Creep-fatigue tests strain-controlled with different strain amplitudes and different hold times at 725 were done on nickel-based alloy 617 as a typical candidate material for turbine rotor of advanced ultra-supercritical power plant. Stress relaxes during the hold time when the strain remains at the tensile peak. The analysis of the stress relaxation during different strain hold times shows that the ratio of the relaxation stress and the maximum stresses has strong correlation with strain amplitude and hold time. The failure life also has a certain dependence on the relaxation stress ratio. The failure life decreases and the relaxation stress ratio increases as the strain amplitude increases. The failure life decreases and the relaxation stress ratio increases as the hold time increases. Therefore the stress relaxation ratio was used as an intermediate variable to obtain the corresponding relationship model by establishing the relationship between the relaxation stress ratio and the strain and the relationship between the relaxation stress ratio and the failure life. This model can be used to predict the creep-fatigue interaction life more simply and directly.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.