Dependence of crack growth rate on cyclic loading period of alloy 625 in high temperature steam and dry gas environments

In order to achieve higher thermal efficiency and decreased emissions, Advanced ultra supercritical (A-USC) steam power plant in which the main steam temperature is raised above 700°C, is developing. Ni-base superalloys have been considered as a candidate material for use in the high-temperature section of an A-USC plant. Since these materials are exposed to the high-temperature steam, it is important to investigate environmental effects on material degradation. There is only limited knowledge about the environmental effects on the cracking behavior of Ni-base superalloys in the temperature range proposed for an A-USC plant. In this study, the crack growth rate of alloy 625 at 750°C in a steam environment was obtained under cyclic loading using a compact tension specimen. Results were compared with those obtained in a dry gas environment, and it was found that crack growth rates in the steam environment were faster than those in air. According to the observation of oxides formed at the crack tip region, it was thought that steam oxidation enhanced intergranular crack growth. In a longer cyclic period, the cause of the enhancement in the growth rate is probably due to the interaction between environmental fatigue and creep.