Microstructural effects of splat cooling a high-speed steel

Abstract

A metallographic investigation of splat-cooled M1 high-speed steel is described. Apart from general microstructural refinement, the splat-cooling process retains in solid solution much greater amounts of the alloying elements than conventional quenching processes. This stabilizes the austenite to such an extent that the M _s temperature is depressed close to or below ambient temperature and no martensite is formed. The morphology of the matrix phases, δ-ferrite and austenite, is shown to depend critically upon the specimen thickness and hence local cooling conditions. Owing to the high solute supersaturation, subsequent tempering of the splat-cooled steel leads to precipitation of the vanadium-rich carbide MC on 100 planes in the δ-ferrite, in sharp contrast to the molybdenum-rich M₂C carbide which is normally responsible for secondary hardening in conventionally processed high-speed steels. In the splat-cooled steel, M₂C appears only at the solute-rich interdendritic boundaries. Sintering studies were carried out to consolidate splat-cooled material into a more useful form. The presence of high concentrations of carbon in solution in the as-splatted material depresses the solidus temperature so much that partial melting occurs upon sintering at 1200°C, while the excess dissolved carbon permits the precipitation of M₂C at 1100°C. The practical application of the knowledge gained from this study to possible future developments in high-speed steel technology is discussed.