Type IV creep damage in fine grained heat affected zone of Mod.9Cr-1Mo steel has been concerned and extensively studied for steam piping of thermal power plants. However, the evaluation of Type IV damage by EBSD measurement is not enough. Serviced materials that have been used for a long time in an actual equipment are considered to effective materials for investigating microstructural changes in a long-term region. In this study, SEM observation and EBSD measurement were performed on the material of the Mod. 9Cr-1Mo steel FGHAZ used on the actual equipment and interrupted test specimen. As a result, as the creep progressed, strain concentration due to dislocation accumulation was observed in the portion near the voids, and recovery due to dislocation reduction was confirmed in the non-void portion, and the mechanism of Type IV damage caused by local strain concentration was confirmed. ΔKAM, which is obtained by subtracting KAMlocal, which is the KAM value near the void portion, and KAM0, which is the KAM value of the nonvoid portion, showed a monotonic increase with respect to the creep damage rate t/tr. The possibility that creep damage can be evaluated by this index was shown. In addition to examining the estimation formula expressing the change of the KAM value, it was found that the macroscopic and local void numbers could be evaluated from the KAM value.
