Steels containing above .25% of carbon (approximately) will become hardened if heated to or above the absorption point and suddenly cooled, in water or by other means. The degree of hardness depends upon (1) the amount of combined carbon, (2) the rapidity of cooling, and (3) upon the range of temperature through which cooling takes place below the absorption point. The rate of cooling of a piece of metal is dependent upon its mass, as well as upon its degree of temperature above that of the cooling medium.

These statements do not apply to steels alloyed with manganese and tungsten. They harden by being heated nearly to melting heat and being cooled fairly rapidly in a blast of air. In this condition they do not lose their hardness when raised to a red heat, and can be used at a red heat for cutting.