Study on Quantitative Relation between Stress and Stress Multiaxiality and Initiation Rate, Growth Rate, and Initiation Start Time of Voids in Type IV Creep Damage of Modified 9Cr-1Mo Steel

Type IV creep damage is a problem in high-temperature steam piping made of high chromium steel at thermal power plants, and a method for evaluating the remaining life is required. In this study, we considered that void’s initiation and growth can be expressed by initiation rate f, growth rate h, and initiation start time t₁, and that stress and TF affect f, h and t₁. We also proposed the method to estimate f, h and t₁ by measuring the change of the distribution of radius of voids during creep test. The creep test conditions are (1) test temperature of 650 C, maximum principal stress σ₁ of 79.5MPa, and TF of 2.5 ~ 3.0, and (2) test temperature of 650C, maximum principal stress of 71.5MPa, and TF of 2.5 ~ 3.0. The influence of σ₁ to f, h and t₁ was quantified by comparing the result of test (1) and that of test (2).