Abstract
Microcrack initiation and growth behavior were observed on the surface of nondegraded and degraded CrMoV cast steels under various strain wave shape loadings at high temperature. The degraded steel was taken from the inside of a main steam valve casing after 140 000 hours of service. No signigicant difference in crack growth behavior was observed under low cycle fatigue for either steel. However, the crack growth rate in the degraded steel was higher than that in the nondegraded one under creep-fatigue resulting from slow/fast straining. This was caused mainly by grown and coarsened grain boundary carbides. A residual life assessment method, based on microcrack growth and the statistical properties of microcrack length distribution, closely predicts the experimental results.
