Creep-fatigue-oxidation interactions in a 9Cr-1Mo martensitic steel, part III. Lifetime prediction

A model devoted to the prediction of the high temperature creep–fatigue lifetime of modified 9Cr–1Mo martensitic steels is proposed. This model is built on the basis of the physical mechanisms responsible for damage due to the interaction of creep, fatigue and oxidation. These mechanisms were identified thanks to detailed observations previously reported in part I and part II of this study. These observations led to the distinction of two main domains, corresponding to two distinct types of interaction between creep, fatigue and oxidation. As no intergranular creep damage can be observed in the tested loading range, the proposed modelling consists in the prediction of the number of cycles necessary for the initiation and the propagation of transgranular fatigue cracks. Propagation rate measurements under high stress low-cycle fatigue conditions were carried out to calibrate the Tomkins model used to predict the life spent in crack propagation, whereas the initiation stage is predicted using the model proposed by Tanaka and Mura. The predictions obtained compare very favorably with the experimental creep–fatigue lifetimes. Finally the extrapolations and limits of the model are discussed.