Effect of Microstructure on the Fracture Toughness of a Vacuum-Heat Treated ESR M2 high-speed steel

摘要

The microstructure of AISI M2 high-speed steel can be substantially modified by vacuum heat treatment in order to optimise the ratio between hardness and fracture toughness. This ratio is significantly affected by the volume fractions of retained austenite and undissolved eutectic carbides, as well as the mean distance between these carbides. Calculated fracture toughness values, which were obtained using a semi-empirical equation, based on the stress-modified critical strain criterion and the quantified microstructural parameters, gave us an opportunity to better understand the influence of the microstructure of investigated AISI M2 high-speed steel on its fracture toughness. Furthermore, a good understanding of the mutual interaction of mechanical and microstructural properties on the fracture toughness KIc, can assist both the materials engineer and the structural designer to choose the optimum composition and processing for high-speed steel (HIP'ed, forged, sintered, ESR or conventional material) and the best heat treatment process (salt bath, fluidised bed or vacuum heat treatment) to obtain an optimum combination of basic characteristics for a given tool application.