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.