Analyses of Material Properties of Nitrided AISI M2 Steel Treated by Plasma Immersion Ion Implantation (PHI) Process

摘要

The AISI M2 steels were treated using the plasma immersion ion implantation (PHI) nitriding process at a high temperature and at various gas pressures. Results show that the thicknesses of nitrided layers and the concentrations of nitrogen atoms at nitrided layers increased when the processed gas pressure was increased and nitriding layer thickness is more than 180 μm. The XRD results show that the main compound phase of iron-nitrogen is transformed from Fe_(24)N_(10) to Fe_4N when the gas pressures are increased from 2.5 mTorr to 6.5 mTorr. In this study, a gas pressure of 4.5 mTorr is the saturation pressure of the M2 steel treated at 530℃. The hardness profile at the nitriding layer does not show monotonous decreases with the penetration depth. There was a secondary peak near the depth of about 100 μm. Area size calculations of educts in TEM photographs show that large educts of solid solution formed at a depth of about 100 μm. The initial decrease in the hardness at small penetration depths can be attributed to the distribution of implanted N ions. The second increase was caused by the filling of carbon and nitrogen atoms in the defects and vacancies of the Fe lattices, which can form big educts of the solid solution able to strengthen M2 steel.