The effect of molybdenum on the characteristics of surface layers of low temperature plasma nitrocarburized austenitic stainless steel

The effect of molybdenum in the surface characteristics on low temperature plasma nitrocarburized layer of austenitic stainless steel was investigated. A low temperature nitrocarburized layer of AISI 316L steel (Fe–17Cr–12Ni–2.5Mo) was compared with that of AISI 304L steel (Fe–19Cr–10Ni) to evaluate the influence of molybdenum on nitrocarburizing. The low temperature plasma nitrocarburizing was performed in a gas mixture of N 2, H 2 and carbon-containing gas such as CH 4. The influence of processing temperature (380–480 °C) on the surface properties of the nitrocarburized layer was investigated. The resultant nitrocarburized layer produced on both 316L steel and 304L steel is a dual-layer structure, which comprises a N-enriched layer ( γ N) with a high nitrogen content on top of a C-enriched layer ( γ C) with a high carbon content, leading to a significant increase in surface hardness (about 1200 HV 0.01). The chromium nitride was formed in the N-enriched layer for 316L steel treated at temperatures above 480 °C compared with 304L steel treated at temperatures above 430 °C. The thickness of the hardened layer without precipitation of chromium nitride of both 316L steel and 304L steel increased with increasing temperature, and reached up to 25 μm in 316L steel at 450 °C, 10 μm in 304L steel at 400 °C, respectively. However, at same treatment temperature, the thickness of the hardened layer formed on nitrocarburized 316L steel was larger than that produced on nitrocarburized 304L steel. The specimens treated at 400 °C showed much enhanced corrosion resistance in terms of lower corrosion current density and a higher corrosion potential as compared to the untreated steel. The loss in corrosion resistance was observed for the specimens treated at 430 °C for 304L steel and 480 °C for 316L steel, due to the formation of chromium nitrides in the nitrogen-enriched layer.