Type IV creep damage occurs in welded joints of high chromium steel in power plant. The method based on the number density of voids is one of the methods to evaluate the residual life. The observed number density of void depends on the observation condition; the observation area and the magnification of observed photograph of metallographic structure. In previous study we had studied influence of them and had proposed how to determine the appropriate observation area for the temporary allowable error. In this paper we experimentally show that the appropriate area does not depend on heats or heat treatments. Next, we proposed the method to evaluate the start time of initiation of voids, the initiation rate of voids and the growth rate of voids. The relation of observed number density of voids and the magnification of observed photograph was experimentally taken. Based on it, the distribution of the radius of voids changing with creep time was evaluated. With it these parameters are evaluated. In the future influence of the stress triaxiality on these parameters will be studied.
