Study on creep-fatigue evaluation procedures for high chromium steels-Part II: Sensitivity to calculated deformation

High-chromium steels containing 9-12% chromium such as ASME SA-213 Grade 91 or Grade 122 are widely used in conventional and combined cycle fossil power plants and are also regarded as candidate structural materials for future nuclear power plants aiming at operation in the creep range. Evaluation of failure life under creep-fatigue conditions constitutes an important part of assessing the structural integrity of these plants. The author has been conducting a series of creep-fatigue tests for three types of high-chromium steels and the validity of life prediction methods has been evaluated using the measured deformation data. Here an additional exercise was carried out in order to evaluate the adequacy of the total life prediction procedure, including the process of predicting stress and strain. After making comparisons of the test data with various existing equations developed for describing deformation and failure behaviour of Grade 91 and Grade 122, prediction of creep-fatigue life was attempted using deformation behaviour analytically estimated by these equations. Many calculations revealed that failure lives predicted by the time fraction approach showed a strong dependency on stress relaxation behaviour, whereas those based on the modified ductility exhaustion method showed a much smaller sensitivity, and therefore some error or uncertainty in stress and strain would be tolerated