Creep-fatigue strength of post-service 2 · 1/4 Cr-1 · Mo steel used for about one hundred-thousand hours in a fossil fuel power plant was studied. The creep-fatigue strength of the post-service material was lower than that of the virgin material, whereas it was comparable to that of thermally aged material, which was artificially exposed at high temperature for a long time so that it had an equivalent value of the Larson-Miller parameter to the post-service material. The nondestructive detection of the long-term degradation damage due to long-term thermal aging, as well as due to creep-fatigue, was also investigated by applying an ultrasonic technique. It was found that the energy attenuation coefficient, α, which is defined by the ratio of input to output energies of a longitudinal ultrasonic wave, had a good correlation with creep-fatigue damage in the virgin, aged and post-service materials; and hence, α was a successful parameter to detect creep-fatigue damage. Based on the results thus obtained, a new remaining life estimation method for creep-fatigue of in-service high-temperature materials was proposed. The application of the method to the post-service material tested gave good predicted results.

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