Solidification of high speed steels

High speed steels are ferrous based alloys of the Fe-C-X multicomponent sys tem where X represents a group of alloying elements comprising mainly Cr, W or Mo, V, and Co. These steels are mainly used for cutting tools, since th ey are characterised by their capacity to retain a high level of hardness w hile cutting metals at high speed. The as cast microstructure of high speed steels consists of dendrites surrounded by a more or less continuous inter dendritic network of eutectic carbides. These are observed even under rapid cooling, at rates as high as 10(6) K s(-1). The main features of the as ca st microstructure are the distribution and morphology of eutectic carbides, owing to their decisive influence on mechanical properties, and on the ser vice performance of the high speed steels, even after hot working. Hence, t he most important improvements on the as cast microstructure that have been obtained through progress in alloy design concern the type, morphology, an d volume fraction of the eutectic carbides, while improvements obtained by progress in solidification processing are, on the other hand, mostly relate d to a significant reduction in the microstructure scale. The development o f the microstructure in high speed steels through solidification is reviewe d, emphasising the effects of the alloy chemical composition and of the coo ling rate. The formation of the eutectic carbides and the techniques used t o control their morphology and distribution, both under normal and rapid co oling, are discussed. Microstructures obtained through surface remelting an d surface alloying using laser and electron beams are described, and the ap plication of highly alloyed high speed steels to the manufacture of cast ho t rolling mill rolls is reviewed.