Axial strain-controlled fatigue tests of a 9Cr-2W and a 12Cr-2W ferritic heat-resisting steels were performed to obtain fatigue life curves (S-N curve) over a wide from10
2 to 10
6 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 show linear at room temperature and 400°C.
However, the relationships have inflection around 10
4 cycles beyond 600°C. There are differences in hystresis loops between in the low-and high cycle regions at these elevated temperatures. Moreover, it is cleared that subgrains became coarse uniformely 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 the relations at the elevated temperatures.
Then, a modified universal slope method is proposed for S-N curves that considered to inflection in Manson-coffin relation.
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