Evaluation of Creep Crack Growth Behavior of NCF800H Alloy Based on Creep Fracture Mechanism

In order to investigate the relationship between microscopical creep fracture mechanism and creep crack growth rate, creep crack growth tests were conducted on CT specimens of NCF800H alloy. Wedge-type intergranular fracture, transgranular one and cavity-type intergranular one were observed at 873K, 923K and 973K, and 1073K, respectively. For an equal value of C* parameter, creep crack growth rate was the fastest under wedge-type fracture condition, and secondary under cavity-type fracture condition and the slowest under transgranular fracture. Similar dependence of crack growth rate on fracture mode had been observed in creep crack propagation of a SUS316 steel. This corresponds with the difference of creep fracture ductility. Under the cavity-type fracture condition, creep crack growth rate increased as the damaged zone ahead of the crack tip increased. This effect could be explained by Riedel's crack growth model based on the grain boundary cavitation. Therefore, it is necessary to take the effect of creep damaged zone into account, when we evaluate the creep crack growth rate of structural components used for long time at high temperature.