The application of linear elastic fracture mechanics to optimize the vacuum heat treatment and nitriding of hot-work tool steels

摘要

Linear elastic fracture mechanics was used to optimise the vacuum-heat-treatment procedure for conventional hot-work AISI H11 tool steel. The fracture toughness was determined with non-standard, circumferentially notched and fatigue-precracked tensile-test specimens. The fracture-testing method is sensitive to changes caused by variations in the microstructure due to the austenitizing and tempering temperature as well as the homogeneity of the material itself. The combined tempering diagram -Rockwell-C hardness, fracture toughness, K_(lc), tempering temperature - was used to help choose the vacuum-heat-treatment parameters, aimed at obtaining the best properties for a given application with respect to the investigated steel. Nitriding treatments are established methods for improving the wear performance of tools and dies from Hll hot-work tool steel. However, the understanding of the relationship between the nitriding process parameters, on the one hand, and the microstructure and fracture behaviour of the surface layers, on the other, is far from complete. Vickers hardness indentations can generate radial fractures in brittle surface layers, and it has been shown that these cracks can be used to provide reliable information about the fracture toughness of these layers. The results suggest that for a sufficiently thick compound layer, this method has the potential to be applied as a pseudo non-destructive method of monitoring the fracture properties of treated surfaces on actual tool parts. The use of Vickers indentations for fracture toughness would give a useful insight into the fracture properties of nitride layers and their likely response to the application conditions involving high shear or impact loading. This could be very useful for investigations and quality control tool for industrial surface engineering applications.