In power plants, Type IV creep damage and the softening structure initiate in FGHAZ (Fine Grain Heat Affected Zone) of high temperature steam piping made of high chromium steel. The evaluation methods of remaining life of the damaged piping are needed. Before the creep, the grain boundaries of FGHAZ are serrated and unclear. With increasing of creep damage, they become simpler lines and clearer. The grain boundaries are the fractal and their complexity are expressed with the fractal dimension. We studied on the method that measured the creep damage in FGHAZ with the fractal dimension. The creep tests were conducted with the notched specimens made of the simulated FGHAZ of Mod.9Cr-1Mo steel. Before the fracture they were observed with SEM and their fractal dimension were measured with the box-counting methods. The fractal dimension decreased with creep damage. The fractal dimension decreased rapidly when the creep strain was small and the stress triaxiality was large. This decrease in fractal dimension was saturated at some value. We defined the fractal dimension decreasing rate by dividing the decreased value of the fractal dimension by creep time. It correlated with the creep rate. Vickers hardness decreased and the average grain size increased with decreasing fractal dimension. Those observations suggested that the re-crystallized grain in FGHAZ grew during the creep. The saturation of the decrease in fractal dimension might represent dynamic re-crystallization that stopped the growth of re-crystallized grain. We made the assumption; the large stress triaxiality made the strain small and made the recovery rapid. It also made growth of re-crystallized grain rapid. Then the fractal dimension decreased rapidly. The sufficiently large re-crystallized structure was the softening structure. Therefore, the fractal dimension is probably useful for the prediction or the inspection of the softened structure.
