Estimation of fracture toughness of nitride compound layers on tool steel by application of the Vickers indentation method

Nitriding and nitrocarburizing treatments are established methods of improving the wear performance of tool and die steels. However, our understanding of the relationship between nitriding process parameters, and microstructure and fracture behaviour of the surface layers is far from complete. This paper describes an investigation of the application of Vickers indentation fracture testing to nitrided and nitrocarburized AISI H11 hot work tool steel. Optical microscopy and microhardness testing were used to characterize structure and mechanical properties of the compound and diffusion layers. The results show that, where a sufficiently thick compound layer has formed, 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. However, the validity of the method appears to be a function of the presence and thickness of the compound layer, and possibly the mechanical properties of the diffusion layer, and further work is proposed to establish the sensitivity of the method for fracture toughness calculations.