Effect of additive alloying element on plasma nitriding and carburizing behavior for austenitic stainless steels

Austenitic stainless steels, well known for their good corrosion resistance, have not been used for industrial components exposed to severe friction because of low hardness and poor friction and wear properties. Low-temperature plasma nitriding or carburizing of austenitic stainless steels can produce a specific nitrided or carburized layer, so-called “S-phase”, with high hardness and good corrosion resistance. In this study, various austenitic stainless steels were low-temperature plasma nitrided and carburized, and the effect of additive alloying elements on the S-phase characteristics was investigated by various analyzing techniques: observation using an optical microscope, transmission electron microscope, X-ray diffraction analysis, an anodic polarization measurement in 5% H2SO4 solution, and a friction and wear test using a ball-on-flat friction apparatus. The thickness of the nitrided and carburized layers increased with an increase in the process temperature, and the thickness of the layer formed on the AISI316 steel is thickest in all substrate steels. While, at above ‘critical temperature’ the corrosion resistances were deteriorated owing to the precipitation of chromium nitride or carbide. The critical temperature was dependent on the substrate material in nitriding. On the other hand, in the case of carburizing, the critical temperatures were almost constant with substrate materials. The corrosion resistances of the S-phase layers on most of the specimens were less than that of the untreated stainless steels, but the AISI316 and JIS-SUS304J3 steels carburized at 400 °C showed the same excellent corrosion resistances as the untreated steels respectively. The wear resistance of every stainless steel was obviously improved by nitriding or carburizing treatment.