Effect of σ-phase Embrittlement on Fatigue Behavior in High-Chromium Ferritic Stainless Steel

The purpose of the present study is to evaluate the effect of σ-phase embrittlement on fatigue behavior in high-chromium ferritic stainless steels. When ferritic stainless steels are exposed to the temperature range of 700∼ 800°C, the precipitation of σ-phase occurs. The precipitation brings about the embrittlement of materials which is known as the σ-phase embrittlement. Vickers hardness of a high-chromium ferritic stainless steel, type 447 with a chromium content of 30.7% increased significantly by the aging at 750°C due to the precipitation of hard σ-phases which are Cr- and Mo-rich inter-metallic compounds. On the other hand, in type 444 whose chromium content was 18.7%, the precipitation of σ-phase and the increase of hardness were not recognized by the aging at 750°C. Axial fatigue tests were performed using type 447 aged at 750°C for two different aging periods of 150h and 300h, and the effect of σ-phase embrittlement on the fatigue behavior was discussed. The aged type 447 exhibited much lower fatigue strengths than the un-aged one, where fatigue strengths decreased with increasing aging time. The brittle fracture of σ-phase and softening of ferritic phase reduced both crack initiation and growth resistances of the aged specimens, resulting in the lower fatigue strengths.