Dissolution of carbon in Cr-prealloyed PM steels: effect of carbon source

Abstract

Modern water-atomised steel powder grades are characterised by the presence of two types of surface oxides: a thin iron oxide layer, covering more than 90% of the powder surface, and more thermodynamically stable particulate oxides. The development of inter-particle necks and carbon dissolution in the iron matrix both require efficient removal of the iron oxide layer. Hence, carbon reactivity strongly affects the surface oxide reduction that determines inter-particle neck development and carbon dissolution, and so microstructure development. An analysis is presented of the effect of three carbon sources – synthetic graphite, natural graphite and carbon black – on microstructure and inter-particle neck development in Cr-alloyed PM steels. Metallographic and fractographic studies indicate that the most significant property of the carbon sources affecting reactivity is the carbon powder size. Carbon black shows the highest reactivity at elevated temperatures but is fully inert at temperatures below 900°C.