Material Behaviour and Development of Microstructure during Thermal-Mechanical Fatigue of a 12% Chromium Steel

Ferritic-carbidic steels with chromium contents ranging from 8 to 12% are important structural materials for thermally and mechanically heavily stressed components which operate at temperatures up to 550 °C and are used e.g. in power generating plants or in the chemical industry. The complex thermal-mechanical loading of these components usually comprises creep, high cycle fatigue and low cycle fatigue thermally induced by start-ups, load changes and shut-downs, producing instationary temperature gradients and hence strain as well as stress fields. In comparison with austenitic steels, high chromium ferritic-carbidic steels have a relatively low thermal expansion and a relatively high thermal conductivity. Therefore, it is to be expected that thermally induced stresses are generally lower than in austenitic steels. Up to now research work is mainly focused on the creep properties of these steels and the degradation of the microstructure during creep [1]. However, some investigations were carried out concerning the relationship between microstructure and thermal-mechanical fatigue behaviour, indicating that continuous cyclic softening occurs during thermal-mechanical fatigue (TMF) life [2, 3] and that it is combined with microstructural changes, which are important because they may change the material’s response to operating stresses.