Microstructural analyses by FE-SEM and TEM were performed on a ferritic heat-resisting steel that contained 12mass% chromium and 2mass% tungsten to characterize its multi-scale structure, consisting of prior austenite grains, packets, blocks, subgrains and precipitates. The size distributions of the block, subgrains and precipitates were quantitatively evaluated before and after a creep-fatigue test to relate them to their creep-fatigue property. Our results showed that the occupancy of precipitates on prior austenite grain boundaries increased markedly and subgrains became coarse during the creep-fatigue test, while block size did not change. It is suggested that the growth of grain boundary precipitates and coarse subgrains plays an important role in the intergranular fracture mechanism caused by creep-fatigue.

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