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.