Investigations on the evaluation of the residual fatigue life-time in austenitic stainless steels

In view of plant life extension of nuclear power plants, many efforts are taken to assess the structural integrity of components affected by service, such as the components of the primary circuit, but also the auxiliary and safety systems. Frequently damage in components during operation is caused by cyclic loading, due to mechanical or thermal fatigue. Fatigue damage often involves loads, which were not taken into account in the design e.g. temperature cycling arising from unforeseen stratification flow conditions. Therefore lifetime calculations should be supported by non-destructive measurements on the components during the operation life to guarantee their integrity, by monitoring of the changes in the microstructure, and the related mechanical and physical material properties, which are due to fatigue damage. Those changes of the microstructure appear in a period before crack initiation, which covers a considerable part of the fatigue life. To gain information on the changes in the microstructure during fatigue loading, samples of the stainless steel materials 1.4541 and 1.4550, which are the representative materials for the majority of auxiliary and safety systems, were strained under static and dynamic conditions at different temperature between RT and 300°C in order to correlate the fatigue loading conditions and residual lifetime with the microstructural phenomena. In particular the formation of deformation induced martensite was analysed, which is accompanied by pronounced changes in the magnetic properties. Non-destructive testing methods (NDT), based on eddy current techniques, are of use to detect these changes in the magnetic properties. The results lead to an assessment scheme for the evaluation of the residual lifetime of components.