[Illustration: FIG. 48.--Slowly cooled high-carbon steel, polished, etched and viewed at 100 magnifications. The dark grains are pearlite, separated by white films of iron carbide (cementite). Photograph by H. S. Rawdon.] In general it may be said that the previous crystalline structure of a steel is entirely obliterated when it passes just through the critical range. At that moment, in fact, the ferrite, cementite or pearlite which previously existed has lost its identity by everything going into the solid solution called austenite. If sufficient time is given, the chemical elements comprising a good steel distribute themselves uniformly through the mass. If the steel be then cooled, the austenite breaks up into new crystals of ferrite, cementite and pearlite; and in general if the temperature has not gone far above the critical, and cooling is not excessively slow, a very fine texture will result. This is called "refining" the grain; or in shop parlance "closing" the grain. However, if the heating has gone above the critical very far, the austenite crystals start to grow; a very short time at an extreme temperature will cause a large grain growth. Subsequent cooling gives a coarse texture, or an arrangement of ferrite, cementite and pearlite grains which is greatly coarsened, reflecting the condition of the austenite crystals from which they were born. 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.