Surface modification of 316L stainless steel with plasma nitriding

Plasma nitriding treatment of 316L austenitic stainless steel has been performed in 80%H-2-20%N-2 gas mixture, for various treatment times (1, 4 and 8 h) and at relatively low temperatures (400, 450 and 500 degrees C). Mechanical and tribological properties of the plasma nitrided 316L stainless steel were examined using X-ray diffraction, microhardness tester, scanning electron microscopy and pin-on-disc tribotester. The results show that the modified surface specimens of nitrided 316L stainless steel at 400 degrees C consist mainly of s-phase and small chromium nitride precipitates. At higher temperatures both CrN and iron (gamma'-Fe4N) nitrides have been observed and their amount increases with increasing treatment temperature. The highest hardness values were attained at 500 degrees C and 8 h, an increase of surface hardness was observed with increasing process time. Also wear resistance was improved after nitriding. The wear rate decreased with increasing process time and temperature. After electrochemical measurement it is observed that the corrosion resistance of nitrided 316L steel is getting worse in the modified layer unless the layer consists completely of s-phase and has an adequate continuous layer which covers whole surface of the specimen.