The role of alloy composition in the precipitation behaviour of high speed steels

The role of alloy composition in determining the microstructure and microchemistry of a series of related high speed steels has been investigated by a combination of analytical electron microscopy and atom-probe field ion microscopy. The four steels which were investigated (M2, ASP 23, ASP 30 and ASP 60) cover a large range of C, V and Co contents. Excepting the Co content, the composition of primary MC and M₆C carbides and as-hardened martensite was similar in all four alloys and the major effect of increasing the content of C and V was to increase the volume fraction of MC primary carbides. Precipitation of proeutectoid carbides (mainly MC and M₂C) occurred during hardening of all four steels and the extent of this was greatest in the highly alloyed ASP 60. Tempering at 560°C resulted in the precipitation of extremely fine dispersions of MC and M₂C secondary carbides with very mixed compositions in all four steels. It was found that, as well as hindering the formation of autotempered M₃C in the as-hardened martensite, additions of Co refined the secondary carbide dispersion and delayed overaging reactions. Overaging at 600°C resulted in the precipitation of M₃C, M₆C and M₂₃C₆ at the expense of the fine MC and M₂C secondary carbide dispersion.