Cyclic stress-strain properties of service-exposed ferritic steels for use in thermal fatigue assessments

This paper is concerned with the laboratory isothermal (low cycle) fatigue response of sixteen ferritic steel samples taken from power station components which have seen between 70,000 h and 170,000 h service exposure to high temperatures. After suitable machining into specimens the cyclic stress-strain properties between 300 and 600degreesC at strain rates between 6 X 10(-4)/s and 5 X 10(-6)/s were established using the multiple step technique (which is discussed in some detail). Ferritic steels generally cyclically soften, and by evolutionary tests undertaken at 550degreesC it is demonstrated that start/up and shut/down procedures in service have accelerated this process in material subjected to temperature transients. The steady-state cyclic stress-strain response has shown a marked loss in strength compared with data on a selection of unexposed steels e.g. material trepanned from turbine casings at the manufacturing stage or from specially cast test blocks. The data have been fitted to the Ramberg-Osgood law and are presented in Tables for direct application to remanant life assessments requiring stress analysis. The effects of artificial ageing are compared with the response of service-exposed material. Full chemical analyses and abbreviated service histories of the materials are presented together with graphical illustrations of trends in the data extracted from the Tables.