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/γ´-Fe₄N and CrN/γ-Fe, while the post-oxidized sample surfaces are comprised of CrO₃, (Fe₃O₄)/ε-Fe₂(N,C), γ′-Fe₄N and S(γ_N)/CrN/α-Fe. Nuclei ε-Fe₂N_1 − 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 (γ_N) move gradually towards higher diffraction angles as increasing depth of 15 μm to 35 μm. An increased content of oxygen is recorded in the post-oxidized surface layer down to the depth of approximately 15 μm, a small peak concentration of carbon occurs at the front of the nitrided layer. Micro-hardness of the post-oxidized samples reaches about 1300 HV_0.1 near the surface region and then reduces sharply across the case/substrate interface.