Relations between fracture toughness, hardness and microstructure of vacuum heat-treated high-speed steel

When modelling the fracture toughness of the investigated AISI M2 high-speed steel, the stress-modified critical strain criterion was used. The very important influence of microstructural parameters such as the volume fraction of undissolved eutectic carbides, their mean diameter, and the mean distance between the carbides, as well as the volume fraction of retained austenite in the matrix, was also taken into account. The influence of yield stress and fracture ductility was expressed in terms of the hardness of the steel. It was found that the plastic zone which develops, during fracture toughness measurements, ahead of the fatigue crack tip, was, as a rule, smaller than the prior austenite grain size, so that, in the case of the investigated high-speed steel, the size of these grains did not have any influence on the measured fracture toughness value. However, importantly, the calculated fracture toughness values, which were derived using a newly developed semi-empirical equation, agreed well with the experimental results obtained by the authors, as well as with results obtained by other authors.