Institute of Metals Division - Effect of Cobalt on Carbon Activity and Diffusivity in Steel

Cobalt clearly increased the activity of carbon in austenite and in ferrite. This effect of cobalt on carbon activity Plausibly accounted for the effect of cobalt on accelerating the austenite to pearlite transformation. Cobalt was found to have only a slight effect on carbon diffisivity in various Fe-Co and Fe-Co-W-Mo austenites, but was found to decrease carbon diffusivity in Fe-Co ferrite. THE effect of cobalt in steel is unique in that it is the only element that reduces hardenability by accelerating the austenite to pearlite reaction. The results of several excellent investigations aimed at explaining the role of cobalt have been published,1"7 but a complete understanding is still lacking. In addition, cobalt significantly enhances the resistance to softening or tempering of highly alloyed highspeed and hot-work tool steels. The resistance to tempering is related to the secondary hardening reactions associated with the fourth stage of tempering during which carbides of molybdenum, tungsten, and/or vanadium form in a martensite matrix. The effect of cobalt on secondary hardening reactions in high-alloy tool steel has received considerable attention, but a full explanation of the reactions involved has not been developed. Inasmuch as austenite to pearlite transformation and secondary hardening involve carbide formation, the present authors believed that the role of cobalt in effecting carbon activity and diffusivity in austenite and in ferrite was important to a fuller understanding of the phenomena involved. TO investigate the effect of cobalt on the activity coefficient of carbon in austenite, an approach was used similar to that used by Darken17 in his experiments on the effect of silicon, manganese, and molybdenum on carbon activity and diffusivity in austenite. TO