Quantitative microstructural analysis of M2 grade high speed steel during high temperature treatment

Abstract

The microstructural changes of M2 grade high speed steel were examined quantitatively to identify the mechanisms responsible for changes during high temperature treatment. Reheating at high temperature leads to major microstructural changes involving phase transformation, spheroidization and coarsening of carbides formed in the as-cast M2 grade high speed steel. The M 2C carbide decomposes rapidly to form MC and M 6C in less than 1h after attaining the reheating temperatures of 1150 or 1200°C. Further reheating brings about the eutectic M 6C carbide spheroidization and general coarsening of both M 6C and MC. Carbide particles situated at the original austenite grain boundaries grow preferentially until impingement of carbides along these boundaries takes place. It is the diffusion of tungsten in γ-Fe which controls spheroidization rate of M 6C carbides, while diffusion of vanadium and tungsten in γ-Fe is the rate controlling mechanism for coarsening of MC and M 6C carbides, respectively. The process of spheroidization of alloy carbides such as M 6C, is somewhat different from that of cementite where impingement of the already spheroidized carbide particles make the analysis more difficult.