Study on Influence of the Maximum Principal Stress and Stress Triaxiality Factor on Initiation Rate and on Growth Rate of the Voids of TypeIV Creep Damage in Mod.9 Cr-1Mo Steel

Type IV creep damage is the phenomenon that many voids initiate, coalesce, become large cracks and break the weldment. It occurs in FGHAZ (Fine Grain Heat Affected Zone) of high temperature steam piping made of high chromium steel in power plants. The method to estimate the remaining life of damaged weldments is needed. The stress and the stress triaxiality affect TypeIV creep damage, but the quantitative influence of them are not clear. We conducted the creep tests on the three conditions of the different maximum principal stress and of the different stress triaxiality factors with simulated FGHAZ of the Mod.9Cr-1Mo steel and got the damaged samples. The test temperature was 650C. In previous research, we had proposed the simple method that estimates the initiation rate and the growth rate of voids by the distribution of radius of void. We applied this method to the samples and quantified the influence of the maximum principal stress and TF (stress Triaxiality Factor) on the initiation rate and on the growth rate of voids. The radii of voids were measured by optical microscope. The results were as follows; under the same TF of 2.9, the maximum principal stress of 79MPa gave the larger initiation rate and the larger growth rate than 73MPa.Under the same stress of 79MPa, TF of 2.9 gave the larger growth rate than TF of 3.5, but the initiation rates on both conditions were almost the same.