Creep-Fatigue Life Evaluation of Mod.9Cr-1Mo Steel Round Notch Bar Specimens

Abstract

Modified 9Cr-1Mo steel is used for high temperature components such as steam turbine rotors because of its superior high-temperature property. Creep-fatigue damage progresses preferentially at stress concentration portions in the steam turbine rotors during cyclic start up- shut down operation. Therefore development of an accurate creep-fatigue failure life assessment method under multiaxial stress states at the stress concentration portions is necessary to maintain reliable operation. In this study, creep-fatigue tests using three kinds of round notch bar specimen on a modified 9Cr-1Mo steel and finite element(FE) creep analyses of the specimens have been conducted to clarify effect of multiaxial stress conditions on failure life. Larger life reduction of all kinds of notch specimen under the creep-fatigue tests than the fatigue lives was observed at lower strain range. Observation of notch root of the creep-fatigue damage specimens by a scanning electron microscope suggested that most of the creep-fatigue life was occupied by crack propagation up to 1mm from the notch root surface. Appropriate life prediction results were not obtained in the case of using the FE analysis results at the notch root surface. On the other hand, creep-fatigue lives of notch specimens were accurately predicted by applying the damage area rate concept, in which average creep-fatigue damage value was calculated within 1mm from the notch root surface, with both nonlinear damage accumulation model and ductility exhaustion theory.