Characterization and modeling of the ratcheting behavior of the ferritic-martensitic steel P91

The ratcheting behavior of the 9%-Cr-1%-Mo ferritic-martensitic (FM) steel P91 is investigated by uniaxial strain- and stress-controlled cyclic loading tests at room temperature and 550 °C. Ratcheting rates under multiple loading conditions are measured to build the database of P91 for the further application in generation IV fission reactors. The unconventional asymmetry of stress under strain-controlled tests at room temperature predicts the non-zero ratcheting with zero mean stress, which is approved in the stress-controlled tests. A unified viscoplastic deformation model for RAFM steels is further modified to adapt the ratcheting behavior of P91. The current model describes cyclic softening in strain-controlled LCF tests very well. However it strongly overestimates the uniaxial ratcheting rates in stress-controlled tests, due to application of the Armstrong-Frederick dynamic recovery rule. Based on further analysis of back stresses (BS), a new rule for dynamic recovery is designed to fit the ratcheting rates under multiple loading conditions.