Affiliations: National Institute for Materials Science, Tsukuba, Japan
Note:  Address for correspondence: Megumi Kimura, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. Tel.: +81 29 859 2246; Fax: +81 29 859 2201; E-mail: KIMURA.Megumi@nims.go.jp
Abstract: Axial strain-controlled fatigue tests of a 9 mass% chromium (Cr)–2 mass% tungsten (W) and a 12Cr-2W ferritic heat-resisting steels were performed to obtain fatigue life curves (S–N curve) over a wide from 102 to 106 cycles at room temperature (RT), 400, 600, 650 and 700°C, respectively. As the results, Manson–Coffin relationships between plastic strain range and fatigue life for the both the steels appear linear at RT and 400°C, however the relationships have inflection around 104 cycles beyond 600°C. Moreover, it is cleared that subgrains became coarse uniformly in the low-cycle regions, while subgrains did coarse locally near grain boundaries in the high-cycle regions. Therefore it is suggested that differences in the mechanical factors and change in subgrain structure cause the inflection of Manson–Coffin relations at the elevated temperatures. Then, a modified universal slope method is proposed for S–N curves that considered to inflection in Manson–Coffin relations.