarkable compressive strength of the chilled surface in discussing the modulus on page 33. But it is endeavored to limit the hardness with its accompanying brittleness to just that which is necessary to prevent a forward movement of any portion of the surface to accommodate the metal displaced by the penetrating projectile. More hardness or rigidity than that is not wanted, there is no use for it ; the metal becomes brittle ; a crack started in the surface will run through it, for its elastic limit is close up to its failing point, so that despite its high tensile strength, it can absorb but little work. It is not wished to obtain a metal with less tensile strength, but one with some elongation, and this is possible, for it is known that the tenacity of the metal is not directly connected with either its hardness or brittleness. This may be said to be the text on which this essay was written. It seems odd that Mr. Howe should think I lay too great stress on hardness, when I believe myself to be the pioneer in advocating its limitation and control in face hardened armor.
Mr. Howe's resolution of the resistance of a plate to penetration is open to objection, especially with regard to the "radial resistance which the face offers to being driven parallel with itself, radially away from the point of impact, as the ogive progressively forces its way farther and farther into the plate." It is not the amount of resistance that the plate can offer which tends to shear off its surface or widen the impact, but the force that can be exerted by the projectile. Resistance may be regarded as inversely as the deformation or work done upon the plate ; in hard, brittle plates, however, this is not the case, as comparatively little energy is expended in breaking them up. Mr. Howe recognizes this fact in ascribing alack of tenacity under impact to quartz and chilled iron. Now, it must be apparent to every one who will press the point of a pencil into a rubber pad that until penetration is effected the surface is brought into tension by the force exerted, and the resistance of the body of the pad tends to prevent the flow of the surface towards the pencil point instead of away from it. When some penetration is effected the tension on the surface is no longer direct, being then due to its connection with a lower layer still before the point. The penetrated surface may then be placed in compression immediately around the point and then, in the case of rubber or soft metal, it will be curled up into a fringe. It is different in the case of a face hardened plate ; its elastic compression and extension is greater than m the case of softer steel, but it cannot get out of the way by flowing and curling up into a fringe. In consequence it resists obstinately and directly to the force acting upon it instead of evading it by flowing radially. Now, the force acting upon the cone of depression is, as has been stated above, one of tension, generally, and compression directly where the face of the ogive is in contact with it. It must therefore be seen that as the ogive penetrates the plate and comes into contact with successive zones' of the cone of depression, a compressive force is brought upon these zones acting in a line normal to the ogive at the point of contact. The hard face is therefore crushed into the body and carried forward, enveloping the head of the projectile. A radial shearing component parallel to the face undoubtedly exists, but it is comparatively unimportant until the ogive is well entered, and the hard face has practically completed its direct work of resistance. At this later period of penetration, the resistance to the shearing of the hard face is increased, not diminished by the gashes, for these form hooks penetrating the tough body, and by their similarity to a net work of girders bound in a tough matrix can neither be sheared nor crumpled up. Lieutenant Chambers in the discussion shows considerable appreciation of this feature. There are, however, in existence claims to the right to use this method as a means of introducing and regulating the percentage not only of carbon, but of chromium and other elements in the various processes of cementation.
Mr. Howe admits that the corrugations would possess greater transverse and compressive strength than the flat plates, yet states " that a 10-inch plate with half-inch gashes would be much weaker than a 9.5 inch ungashed plate." His error lies in failing to see that the plate resists as a whole ; the resistance of the hard face and that of the tough body and back cannot be separately considered ; they are inextricably involved with each other.
As to the cost, the required depressions, ridges, or corrugations, may be put in the plate by a single passage through the rolls. They must be introduced in tapered plates under the press or hammer ; in either case the last forging heat may be utilized for the purpose. The Carnegie Steel Company has recently shown that by carburizing a plate 17 inches thick, then forging it down to 14 inches and quenching, its resistance was greatly increased. It is evident that if such a plate was deeply corrugated or gashed at 17 inches, the depth and percentage of carbon could be regulated and the ridges flattened out in the subsequent forging; moreover, this could be done without the long exposure to a high temperature in the cementation furnace which returns the steel to a condition somewhat akin to that of cast metal. There is an expense connected with the additional heat, but it is saved many times over in the shorter time required for cementation. It would be necessary to make cast iron dies for gashing the plate, but this would be done before bending, and with a single flat die, adapted to all varieties of plates. This expense would be trifling.
Pluck and perseverance are always appreciated, and it is hoped the plate SO highly spoken of by Lieutenant Chambers may become as great a success as it deserves to be. One cannot help recalling, however, the way in which the French veteran conjured hot water into nutritious soup. It is true he borrowed a few vegetables and a bone or two from his admiring audience, but then the soup was a wonderful success. It is feared, now that it is found necessary to forge the Chase-Gantt plate down just as much as any other plate, that its great recorhmendation of cheapness has been lost. Later it is thought that the other ingredients which go so far to make up the cost and uncertainty of manufacture of modern armor will all have to be incorporated. As has already been stated, chromium may be introduced by means of pockets or gashes to any desired depth in a process akin to cementation.
I do not understand the word absorption which Lieutenant Chambers has emphasized. Doubtless it may be used either as a generic or specific term. In the latter case, as indicating an inhalation or imbibition without combination, it does not convey a true account of the action which takes place, for it is claimed that the chromium and carbon at the face of the plate are not mere intruders, but actually blended, fused, incorporated, and combined with the cast metal, thus forming a high carbon chrome steel. The generic term is in fact used to describe a specific action which may be more exactly described as that of cementation. The underlying objection to the process is its uncertainty; it also has the defect peculiar to the Harvey process, that is, that in order to get a desired percentage of carbon at the surface it is necessary to run it deeper into the plate than is desired.
The information and corrections which Mr. Wm. Allen Smith, the Secretary of the Harvey Steel Company, has added to the paper are acknowledged with pleasure ; it is to be regretted that he has been unable to discuss the process at length.
Thanks are also again due to the gentlemen who have had the energy to express their convictions either for or against the proposed armor plate.