Improvement of Fatigue Properties by Solution Treatment in Nitrided Type 304 Stainless Steel

In order to stabilize the metastable γ-phase and to improve the strength in type 304 steel by the solid solution of nitrogen, the solution treatment after nitriding (STAN) was performed. The static strength and the hardness were improved by the STAN. In laboratory air, the fatigue strength of the STAN specimen increased as compared to that of the untreated specimen, but was lower than that of the nitride specimen. On the other hand, in 3%NaCl solution, the fatigue strength of the untreated specimen was almost similar to the result in laboratory air, while the fatigue strength of the nitrided and the STAN specimens decreased significantly compared to those in laboratory air. This behavior is attributed to the sensitization resulted from the precipitation of CrN. In the STAN specimen, the strain-induced martensitic transformation was not detected in run-out specimen at fatigue limit by X-ray diffraction method, indicating that the γ-phase was stabilized by the solid solution of nitrogen. Similarly, also in EBSD analysis, the strain induced martensitic transformation was not seen at the wake of crack in the STAN specimen.