Engineering Concepts: Strength of Tempered Martensite

One of the remarkable aspects of steel is that a wide range of strength properties can be obtained by quench hardening and subsequent tempering. The yield strength of AISI 8620 steel can be made equivalent to that of AISI 1045 steel by choosing the appropriate tempering temperatures for each. To fully explain this behavior, a short discussion of crystal dislocation mechanics is required.

Yielding is essentially a result of dislocation motion and in soft metals the dislocation can move long distances without interruption. The yield strength is dependent upon a combination of solid solution (or solute) strengthening and grain size. For single-phase alloys, such as brass and austenitic stainless steel, a decrease in the grain diameter will produce an increase in the yield strength. Thus, decreasing the mean free path of the dislocation produces strengthening.