Study On Processing, Microstructure And Properties Of Surface Layer Of Plasma Re Nitrocarburized 2Cr13 Steel

In order to improve corrosion resistance and modify steep hardness distribution of surface layer conventionally nitrided for 2Cr13 martensitic stainless steel, the pulse plasma RE nitrocarburizing was carried out at low-temperatures (360°C⁴60°C) for preparing surface layer with advanced corrosion resistance and at temperature of 560°C for obtaining layer with gentle hardness distribution. Systematic microstructure characterization of plasma nitrided 2Cr13 steel was carried out using OM, SEM, XRD, TEM and EDS. Additionally, the microhardness profile, wear resistance and corrosion resistance of the layer were also measured by microhardness, wear and anodic polarization tests.The results of research on processing at low-temperatures have shown that precipitation of the CrN phase wouldn't be found when treated at 430°C. The optimal flux of RE-C and NH3 are 0.05L·min^-1 and 0.4L·min^-1, respectively, at 430°C. RE increase the rate of nitrocarburizing obviously, and the thickness of modified layer with RE increases by 53% comparing with that of layer without RE.The results of XRD and TEM show that the nitrogen supersaturated martensite (α'N-expanded martensite) can be formed in layer treated at low temperature, and its microstructure and distribution were observed. The results show that the layer can be divided into three layers by the changing of phase constituents: theε-Fe_2-3N compound layer is formed on the surface, the sub-surface is consisted ofα'N,ε-Fe_2-3N andγ′-Fe4N phase, the third layer is consisted ofα'N andα'N-expanded phase. Moreover, the texture have been significantly refined and the amorphous is found in partially region of layer.Hardness tests show that the hardness of low-temperatures plasma surface alloyed cases is about 1350HV, and the values of hardness fluctuate with the depth for the change of phase constituents. In the frictional wear tests, the RE nitrocarburized samples show lower wear rate and friction coefficient than untreated one. In the electrochemical corrosion tests, the nitrocarburized samples showed higher corrosion potentials, higher pitting potentials and wider passive region than untreated one.The results of plasma RE nitrocarburizing at 560°C have shown that the reducing of superficial nitrogen concentration help to gentle hardness distribution, and the process is related to the role of carbon. A new model based on the interaction between nitrogen and carbon was build to explain the mechanism and discuss the factors affecting hardness gradient.