Hardening behavior of dilute Mo–Ti alloys by two-step heat-treatment

Two dilute Mo–Ti alloys were carburized by two-step heat-treatment. In the first step, specimens were heated with graphite powders at 1773 K for 1.2 ks. In the second step, these specimens were further heated without graphite powders at a temperature between 1473 and 1673 K for 240–360 ks. Change in hardness from surface to center of the carburized specimen was measured and compared with that of the specimen before carburizing. Results are summarized as follows. (1) Hardness of the specimen before carburizing increased almost linearly with the Ti content. Such hardening is attributed to solution effect. (2) In the carburized specimen, hardening was recognized in the region inside from the surface hard layer. Such hardening is attributed to the preferred combination between Ti atoms and C atoms in addition to solution effect and Mo2C particles. (3) Largest hardening was obtained by heating a specimen with graphite powders at 1773 K for 1.2 ks followed by heating without graphite powders at 1573 K for 360 ks. (4) Carbon supply into the bulk is predominantly by diffusion of dissolved C atoms at the interface between the surface hard layer and the bulk.