Plasma nitriding was performed on the 2205 duplex stainless steel samples at 400 V with a gas mixture of H2 and N2 for 15 hrs with changing N2 percent, temperature and adding various amounts of CH4. After treatment the behavior of the surface layer was investigated by optical microscopy, X-ray diffraction, GDOES analysis and micro-hardness testing. Potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. With increasing both N2 percentage from 10% to 25% and nitriding temperature from 370°C to 430°C, the thickness of nitrogen expanded austenite (S-phase) layer and surface hardness increase up to 16 μm and 1200 HV0.1 at the treatment temperature of 430°C with 25% N2, but decreases the corrosion resistance due to the formation of Cr2N and γ`(Fe, Cr)4N. Thus in order to further increase the thickness of S-phase layer and the corrosion resistance, the influence of adding various amount of CH4 (1% to 5%) in the nitriding atmosphere was investigated. Adding CH4 in the nitriding atmosphere increases the layer thickness compared with that of nitrided sample. The highest thickness can be obtained at 1 % CH4, but addition of CH4 beyond 1 % slightly decreases the layer thickness. Moreover, when nitrided at 400°C with 10% N2 and 5% CH4 content, best corrosion behavior is obtained which also have around 10 μm layer thickness and about 870 HV0.1 surface hardness.
The Phenomenon of High Hardness Values on the S-Phase Layer of Austenitic Stainless Steel via Screen Plasma Nitriding Process