Microstructure analysis of 304L austenitic stainless steel by QPQ complex salt bath treatment

The microstructure of nitro-carburized and post-oxidized 304L austenitic stainless steel (SS) produced by a Quench–Polish–Quench complex salt bath process (QPQ) was investigated. The nitro-carburized coating as a whole consists of CrN, γ′-Fe4N and expanded austenite S(γN). On the post-oxidized surfaces, XRD analyses revealed the existence of a superficial compound ξ-Fe2(N,C) sub-layer, the presence of magnetite (Fe3O4) and a small amount of FeO. At greater depth from the surface of the post-oxidized surface, S(γN) is formed and subsequently transforms partially into CrN and ferrite/martensite (αN). A modestly higher sub-surface hardness and a superior hardness profile were obtained by post-oxidizing, although its surface hardness was lower than that produced by nitro-carburizing alone.