Effect of -Phase Embrittlement on Fatigue Behavior in High-Chromium Ferritic Stainless Steel
Masayuki AKITA, Yoshihiko UEMATSU, Toshifumi KAKIUCHI, Keiro TOKAJI and Masaki NAKAJIMA
Abstract:The purpose of the present study is to evaluate the effect of -phase embrittlement on fatigue behavior in high-chromium ferritic stainless steels. When ferritic stainless steels are exposed to the temperature range of 700-800C, the precipitation of -phase occurs. The precipitation brings about the embrittlement of materials which is known as the -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 750C due to the precipitation of hard -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 -phase and the increase of hardness were not recognized by the aging at 750C. Axial fatigue tests were performed using type 447 aged at 750C for two different aging periods of 150h and 300h, and the effect of -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 -phase and softening of ferritic phase reduced both crack initiation and growth resistances of the aged specimens, resulting in the lower fatigue strengths. Key Words:Fatigue, Ferritic stainless steel, -phase embrittlement, Microstructure, Crack initiation, Crack growth behavior