Plasma nitriding of AISI 304 stainless steel: Role of surface mechanical attrition treatment

• SMAT of 304 SS increases the surface roughness and promotes martensite phase.
• Martensite phase surpasses the influence of surface roughness during nitriding.
• The case depth of nitrided layer is higher for SMATed 304 SS than the untreated one.
• Migration of chromium and formation of CrN phase are higher in SMATed 304 SS.
• Hardness of nitrided layer formed on SMATed 304 SS is higher than the untreated one.

The effect of surface roughness, volume fraction of martensite and the tendency to form CrN phase during plasma nitriding of AISI 304 stainless steel pretreated by surface mechanical attrition treatment (SMAT) is studied and compared with those formed on its untreated counterpart. The study reveals that the deleterious influence of surface roughness induced by SMAT on the case depth of the nitrided layer, if any, is surpassed by the favourable influence of martensite phase formed during SMAT in promoting the diffusion of nitrogen. Surface nanocrystallization with slip bands and dislocations in 304 stainless steel samples subjected to SMAT promotes migration of chromium, resulting in the formation of higher amounts of CrN phase than the untreated one under similar conditions of nitriding. The difference in the hardness of the nitrided layer formed on untreated 304 SS and those subjected to SMAT at the top surface is ~ 600 VHN25gf.