Modeling the effect of load oscillations on creep curves and microstructural evolution of martensitic steels

:In this work, we evaluate a comprehensive physics-based mean-field creep model with respect to its capability to describe creep behavior under varying stress conditions. Accurately accounting for stress changes is essential for extending models calibrated using standardized constant-stress creep tests to industrially relevant loading scenarios involving variable stresses. Since the microstructure and its temporal evolution are intrinsic components of the model, external parameters such as stress can be modified at every calculation timestep.Until recently, the lack of suitable experimental data has hindered a thorough validation of the model under variable-stress conditions. A high-quality experimental dataset has now been provided by Kobayashi et al., who investigated the creep behavior of the widely used martensitic 9–10% Cr steel P91 at 650 °C under both constant and variable stress conditions, while continuously recording the creep strain in situ.Comparison of the experimental and simulation results shows that creep deformation during stress variations can be reproduced accurately, provided that the maximum applied stress remains significantly below the yield stress at the respective temperature (<60%). Exceeding this threshold appears to induce a change in the governing creep mechanisms, which is not yet fully captured by the current model.