Study on micro-structure of a QPQ complex salt bath heat-treated 17-4PH stainless steel

The micro-structue of the treated layer of 17-4PH stainless steel (SS) by the QPQ (Quench-Polish-Quench) complex salt bath heat-treatment was investigated, using X-ray diffractometer (XRD) for microstrucrure analysis and the scanning electronic microscopy (SEM) with EDS (energy dispersive spectrum) for the profile of carbon, nitrogen and oxygen of the treated layer. The results show that the QPQ heat-treated layer is approximately 60μm, of which the surface zone was the oxide Fe3O4; the subsurface zone is the compound ξ-Fe2 (N, C) and the inner zone is the mixture of CrN and αN. The compound nitride γ′-Fe4N exists between 15μm and 25μm. The interface between the oxide (magnetite) and the compound γ′-Fe4N was supposed to be the place of the ξ-Fe2N (N, C)1-x nuclei. The oxygen concentration reveals the highest value at approximately 3μm from the surface, while the nitrogen and carbon concentration orderly reaches the high peak value at subsurface. After the oxidation layer was removed, the micro-hardness of the treated layer is observed to be first increasing and then declining with the depth increasing.