Microstructure and mechanical properties from an attractive combination of plasma nitriding and secondary hardening of M50 steel

Microstructure and mechanical properties of a quenched M50 steel plasma nitrided for 8 h at temperatures ranging from 480 to 540 °C in a gas mixture of 25% N ₂ + 75% H ₂ were investigated in this paper. Results show that, compared with the original specimen, microhardness and wear resistance of M50 steel after nitriding treatment are remarkably improved at various temperatures, wherein the N500 specimen achieved the optimum combination of surface hardness (1186 HV), substrate hardness (643HV) and wear resistance (wear rate reduced by 90% more than original specimen). Such significantly improvement could mainly be attributed to the formation of nanophase FeN _0.076 , occurrence of further secondary hardening of substrate and generation of a nitrided surface layer of M50 steel during nitriding process. Two types of carbides, identified as MC and M ₆ C, were found to be the stable precipitates present in substrate of N500 specimen. These results provide us with an attractive method of combining nitriding with secondary hardening to achieve a comprehensive performance from surface to substrate of M50 steel.