Heat treatment response of sintered M3/2 high speed steel composites containing additions of manganese sulphide, niobium carbide, and titanium carbide

Metal matrix composites based on a sintered M3/2 high speed steel (HSS) and containing either single additions of TIG, NbC, or MnS particles or combined additions of MnS + TIC or MnS + NbC were sintered to full density and subjected to a conventional hardening heat treatment cycle of austenitisation, quenching, and double tempering. Relatively poor hardnesses and strengths were obtained in any composites that contained MnS particles, principally because of the weakening effects of MnS particles in the structure but also as a result of decarburisation of the steel matrix when oxygen introduced by the MnS particles reacted with carbon. Composites that contained TIC were changed in structure so that matrix M(6)C carbides were replaced by the less soluble MC carbide. They consequently showed slightly reduced hardening response compared to the baseline M3/2 HSS after heat treatment. Ball milling was necessary ill powder mixtures that contained NbC to break up particle agglomerates. The resultant fine grain sized M3/2 + NbC materials produced by sintering and heat treatment gave superior properties to both the normal M3/2 HSS and the other composites.