SOLIDIFICATION MICROSTRUCTURE AND M2C CARBIDE DECOMPOSITION IN A SPRAY-FORMED HIGH-SPEED STEEL

The solidified carbide morphology, the decomposition behavior of the M 2C carbide, and the carbide distribution after forging of an Fe-1.28C- 6.4W-5.OMo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spra y forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M2C carb ides and a uniform distribution of fine, spherical MC carbides. The me tastable M2C carbides formed during solidification have been fully dec omposed into MC and M6C carbides after sufficient annealing at high te mperatures. Initially, the M6C carbides nucleate at M2C/austenite inte rfaces and proceed to grow. In the second stage, the MC carbides form either inside the M6C carbides or at the interfaces between M6C carbid es. With this increasing degree of decomposition of the M2C carbide, t he carbides become more uniformly distributed through hot forging, whi ch produces a significant increase in ultimate bend strength. The deco mposition treatment of M2C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forgi ng.