Effect of the amount of CH4 gas on the characteristics of surface layers of low temperature plasma nitrided martensitic precipitation-hardening stainless steel

Plasma nitriding was performed on AISI 630 martensitic precipitation hardening stainless steel samples at 400 V with a gas mixture of H2 and N2 for 19 h with changing N2 percentage, temperature and adding various percentages of CH4. After treatment the behavior of the surface layer was investigated by optical microscopy, X-ray diffraction, GDOES analysis and micro-hardness testing. A potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. With increasing N2 percentage from 15% to 35% at 380 °C, the thickness of the expanded martensite (α′N) layer increases to about 20 μm and surface hardness also increases to about 1280 HV0.05 but the corrosion behavior is worse than the untreated sample. When nitrided with 25% N2, increasing nitriding temperature from 380 °C to 460 °C also increases the α′N layer thickness to around 38 μm and surface hardness to around 1270 HV0.05 (at 460 °C), but decreases the corrosion resistance. Thus in order to improve the corrosion resistance, various amounts of CH4 (0% to 6%) were introduced in the nitriding atmosphere at 400 °C with 25% N2 percentage. Increasing CH4 percentage slightly decreases the α′N layer thickness but greatly improves corrosion behavior. Moreover, with 4% CH4 percentage, the best corrosion behavior is obtained which also has around 11 μm α′N layer thickness and about 1290 HV0.05 surface hardness. Therefore, the conclusion can be drawn that, the highly suitable condition for plasma nitriding for a thicker expanded martensite layer, higher surface hardness and good corrosion resistance is plasma nitriding at 400 °C with 25% N2 and 4% CH4 for 19 h.