Type IV creep damage initiates in Fine Grain Heat Affected Zone (FGHAZ) in welded joints that has been subjected to the stress at high temperature for years. The shape of grain boundaries in FGHAZ is complex and complicated before it was subjected to creep. As creep goes on, the shape of grain boundaries becomes simpler and the average grain size becomes larger because of the recovery and the recrystallization. However average grain size is one of indications of creep damage, the complexity of grain boundary shape has not been researched as the indication of creep damage. The grain boundary is the fractal and the complexity of the shape of grain boundaries is represented by the fractal dimension. In this study, we carried on some creep tests with the simulated FGHAZ of Mod. 9Cr-1Mo steel and measured the fractal dimension of grain boundaries of them. The shapes of the test piece were the notched bar, the smooth bar and the smooth square bar. An optical microscope and SEM were used to observe the metallographic structure. Box-counting method was used to measure the fractal dimension.In the smooth test pieces, the average grain size became larger and the fractal dimension became smaller with creep. In the notched test piece, the average grain size became larger but the fractal dimension did not change with creep. The fractal dimension can be an indication of creep damage under uniaxial stress. As the indication of type IV creep that initiates under multi-axial stress, the fractal dimension of grain boundaries needs to be more researched.
