Study on observation condition and on simple evaluation method of TypeIV creep damage of Mod.9Cr-1Mo steel

Type IV creep damage is the phenomena that a large number of voids in micron size initiate, grow, coalesce each other and become large cracks. They initiate in welded joints of high chromium steel in power plant. The density of voids, the number of voids per a unit area, is used to evaluate the residual life. The observed density of voids depends on the observation conditions; the observation area and the magnification of observed photograph of metallographic structure, because voids do not distribute uniformly and the small magnification misses the small voids. In previous study, we studied the influence of them with simulated fine-grain HAZ of Mod.9Cr-1Mo steel, and proposed how to determine the appropriate observation area for the temporary allowable error. We also proposed the method to evaluate the start time of initiation of voids, the initiation rate of voids and the growth rate of voids based on the relation between the observed density of voids and the magnification. But the experimental data was short. In this paper we showed new data, but they were yet not sufficient. We used FEM analysis and considered why enough data had been not taken.