Effect of surface roughness on the properties of the layer formed on AISI 304 stainless steel after plasma nitriding

Experiments were performed with an aim of studying an effect of initial surface roughness with different gas compositions in plasma nitriding, using pulse D.C. glow discharge plasma in presence of nitrogen and hydrogen gas mixtures. Samples were prepared with different mechanical treatments: polishing, rough polishing, machining and grinding. Plasma nitriding was carried out on AISI 304 stainless steel at 560 °C under 4-mbar pressures for 24 h in presence of N2 : H2 in 20 : 80 and 80 : 20 ratios. After plasma nitriding, surface roughness, micro hardness, case depth and phase formation were evaluated by using stylus profilometer, Vickers micro hardness tester, optical microscope and X-ray diffraction techniques, respectively. After plasma nitriding, hardness and case depth variation are observed with variation in surface roughness as well as gas compositions. Maximum hardness i.e. 1325 HV and case depth i.e. 110 μm are achieved on mirror polished samples at 80N2 : 20H2. The diffraction patterns show the most dominant phase formation of CrN, Fe4N and Fe3N which is responsible for this increase.