Metallography.—This science has to do with the art of metals and metal-working. By means of it the structure (anatomy) of metals is bared to the microscopist, and the information thus obtained is to a large extent the responsible factor in the rapid strides which are being made in the manufacture of materials of construction.
Photomicrography.—This is the essential means whereby a photographic-microscopic record is made of the structure of metals under inspection or investigation; a number of photomicrographs accompany this paper.
Testing Metals.—For many years the quality of metals was largely determined by physical testing and chemical analysis. Standards of tensile strength, elastic limit, reduction of area, etc., were created; and, limiting amounts of phosphorus, sulphur, and so on, specified. These tests, by themselves, were not sufficient to gage the actual quality of the material for it frequently happened that important structural members, pronounced satisfactory after testing, were found to fail in service. It was here that metallography stepped in as an aid to the inspection and testing of metals. The ideal structure of a metal, as shown by the microscope, is fine-grained and homogeneous. (See Fig. 4.)
How it is Used.—Until very recently, the microscopic investigation of steel was made on small pieces taken from various parts of the member. These were carefully polished, using a grinder or file to prepare the surface, which was afterwards rubbed down with emery of increasing fineness and finally polished with tripoli powder and jewelers' rouge. The piece was then etched in a 5 or 10 per cent solution of nitric or picric acid in alcohol. When such a piece is placed under the microscope the polygonal net work shown on Fig. 1 is revealed.
It does not always happen that small pieces may be taken for examination, and the latest practice in such cases is to prepare a number of places on the member being inspected and to mount the microscope over these areas. As a rule, however, the pieces cut for physical testing will furnish ample material for microscopic work. This does not mean that the finished product should be neglected even if samples taken therefrom should indicate a satisfactory condition. All important metal members such as crank-shafts and the like should be examined critically before acceptance.
The structure of steel will be different depending on the carbon content, heat treatment, and content of special alloys. As an example of the first, Fig. 1, containing about 0.30 per cent of carbon may be compared with Fig. 2, containing about 0.35 per cent.
In the revealed structure, or anatomy, of a metal, Fig. 2 is an example of the value of metallography in that a decidedly faulty structure is shown to exist. A great streak, or ghost. passes through the center of the mass. This weakens the material considerably as will be understood when it is known that the tensile strength of the constituent forming the white streak is probably not more than 45,000 pounds as compared with 65,000 or 70,000 pounds for the outlying regions. A chemical analysis of this material would probably show a normal condition. If the piece were to be tested for tensile strength and the pull be in the direction of the streak, it is quite likely that no appreciable difference would result because of its presence. Were the test specimen to be cut so that the pull would come in the contrary direction, the piece would fail.
Steel if improperly heat-treated will be injured and rendered unfit for the use intended. For this reason every inspector of steel for the navy should carefully study the effect of heat treatment in order that he may be qualified to determine proper methods as compared with improper ones. Once a question of experience and shop practice, heat treatment has now become a more exact science. The period of guesswork, or glance of eye, is past, and precision heat-measuring instruments used instead.
An example of improper heat treatment is shown in Fig. 3. In the figure continuous streaks are shown.
In Fig. 3 it is seen that certain zones are rich in iron (ferrite), and impoverished in carbon, while other zones are rich in carbon and impoverished in iron. A simultaneous outflow of the richer constituent to the impoverished one would create a better balance, remove the weaknesses indicated by the segregated ferrite, and produce strong and durable metal. It does not follow that this metal is injured beyond repair, for by following precision methods a further and correct heat treatment may be given to restore the quality of the structure.
Such a restoration is shown in Fig. 4. Here the metal has been retreated, and a better balance effected. The objectionable streaks have been eliminated and the whole field presents a finely developed structure indicative of durable and satisfactory metal.
In addition to showing the effect of improper heat treatment, and of the proper effect when the steel has been rightly handled, the microscope in the hands of the metallographist will reveal inclusions of slag, manganese and iron sulphides, or other impurities. These inclusions are shown in Figs. 5 and 6.
The first shows a low carbon steel, such as would be used for boiler drums, etc., and the second a nickel steel forging such as is used for crank-shafts. These figures emphasize the importance of the microscope for it is quite likely that the test specimens from which they were taken cut from certain steel members, during manufacture, may have been free from these impurities.
It may show why supposedly sound forging of engine crank-shafts fail in service even though these shafts were carefully inspected; and may be of the utmost value in determining the rejection of such material when submitted for acceptance.
A further instance of the value of the microscope is shown in Fig. 7. The photomicrograph was taken from a section of boiler tube. Every engineer has to expand a boiler tube now and then and the purpose of introducing this figure is to show what takes place during the cold working (expanding) of the tube. The part next the expanding tool is crushed back, distorting the crystal formation until the normal condition of the metal is impaired. This crushing back of the crystals cannot be avoided, and no harm is done so long as the outer boundary is not injured. If the tube be rolled until the whole thickness of metal is injured, it is quite likely that the tube end will eventually split or at least reach that point where further expanding will not avail to prevent a leaky joint.
Heat Treatment.—It is not intended to take up the question of heat treatment at length in this paper, but merely to give a short introduction to a very important subject. With modern appliances such as may be obtained at this time, the practice of putting a tool in the fire, pulling it out and squinting at it until the right temper color has been obtained, then plunging it into water, or oil, may answer very well for a few cold chisels or the like, but not for high-speed cutting tools. Nor can the "expert" handle tempering or annealing by eye any longer.
Handy and properly made furnaces with recording pyrometers: absolute knowledge of the proper temperatures to use; cooling periods to produce the result desired; are at this time the implements that have driven the "expert" out of business.
A simple diagram will serve to show that for the heat treatment of steel, a temperature absolutely suited to one grade of metal is entirely unsuited to another grade. This can be seen in the following diagram (Fig. 8), where ordinates are temperatures in degrees centigrade and abscissæ are varying carbon contents. The lines within the axes indicate co-ordinate temperatures and carbon contents at which a marked change takes place in the structure of the metal, as well in a descending temperature as in a rising temperature.
If it be shown that a steel of 0.50 carbon when heated up to 750° C. has resolved into a solid solution of Austenite, heating a steel having 0.30 carbon to the same temperature will, according to the same diagram, have just entered the Beta zone and will not wholly resolve into Austenite until a temperature of 800° C. or above, is reached. And a steel having 0.85 carbon will resolve into Austenite at 650° C. It is clearly seen that the lower the carbon content the higher must be the temperature to produce the same effect; it is evident that the eye expert cannot determine this.
Every steel should be investigated to determine its thermocritical points, i. e., the points where the constituents pass from region to region. These may be determined in a small handy furnace, with pyrometer fitted, and a time-temperature curve established as shown below; the curve is not exact, being intended merely as an illustration. The horizontal parts of the curve occur at temperatures coincident with the changes of structure shown in Fig. 8 and are due to a phenomenon allied to that of latent heat in the change from steam to water, water to ice, etc.
The steel worker is now in possession of facts which enable him to handle his heat factors intelligently and produce the results desired. Microscopic examinations keep him informed and photomicrographs make a perfect record of his finished product.
It is hoped that this brief account will serve to show the means whereby our ships are now being made structurally safer, our engines more dependable, and our guns perhaps of longer life.
There are a number of excellent books dealing with the above subject, e. g.,
Howe on Iron and Steel,
Metallography of Iron and Steel (Sauveur),
Composition and Heat Treatment of Iron and Steel (Lake), any one of which would be a desirable addition to the engineer's book shelf.
The photomicrographs here shown were made at the Engineering Experiment Station, Annapolis, Maryland, from metals sent from various ships in the navy for microscopic examination.