Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

The solidified carbide morphology, the decomposition behavior of the M₂C carbide, and the carbide distribution after forging of an Fe-1.28C-6.4W-5.0Mo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spray forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M₂C carbides and a uniform distribution of fine, spherical MC carbides. The metastable M₂C carbides formed during solidification have been fully decomposed into MC and M₆C carbides after sufficient annealing at high temperatures. Initially, the M₆C carbides nucleate at M₂C/austenite interfaces and proceed to grow. In the second stage, the MC carbides form either inside the M₆C carbides or at the interfaces between M₆C carbides. With this increasing degree of decomposition of the M₂C carbide, the carbides become more uniformly distributed through hot forging, which produces a significant increase in ultimate bend strength. The decomposition treatment of M₂C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forging.