Microstructure and properties of M2 high-speed steel cast by the gravity and vacuum investment casting

In this work the microstructure and properties of M2 high-speed steel cast into ceramic moulds using gravity and vacuum casting and subject to heat treatment were studied. The microstructure of castings after solidification and after heat treatment was subjected to detailed characterisation. The surface roughness and content of gases and solid non-metallic impurities of the samples were also evaluated together with mechanical tests for hardness, red hardness, impact toughness and bending strength. The results established that investment casting under vacuum enhanced the cleanliness of the metal and improved the microstructure and properties. The relationship between the microstructure and the properties is discussed. Some advantage of the vacuum cast high-speed steel with respect to the red hardness was attributed to the stronger secondary hardening of the steel during tempering. The refinement of the eutectic carbides and colonies as well as the formation of the more discontinuous and thinner interdendritic network of eutectic carbides and the metal cleanliness were the determining factors which provided enhanced toughness and strength properties of the vacuum cast high-speed steel compared with that of the gravity cast one.