2019 年 68 巻 2 号 p. 136-141
Header stub welds for thermal power plants undergo creep-fatigue damage due to thermal expansion and contraction, which are caused by cyclic startup and shutdown of the plant. In this study, creep-fatigue tests were conducted using header stub mock-up specimens of 47Ni-23Cr-23Fe-7W alloy to investigate the creep-fatigue damage process. It was discovered that the cracks were initiated from the outer surface of the tube near the bond line, and they propagated toward the inner side in both fatigue and creep-fatigue tests. Transgranular cracks were observed in fatigue tests, whereas cracks were found to be progressed along the grain boundary in creep-fatigue tests. The failure mechanism of 47Ni-23Cr-23Fe-7W alloy header stub mock-up specimens was composed of two steps. At first, cracks progressed on the surface near the bond line, and then cracks progressed toward the inner surface. Crack initiation and propagation behaviors of the 47Ni-23Cr-23Fe-7W alloy header stub mock-up specimen were the same as those of the 2.25Cr-1Mo steel header mock-up specimen.