An investigation on the effect of aging on creep deformation of Alloy 800H/HT at 700 °C

High Temperature Gas Reactors (HTGRs) operate at 700–950 °C, offering advantages in efficiency and process heat. Alloy 800H/HT, approved for use up to 760 °C, is a key material for these reactors. However, its behaviour under abnormal thermo-mechanical loads remains poorly understood. For example, our recent study in the University of Bristol (UoB) showed that aging treatment of Alloy 800H/HT at 700 °C for 100 hours prior to tensile and creep testing at the same temperature significantly reduces the creep resistance of the alloy compared to solution-annealed (SA) condition—findings that are consistent with the literature. The higher creep resistance in solution-annealed samples was attributed to the precipitation of M23C6 carbides during the early stages of creep. Nevertheless, standard modelling techniques (e.g., FEM) using empirically based continuum models such as RCC-MR, which are traditionally employed to predict power plant component lifespans, do not account for such metallurgical changes. This is a recognized limitation of current modelling approaches, particularly as safety requirements tighten and component lifetimes are extended. Therefore, the aim of this study is to capture the interaction between carbide precipitates with the lattice strain evolution in several grain families during the creep regime in Alloy 800H/HT by performing in-situ creep testing in three different heat treatments.