Low-pressure arc plasma-assisted nitriding of AISI 304 stainless steel

AISI 304 stainless was modified by low-pressure arc plasma-assisted nitriding (PAN) process at different temperatures. The structure, micrographs and composition depth profile were investigated by X-ray diffraction (XRD), optical microscope, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Microhardness profiles and corrosion behavior in 3.5% NaCl solution were measured by Vicker microhardness tester and potentiodynamic polarization curves, respectively. The results showed that the nitriding temperature had much effect on the phase composition, thickness and microhardness profiles of the nitrided layers. At lower temperature than 430 °C, expanded austenite phase was the major phase, while for higher temperature some iron nitride and CrN phases precipitated. The thickness of the nitrided layers increased with the nitriding temperature. The surface nitrogen concentration obtained was in the range of 23 and 27 at.%. The nitriding produced a surface hardening that can reach microhardness value up to 1300 HV0.05 and the hardening effect was analyzed. For lower temperature than 430 °C, the corrosion resistance of nitrided samples was not deteriorated.