Characterization of Multiaxial Creep Behaviors of 16MND5 Steel at Pre- and Post-phase Transformation

In-vessel retention (IVR) constitutes a pivotal strategy aimed at ensuring the structural integrity of the reactor pressure vessel (RPV). The lower cylinder is subjected to a high-temperature gradient, and the main failure mode of the RPV under IVR condition is attributed to multiaxial creep. At elevated temperatures, the pressure vessel steel 16MND5 undergoes a phase transformation from bainite to austenite, which significantly affects its creep behavior. The phase transformation temperature is distributed across the RPV wall thickness under the accident condition. Therefore, this study undertakes a comparative investigation of the macro- and micro-creep behavior of 16MND5 steel at pre- and post-phase transformation. The phase transformation temperature of the 16MND5 steel is determined to be 1024 K. Localized ablation and peeling of the RPV wall induce structural discontinuity, resulting in multi-axial stress. Accordingly, round bar specimens featuring varying U-notches were utilized for multi-axial creep test, and compared with uniaxial creep tests. Furthermore, the multi-axial creep stress of 16MND5 steel was analyzed by using finite element method, while the creep life was predicted by “skeletal-point stress method”. A good agreement is achieved between theoretical and experimental results.