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

The ratcheting behavior of the 9%-Cr-1%-Mo ferriticemartensitic (FM) steel P91 is investigated by uniaxial strain-and stress-controlled cyclic loading tests at room temperature and 550 degrees 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. (C) 2015 Elsevier B.V. All rights reserved.