Creep-fatigue assessment of EUROFER taking into account the impact of cyclic softening on its creep behavior

Recently, creep-fatigue (CF) rules in existing design criteria were modified for cyclic softening steels, e.g. EUROFER, taking into account the impact of cyclic softening on the evolution of creep damage and, hence, creep lifetime. Therefore, a cyclic softening stress factor was introduced whose dependence on cyclic softening is described by a power function and an additional material and temperature dependent parameter. For determining this parameter creep tests on material specimens pre-cycled/pre-softened to different levels need to be conducted which was successfully done for EUROFER at 550°C. In this work the creep tests on pre-softened EUROFER are extended to 450°C demonstrating similar remarkable effect of cyclic softening on creep properties as at 550°C. Moreover, the Monkman-Grant relationship between creep lifetime and minimum creep rate was also found being independent on cyclic softening and even independent on temperature indicating no change in creep damage mechanism within the temperature range investigated so far.
To verify the modified CF rules low cycle fatigue (LCF) tests with long relaxation as well as long creep holdtimes are conducted at 450°C and 550°C, respectively. ... mehrEvaluating the results in comparison lo the existing CF rules in the ASME-BPV and RCC-MRx design codes demonstrates once more the applicability of the modified CF rules to cyclic softening steels without being overly conservative. •