Effects of Strain Hardening and Fine Structure on Strength and Toughness of Tempered Martensite in Carbon Steels

A series of medium-carbon low alloy steels was quenched to martensite and tempered at 150 C. This low-temperature-tempered (LTT) martensite was subjected to uniaxial tensile and impact testing. Yield and ultimate tensile strength and uniform elongation increased, and post uniform and total elongation and fracture toughness decreased, with increasing carbon concentration. The carbon dependence of the mechanical properties is attributed to the dislocation/transition carbide substructure of the L l T martensite. As carbon content increases, the density of the transition carbide arrays increases, and the rate of strain hardening increases, resulting in the noted property changes. The strain hardening mechanisms and questions remaining about the evolution and characterization of the transition carbide structures are discussed.