The Structures of Quenched and Unquenched Steel
We saw that the lag or tardiness is greater the more rapid the cooling. Along with this very great lag which is brought about by very rapid cooling comes increasing slowness, i.e., less ability to catch up, as the temperature is lowered. Hence quenching produces such a wide lag and so slows the changes which should take place that they do not take place at all, i.e., the structure which the piece had at the higher temperatures cannot change but is set or fastened by the quickness of the cooling.
Though no degree of suddenness is sufficient to set completely the structure existing at very high temperatures, for our present purposes we can say that by quenching in cold water we can freeze or fix any structure. Then after we have quenched a piece of steel, it will have when cold, the structure which corresponded with or resulted from the temperature which it had at the moment before the quenching.
If so, the microscope should give us aid.
Illumination of the Sample under the Microscope
By breaking off pieces of a quenched piece and very carefully and slowly grinding and polishing without heating a surface which was an interior part we find after etching that we can actually see the kind of structure which corresponded with the temperature from which the piece was quenched.
Photomicrograph No. 80 shows the appearance of a piece of hardened carbon steel. Note the needle-like structure under the microscope at magnification of 400 diameters. This structure is characteristic.
The constituent, having this needle-like appearance has been named “martensite” in memory of a distinguished European metallurgist, A. Martens. It is supposed to be “beta” iron, much the hardest allotropic variety of iron, and to hold in solution the carbon of the alloy, either as carbon alone or as the extremely hard chemical compound, iron carbide, Fe3C.
Martensite, then, is the extremely hard structure, necessarily containing considerable carbon or iron carbide in solution which gives to our carbon tool steels their hardness and great usefulness.
Unhardened steels never look like this. Their appearance is shown in photomicrographs Nos. 3b, 5, 22 and 24a.
No. 80. Martensite, the Constituent of Hardened Steel
(Magnification 400 Diameters)
In unhardened steels having less than .90% of carbon we find two constituents.
“Ferrite” is the name which has been given to one, the soft and ductile constituent, pure iron. With ordinary etching the ferrite usually shows as light-colored or white grains bounded by black lines, which, if the patch is large enough, give a fish-net appearance. It is soft and ductile like copper, for pure iron and pure copper are not so greatly different in malleability and ductility as one might suppose.
The darker and more or less triangular patches at the corners of the ferrite grains are “pearlite,” a name originating because of their “pearly” appearance under the microscope. How this pearly appearance comes about will be readily understood from photomicrograph No. 23e which was taken at a magnification of 400 diameters. It is seen that it results from alternate black and white layers.
Again we must give up the idea of any finality in the things we learn or think we have learned. We just learned, for instance, that ferrite usually was light or white in color. Well, in pearlite, as shown in photomicrograph No. 23e, every other plate is of ferrite but they are not the white but the black ones.
No. 23e. Pearlite at Magnification of 400 Diameters
You may not have understood before that color as shown under the metallographic microscope depends not so much upon actual color of the material itself as upon its ability to reflect light. For metallographic observations it is necessary to have very strong illumination. Usually the powerful beam from an electric arc is concentrated by means of condensing lenses upon a thin disc of glass called an oblique reflector which directs the beam upon the polished and etched specimen beneath the objective of the microscope. Often a prism is used. The rays of light returning from this highly illuminated “field” under observation return up through the tube and eye piece of the microscope and can be focused upon a small screen convenient for observation or upon the ground glass of the attached camera by means of which the pictures are taken. Unless the surface of the specimen being examined is perfectly plain and level, not all of the vertical rays thrown down upon it will be reflected back up through the tube and eye piece. Those portions of the field which are absolutely at right angles to the vertical rays appear at the eye piece or upon the screen as white or light-colored portions, while those which, during the polishing or etching have been dug or eaten away reflect the light imperfectly or in directions other than up the tube of the microscope, wherefore such portions show as darker or black sections.
The pearlite, then, is made up of little plates of soft ferrite alternating with others of a very much harder constituent. The harder plates are much less affected during polishing and etching than are those of the softer ferrite, hence they stand out in relief and reflect abundant rays of light, whereas the “dug-out” ferrite plates reflect the light imperfectly or not at all and therefore appear as dark lines.
