Influence of a post-tempering cryogenic treatment on the toughness of the AISI M2 steel

A deep cryogenic treatment, carried out after quenching, can improve several mechanical properties of many metallic materials, but there are few reports on the effect of post-tempering deep cryogenic treatments on steels. The main aim of the present research work is the study of the effects of a post-tempering cryogenic treatment on microstructural and mechanical properties of the AISI M2 tool steel. In particular, the results of impact strength, fracture toughness, scratch test and microhardness show that the microstructural changes, induced by the post-tempering cryogenic treatment, promote a relief of residual stresses and a little improvement of mechanical properties, except for the hardness. Microstructural analysis performed by means of Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), equipped with an Energy Dispersive X-ray Spectroscopy (EDS) microprobe, confirmed the precipitation of small and homogeneously dispersed secondary carbides. The induced precipitation of fine and homogeneously dispersed sub-micrometric carbides does not alter hardness but reduces residual stresses and increase fracture toughness. Finally, scratch test proved to be an alternative simple technique to determine fracture toughness of cryogenically treated steels