Degradation of remaining mechanical properties of bainitic 2.25Cr1Mo steel during creep

Abstract

In this work, the microstructure and remaining mechanical properties of bainitic 2.25Cr1Mo steel plate used in steam generator of sodium cooled fast reactor during creep were systematically examined. Creep tests at 530 °C with a rupture time up to 18800 h reveals that the creep stress correlates linearly with the Larson-Miller parameter (PLM), while the creep plasticity is rather independent on PLM. The remaining strength parameters, such as yield strength (RP0.2), ultimate tensile strength (Rm) and hardness (H), are found to linearly decrease with PLM, while the remaining plasticity is seen to increase. It suggests that the application of creep stress to the steel accelerates the softening of the material. During the creep process, the dislocation density in bainitic matrix decreases, while the number density of carbides increases, sacrificing the solution strengthening and leading to a continual reduction in the lattice parameter. Although abundant fine carbides precipitate after creep rupture, the remaining strength is however reduced. Instead, the weakening effect due to reduction in dislocation and solute content overwhelms the strengthening of precipitation, resulting in a degradation of the remaining strength. The obtained remaining mechanical properties are attempted to be applied in the design and operation assessment of steam generator.