Characterization of high temperature long-term fatigue properties for advanced ferritic heat-resisting steels

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.