Influence of salt bath nitrocarburizing and post-oxidation process on surface microstructure evolution of 17-4PH stainless steel

The effect of Quench–Polish–Quench complex salt bath heat-treatment (QPQ) on surface microstructure of the 17-4PH stainless steel (SS) was investigated. A bright zone (compound ɛ-Fe2(N,C)) is formed at the outermost surface of the nitrocarburized sample, followed by a zone (mixture Fe3N/Fe4N/CrN/α-Fe) which is slightly etched. A thin oxides zone (Fe3O4/FeO) is formed at the outer surface of the post-oxidized sample, followed by a zone (mixture Cr2N/ɛ-Fe2(N,C) and the inner zone (CrN/αN). A significantly high amount of oxygen is found at the outermost surface of the post-oxidized sample. The push-effect of oxygen leads to the peak nitrogen concentration, while the peak carbon concentration at the front of the nitrided layer is due to the push-effect of nitrogen. High microhardness values (>1200 HV0.1) is obtained at the surface and then drops abruptly at the case/core interface to substrate microhardness value for both of the nitrocarburized and post-oxidized samples.