Effect of σ-phase embrittlement on fatigue behaviour in high-chromium ferritic stainless steel

When ferritic stainless steels are exposed to the temperature range of 700–800 °C, embrittlement takes place, which is known as the σ-phase embrittlement. In the present study, axial fatigue tests were conducted using a high-chromium ferritic stainless steel, type 447 with the chromium content of 30.7%, which was aged at 750 °C for 150h and 300h, and the effect of σ-phase embrittlement on the fatigue behaviour was discussed. Vickers hardness increased significantly by aging due to the precipitation of hard σ-phase. The aged specimens exhibited much lower fatigue strengths than the un-aged one, where fatigue strength decreased with increasing aging time. The precipitation of brittle σ-phase reduced the crack initiation and growth resistances, resulting in the lower fatigue strengths of the aged specimens.