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. [Illustration: No. 73. ANNEALED STEEL HAS FINE GRAIN (_Magnification 70 Diameters_) ] Tempering or Drawing “Tempering” is done to relieve the intense brittleness of steel after quenching to martensite. While we dislike to sacrifice any of the hardness, it pays to temper or “toughen” the steel, as the toolmaker calls it, by reheating it to somewhere between 400° and 570° F. The higher the temperature, the freer and quicker is the change from one structure to another, as, for instance, the austenite to martensite. At the low drawing temperatures the changes from martensite to the pearlitic structure may be said to just creep along. A second quenching then fastens it at the new structure which gives a trifle less hard but a tougher steel. As you would guess, the microscope shows on these what we may term a “transition” or “breaking-down” appearance and structures not at all definite. These, of course, give to the steels the various degrees of hardness and brittleness and other qualities which are so desirable from the practical standpoint. The production of these fine shades of temper by the practical tool maker or blacksmith may almost be considered a fine art. How and Why Do the Steels Harden? Now, from all of these facts, what, shall we say, is the cause of the hardening of steel?