Effect of CH4 addition on plasma nitrocarburizing of austenitic stainless steel

This article evaluates the influence of the plasma atmosphere composed of different ratios of N2/H2/CH4 on the micro-structures, wear resistance and corrosion resistance of 304L and 316L austenitic stainless steels. DC power with a bias voltage in the range of 250–350 V was applied between the vessel (anode) and the work (cathode). The results show that the thickness of the hardened layers decreased with increasing methane addition and dramatically decreased when methane addition reached 5%. The wear resistance behavior was significantly improved, especially in the nitrocarburized specimens. The corrosion resistance of nitrided and nitrocarburized layers, in general, as inferior to that of the bare materials when they were tested in an imitation gastric juice. However, the specimen of 304L nitrocarburized with 5% addition of CH4 exhibited excellent improvement to corrosion resistance, even better than the bare material. This excellent corrosion resistance is attributed to the formation of chromium nitrides and chromium carbides, such as CrN, Cr7C3 and Cr2N instead of Fe4N.