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)
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?
The explanation most generally accepted seems to be, that, of the three allotropic forms of iron the gamma and beta varieties are very much harder than alpha iron, which is the one which we have in annealed steel at ordinary temperatures, and, of the two, the beta is harder than the gamma variety. It is thought also, that carbon, perhaps as carbide of iron, is held in solution in gamma or beta iron after the quenching, and increases the hardness proportionately with increase of the carbon content of the steel. While this carbon or iron-carbide solid solution may and probably does itself confer additional hardness to the steel, its main function is to retard or slow down the change from gamma into beta and alpha iron, which change in carbonless iron and low carbon steels is so insistent and extremely rapid that not even the most severe quenching, as in ice-water or liquid air, can prevent or stop it. Not only do we fail to get austenite, which is gamma iron, until we get 1% or more of carbon present and quench from very high to very low temperatures, but we cannot even stop the transition at beta iron, the next lower allotropic variety until we have at least .30% of carbon, and, for serviceable hardening fully .60% of carbon in the steel.
Fortunately for us, this beta form of iron is the one we want, for it is harder and more useful than the gamma form.