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.OMo-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.