Abstract
A high-efficiency coal-fired power plant, calld an Advanced Ultra Supercritical (A-USC) is being developed. A-USC improves the efficiency to 46 to 48% and reduces CO2 emissions by 10% or more by improving main steam conditions by 100 °C or more compared to conventional USC. Ni-base superalloys is one of the possible candidates to replace the ferritic steels. In addition, lately, with the introduction of a large amount of renewable energy, it is suggested that coal-fired power plant requires the output adjustment operation to adjust supply and demand in the future. However, it is expected that the output adjustment would induce fatigue damage in which the oxidation of material interacts. So far, it has been clarified that the fatigue crack growth rate of alloy 617 in the 750 °C steam environment depends on the cyclic loading frequency, and it was found that crack propagated along grain boundary under lower cyclic loading frequency. Since selective oxidation of grain boundaries was observed on the fractured surface, it is thought that such oxides contributed to the reduction of grain boundary strength and cracks progressed selectively at grain boundaries under slower cyclic loading frequency.
