Evaluation of the sliding wear and corrosion performance of triode-plasma nitrided Fe-17Cr-20Mn-0.5N high-manganese and Fe-17Cr-35Ni-1.2Si high-nickel austenitic stainless steels

Low-temperature plasma nitriding has been widely studied and applied, in enhancing the wear performance of austenitic stainless steels (ASSs) without losing corrosion resistance. In this work the wear and corrosion behaviours of two specialty ASSs, i.e. Staballoy® AG17 (Fe-17Cr-20Mn-0.5N, in wt%) and RA330® (Fe-19Cr-35Ni-1.2Si, in wt%), were evaluated – and compared to AISI 304 – before and after low-temperature triode plasma nitriding (TPN) at 400 °C and 450 °C. A nitrogen interstitially-supersaturated expanded austenite layer (γN) was developed for all three ASSs after TPN treatment at 400 °C, which led to a) an approximately 4-fold increase in surface hardness, b) a reduction in specific wear rate of at least 2 orders of magnitude in unlubricated dry-sliding, and c) an improved resistance to pitting in 3.5 wt% NaCl aqueous solution. Large numbers of ‘linear defects’ (identified in TEM studies as strips of HCP-εN) were seen in the γN-AG17 layer, that could be correlated to comparatively higher surface hardness and better wear resistance. Several slip/shear bands were also seen in the γN-330 layer, where short-range Cr-segregation could occur, leading to localised corrosion. More importantly, after TPN treatment at 450 °C, alloys AISI 304 and AG17 presented a deterioration in corrosion performance, whereas good corrosion performance was maintained for alloy RA330. Redistribution of Si (in preference to Cr) was revealed in γN-330 after TPN treatment at 450 °C, whereby Si-alloying at a significantly higher level than in the other two alloys investigated appears (in addition to the high Ni content in alloy 330) to be beneficial in delaying CrN precipitation, and thus in maintaining the good corrosion performance of γN after nitriding at low-to-intermediate temperatures.