In order to establish creep damage assessment method for Gr.91 steel longitudinal welds by the ultrasonic phased array (UPA) method, creep rupture tests with large test specimens were carried out under 650°C, 50-60 MPa. All creep specimens ruptured at heat affected zone (HAZ). The UPA method was performed using interrupted creep specimens and ruptured specimens. The UPA indication was recognized near HAZ of the root part at creep life ratio of about 0.7. After that, the UPA indication was locally propagated thorough the wall thickness direction with increasing creep life ratio. Next, the relationship between UPA indication and microstructure was investigated. Many creep voids were observed at UPA indication position. Also, creep voids were initiated at recovery area of microstructure near HAZ of the root part. On the other hands, there were hardly creep voids at non-indication positions. It is thought that UPA method can identify the area of dense creep voids. This result suggests that the application of UPA method for Gr.91 steel longitudinal welds is effective as creep damage assessment like creep voids. Finally, in order to clarify the reason why recovery of microstructure progressed at the root part, EBSD (Electron Back Scatter Diffraction) method and ECCI (Electron Channeling Contrast Imaging) method were carried out. As a result, it was found that recovery and recrystallization of microstructure is easy to occur preferentially at the root part by stress concentrations.
