Microstructural changes in as-cast M2 grade high speed steel during high temperature treatment

An optical and electron metallographic study has been carried out on chill and sand cast M2 grade high speed steel in the as-cast condition and after reheating to temperatures typical of those for hot working for times up to 200 h. The as-cast material contains M₂C, MC, and M₆C particles, but the M₂C decomposes rapidly on reheating. M₂₃C₆ was not detected after any of the treatments. Coarse carbides are found both at the centres of dendrites and as interdendritic eutectic pools, with small carbides distributed throughout the as-cast material. Long term treatment causes spheroidization of the eutectic carbides and general coarsening, with preferential growth and impingement of carbides situated on original austenite grain boundaries. The majority of small carbides are dissolved but a few grow preferentially. The structure remains heterogeneous even after long treatment times.