Field-ion microscopy of tempered martensite

Abstract

Hydrogen-ion microscopy has been used to study microstructural features of tempered martensite in a commercial FeO8% C-0·4% Mn steel, and a comparison between microstructure and measured microhardness has been made. At peak hardness in the early first stage of tempering, a high density of carbides with {100} habits and primarily in the form of very thin platelets is present. Many of these particles are coherent and some are associated with dislocations. As the hardness decreases, the density of particles decreases and their length and width increase rapidly. Their thickness, however, increases very slowly and coherency is maintained for a large number of particles throughout the first stage of tempering. In the late first stage of tempering large incoherent carbide platelets with {112} and {110} habits begin to form; and in the third stage of tempering where the hardness drops more rapidly, a comparatively low density of these large incoherent carbides is found. These results support the theory that dispersion hardening is primarily responsible for peak hardness in the early first stage of tempering for this steel.