Cyclic softening of tempered ferritic-martensitic steel P91

For the ferritic-martensitic steel P91 cyclic softening has been identified as important factor in the assessments of creep-fatigue in nuclear components. Better understanding of the cyclic softening is needed as it is believed to be the main contributor for the complex material behavior encountered in creep-fatigue testing. In fast reactor components the main life shortening load cycles are from thermal fluctuations causing through-wall stress gradients. The main objective of this work (as a Task in the European project MATISSE) is to perform tailored experiments and to develop models for understanding and predicting the cyclic-softening of P91. The supporting experimental program produces data for calibration and validations of the developed models. An existing elasto-viscoplasticity model simulates the key mechanisms such as decrease in dislocation density, increase in sub-grain size and recovering phenomena. The model will be modified to also incorporate the effect of hold time on the cyclic softening.