Vacuum heat treatment, deep cryogenic treatment and simultaneous pulse plasma nitriding and tempering of P/M S390MC steel

Vacuum heat treatment, deep cryogenic treatment and pulse plasma nitriding are efficient techniques to improve the properties of tool and high speed steels. Sometimes the influence of subzero treatment could be directly ascribed to a specific metallurgical transformation. It is the case of the transformation of retained austenite into martensite, causing a general increase in hardness and higher wear resistance. In other cases, however, the increase in wear resistance is not supported by higher hardness and several theories were proposed to explain the observed results. However, poor experimental evidence was reported in the literature for this phenomenon. Specific attention is paid to the influence of subzero treatment just after quenching and solubilization in the vacuum heat treatment or simultaneous pulse plasma nitriding and tempering (PPNT) cycle of the P/M S390MC high speed steel, respectively. Special emphasis was put on resistance to galling and abrasive wear resistance under dry sliding conditions. From obtained results it can be concluded, that the application of deep-cryogenic treatment results in a significantly higher wear resistance of high speed steels, but no significant improvements in fracture toughness have been noticed.