Surface microstructure of 316L austenitic stainless steel by the salt bath nitrocarburizing and post-oxidation process known as QPQ

Systematic materials characterization of the Quench-Polish-Quench complex salt bath heat-treatment process (QPQ) surface modified 316L steel was investigated. The results reveal that the nitro-carburized sample surfaces consist of Cr2N/gamma'-Fe4N and CrN/gamma-Fe, while the post-oxidized sample surfaces are comprised of CrO3, (Fe3O4)/epsilon-Fe-2(N,C), gamma'-Fe4N and S(gamma(N))/CrN/alpha-Fe. Nuclei epsilon-Fe2N1-x accumulates at the interface between oxide layer and nitride compound layer by the help of post-oxidation. The diffraction peak lines of S-phase (gamma(N)) move gradually towards higher diffraction angles as increasing depth of 15 gm to 35 gm. An increased content of oxygen is recorded in the post-oxidized surface layer down to the depth of approximately 15 gm, a small peak concentration of carbon occurs at the front of the nitrided layer. Micro-hardness of the post-oxidized samples reaches about 1300 HV0.1 near the surface region and then reduces sharply across the case/substrate interface. (C) 2007 Elsevier B.V. All rights reserved.