Influence of γ'N and ɛ'N phases on the properties of AISI 304L after low-temperature plasma nitrocarburizing

Abstract

The surface of austenitic stainless steel AISI 304L has been hardened by adding nitrogen and carbon through the plasma nitrocarburizing process. Plasma nitrocarburizing at low temperatures eliminates the formation of nitrides and carbides of chromium, thereby increasing the surface hardness without compromising its corrosion resistance. This is attributed to the presence of a hard case consisting of a solid-solution of nitrogen and carbon occupying interstitial sites in an “expanded” austenite phase. The expanded austenite is further characterized as nitrogen-expanded austenite (γN) and carbon-expanded austenite (γC). In the present work, austenitic stainless steel AISI 304L samples were plasma nitrocarburized by varying the N2 content (%N2) in N2-H2-C2H2 feed gas mixture and treatment times (3, 5, and 7 h) at 673 K and 500 Pa. The samples were characterized by means of XRD, microindentation, XPS, EIS, and potentiodynamic polarization testing, for studying the microstructural, mechanical, and electrochemical properties. The XRD analysis revealed the presence of ɛ'N and γ'N in addition to γN and γC when treated with higher %N2 gas for 5 and 7 h. From XPS, the binding energies of undefined FexN were obtained which were found to lie very close to the defined FexN for higher %N2 gas. Surface hardness significantly improved after the plasma nitrocarburizing processes irrespective of the %N2 and treatment times compared to the untreated AISI 304L. The presence of CrN precipitation on the surface of samples treated with the 75% N2 for 5 and 7 h were observed from XPS, which caused higher corrosion rate. Among the treated samples, the process with 50% N2 and 5 h exhibited the best corrosion resistance with high surface hardness.