Effect of solution treatment after nitriding on fatigue properties in type 304 stainless steel

• Fatigue tests of type 304 steel solution-treated after nitriding in laboratory air and in 3% NaCl solution.
• Improvement of static strength and hardness by this treatment.
• Increase in fatigue strength of the treated specimen in laboratory air and significant decrease in 3% NaCl solution.
• The deterioration due to the formation of chromium-depleted zone resulted from the precipitation of CrN.
• Stabilization of metastable austenitic phase in type 304 steel by the solid solution of nitrogen.

The solution treatment after nitriding (STAN) was performed to improve the fatigue strength in type 304 steel by means of the solid solution of nitrogen. The static strength and the hardness were improved by the STAN. In laboratory air, the fatigue strength of the STAN specimen increased compared to that of the untreated specimen but was lower than that of the nitrided 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 strengths of the nitrided and the STAN specimens decreased significantly compared to those in laboratory air. This behavior is attributed to the sensitization by the formation of chromium-depleted zone resulted from the precipitation of CrN. In the STAN specimen, the strain-induced martensitic transformation in the run-out specimen at a stress level of fatigue limit was not detected 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 crack wake in the STAN specimen.