Improvement of fatigue properties in type 304 stainless steel by annealing treatment in nitrogen gas

The effects of annealing in nitrogen gas on the fatigue and static mechanical properties were studied using type 304 austenitic stainless steel. Annealing in nitrogen gas was performed at 1100 °C and at 1200 °C for 30 h, resulting in the precipitation of nitrides at grain boundaries and inside grains at both temperatures. The nitrides were identified as a chromium nitride by EDX analysis. There were fewer nitrides present in specimens treated at 1200 °C than at 1100 °C. The higher solubility of nitrogen at 1200 °C lowered the amount of chromium nitride. The hardness and tensile tests revealed that the surface hardness and the static strength were significantly improved by annealing in nitrogen gas. The fatigue strengths of the annealed specimens significantly increased compared with that of the untreated specimen, and the specimen annealed at 1100 °C obtained a higher fatigue strength than that of the specimen annealed at 1200 °C. After fatigue tests at fatigue limit stress levels, a strain-induced martensitic transformation was observed in the untreated specimen because the austenite (γ) phase of type 304 steel is metastable. However, in the annealed specimens, the strain-induced martensitic transformation did not occur during fatigue tests at fatigue limit stress levels, indicating that γ was stabilized by nitrogen in the solid solution.