It maybe noted in passing that the coarse crystals of cast metal cannot generally be refined by heat treatment unless some forging or rolling has been done in the meantime. Heat treatment alone does not seem to be able to break up the crystals of an ingot structure.
HARDENING
Steel is hardened by quenching from above the upper critical. Apparently the quick cooling prevents the normal change back to definite and sizeable crystals of ferrite and cementite. Hardness is associated with this suppressed change. If the change is allowed to continue by a moderate reheating, like a tempering, the hardness decreases.
If a piece of steel could be cooled instantly, doubtless austenite could be preserved and examined. In the ordinary practice of hardening steels, the quenching is not so drastic, and the transformation of austenite back to ferrite and cementite is more or less completely effected, giving rise to certain transitory forms which are known as "martensite," "troostite," "sorbite," and finally, pearlite.
Austenite has been defined as a solid solution of cementite (Fe3C) in gamma iron. It is stable at various temperatures dependent upon its carbon content, which may be any amount up to the saturated solution containing 1.7 per cent. Austenite is not nearly as hard as martensite, owing to its content of the soft gamma iron. Fig. 49 shows austenite to possess the typical appearance of any pure, crystallized substance.
In the most quickly quenched high carbon steels, austenite commonly forms the ground mass which is interspersed with martensite, a large field of which is illustrated in Fig. 50. Martensite is usually considered to be a solid solution of cementite in beta iron. It represents an unstable condition in which the metal is caught during rapid cooling. It is very hard, and is the chief constituent of hardened high-carbon steels, and of medium-carbon nickel-steel and manganese-steel.
Troostite is of doubtful composition, but possibly is an unstable mixture of untransformed martensite with sorbite. It contains more or less untransformed material, as it is too hard to be composed entirely of the soft alpha modification, and it can also be tempered more or less without changing in appearance. Its normal appearance as rounded grains is given in Fig. 51; larger patches show practically no relief in their structure, and a photograph merely shows a dark, structureless area.
FIG. 49.—Coarse-grained martensite, polished and etched with nitric acid and magnified 50 times. Made by Prof. Chas. Y. Clayton.
Sorbite is believed to be an early stage in the formation of pearlite, when the iron and iron carbide originally constituting the solid solution (austenite) have had an opportunity to separate from each other, and the iron has entirely passed into the alpha modification, but the particles are yet too small to be distinguishable under the microscope. It also, possibly, contains some incompletely transformed matter. Sorbite is softer and tougher than troostite, and is habitually associated with pearlite. Its components are tending to coagulate into pearlite, and will do so in a fairly short time at temperatures near the lower critical, which heat will furnish the necessary molecular freedom. The normal appearance, however, is the cloudy mass shown in Fig. 52.