Solidification of high speed steels

Abstract

High speed steels are ferrous based alloys of the Fe–C–X multicomponent system 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 they are characterised by their capacity to retain a high level of hardness while cutting metals at high speed. The as cast microstructure of high speed steels consists of dendrites surrounded by a more or less continuous interdendritic network of eutectic carbides. These are observed even under rapid cooling, at rates as high as 10⁶ K s^-1. The main features of the as cast microstructure are the distribution and morphology of eutectic carbides, owing to their decisive influence on mechanical properties, and on the service performance of the high speed steels, even after hot working. Hence, the most important improvements on the as cast microstructure that have been obtained through progress in alloy design concern the type, morphology, and volume fraction of the eutectic carbides, while improvements obtained by progress in solidification processing are, on the other hand, mostly related to a significant reduction in the microstructure scale. The development of the microstructure in high speed steels through solidification is reviewed, emphasising the effects of the alloy chemical composition and of the cooling rate. The formation of the eutectic carbides and the techniques used to control their morphology and distribution, both under normal and rapid cooling, are discussed. Microstructures obtained through surface remelting and surface alloying using laser and electron beams are described, and the application of highly alloyed high speed steels to the manufacture of cast hot rolling mill rolls is reviewed.