These white, hard plates of the pearlite contain all of the carbon of the low carbon alloys. They are this other constituent, “cementite,” so named because it was first discovered in steel made by the “cementation” process. It is a very hard and brittle substance, hard enough to scratch glass. It is the chemical compound (Fe3C), unvarying in composition as chemical compounds always are. It consists of just three atoms of iron (93.4% by weight) and one of carbon (6.6%).
Pearlite, therefore, is a sort of mechanical mixture of two separate constituents, ferrite or pure iron, and this chemical compound, carbide of iron, which is called cementite. Pearlite is common to all unhardened steels whether of low, medium or high carbon content and may be considered characteristic.
That we may understand clearly the structures of the annealed steels, let us start with pure iron and gradually change it into higher and higher carbon steels by gradual addition of carbon. Pages [328] and [329] show such a series.
Photomicrograph No. 99b is open-hearth iron which is entirely made up of free ferrite. In No. 3b there is considerable pearlite, here appearing black, though the sample of steel yet contains but .10% of carbon. In No. 5, which is of a steel containing .30% of carbon, we have more pearlite and in No. 22c with .50% carbon we have yet more. Manifestly at this rate the comparative pearlite areas are growing so that there will soon be room for no ferrite at all. In No. 23g this has occurred. This, the photomicrograph of a steel containing .86% of carbon is one of the steels in which we found that the point of recalescence, loss of magnetism, decrease in electrical conductivity and rate of expansion take place all at the one point.
No. 99b. Carbonless Iron
No. 3b. Steel with .1 Per Cent Carbon
No. 5. Steel with .3 Per Cent Carbon
No. 22c. Steel with .5 Per Cent Carbon
(Magnification 60 Diameters.)
Now as we go still farther on up in percentage of carbon content, i.e. (beyond .86%, we have a white constituent beginning to appear as cell walls around the grains of the pearlite and this increases with increase of carbon until, with alloys having carbon around 3%, we have a proportionately small amount of pearlite while the white areas have so increased that it appears that the more or less round patches of pearlite float in a lake of white. This white which appears first as cell walls, and later in greater and greater quantity is free cementite.)
No. 23g. Steel with .9 Per Cent Carbon
No. 24a. Steel with 1.25 Per Cent Carbon
No. 36b. Steel with 2 Per Cent Carbon
No. 109. White Cast Iron with 3 Per Cent Carbon
(Magnification 60 Diameters.)
Such are illustrated in photomicrographs Nos. 24a, 36b and 109 which contain 1.25%, 1.98% and 3.00% of carbon respectively. While steels with the typical white, free ferrite areas are so soft that a needle-point will plow furrows across them, those with over 1.25% of carbon have such excess of free cementite that they are very hard to scratch and too brittle to use except for special purposes.
Austenite (White) and Martensite (Dark) Magnified 1,000 Times Their Actual Size
So during ordinary cooling from the molten alloy or the slower cooling of the steel during the annealing process, the martensitic structure breaks down at the recalescent temperature into pearlite and ferrite (soft iron) if the carbon content of the steel is lower than about .90%, or pearlite and the other and very hard constituent, “cementite,” if the steel has more than .90% of carbon. If the carbon content happens to be just .90%, or thereabouts, there is exactly sufficient pearlite to make up the total area of the field shown under the microscope.
Another constituent which is of great interest scientifically, though not at all commercially, is “austenite.” By quenching very high carbon steels from a very high temperature very suddenly and completely, we can fasten the “austenite” structure, which exists only at temperatures higher than martensite, i.e., austenite is our gamma iron with the carbon of the alloy in solid solution, perhaps as iron carbide, while martensite is thought to be the beta iron solid solution, perhaps with some gamma iron mixed with it.
While ordinary quenching fastens structures pretty well, it is not usually quick enough to prevent the austenite from sliding along down into martensite. However, carbon discourages such slipping, so, with high carbon to act as a brake, we can fasten some of it by chilling very suddenly and completely from a very high temperature. Steels with 1.5% of carbon and temperatures of 2000° F., or over, are usually necessary to accomplish it.
However, austenite, after we get it, is not as hard as martensite and we have little use for it commercially. As was stated before, martensite is the useful and proper structure for carbon steel tools.
No. 73. Annealed Steel has Fine Grain
(Magnification 70 Diameters)