Evaluation of crack growth behavior of alloy 625 under cyclic loading in a steam environment at 750°C

Advanced ultra supercritical (A-USC) steam power generation, in which high-pressure steam is raised to beyond 700°C, is being studied internationally. The creep strength of Ni-based super alloys evaluated at these high temperatures in an air environment makes these materials promising candidates for the material to be used for the structural components of these generators. Since they are exposed to high temperature steam, it is important that the effect of the environment on the degradation of these materials is investigated. In this investigation, the crack growth rate under cyclic loading in a 750°C steam environment using a compact tension specimen was evaluated. Crack length monitoring using the direct current potential drop technique was applied to the growing crack in a high temperature environment in order to evaluate the time-dependent behavior of the crack growth. The dependence of the loading rate and amplitude in terms of the stress intensity factor was obtained. The crack growth rate increased with decreasing loading rate and increasing amplitude. Multiple loading patterns were applied to a single specimen during crack length monitoring. When the loading pattern was changed to a different pattern, in most of the cases, the crack growth rate started to change and then became stable aftera transition period. The influence of intermetallics and different phases on the crack growth behavior is discussed based on the oxidation rate of these phases.