Creep deformation phenomena in near-surface carburised layers of 316H stainless steels

The local creep-induced phenomena occurring within the near-surface carburised layer in two pre-carburised type 316H austenitic stainless steels, containing 0.98 or 1.52 wt% Mn, was assessed at 550 °C. The pre-carburisation exposed both steel grades at either 550 °C (8000 h) or 600 °C (3000 h) to simulated CO₂-rich gas environment of UK Advanced Gas-cooled Reactors. The local properties within the carburised layer causes a reduction in minimum creep rate as compared to the non-carburised condition. In both steels carburised at 600 °C, ferrite is observed to form in the most heavily carburised regions, including slip bands, during the initial plastic deformation to the target applied stress. The ferrite area fraction further increases with creep strain. Within the ferritic areas, new fine grains are observed in the creep tested specimens and the fraction of high angle grain boundaries is >60%. Those fine grains have a lower degree of lattice strain compared to the surrounding ferrite area and especially to the austenite matrix. The lattice strain localises within the carburised layer, and the formation of ferrite from the strained austenite, and potentially a dynamic recrystallization within the ferrite regions, pinpoints to a dominant strain relief-driven mechanism. Lattice strain localises primarily in the pre-carburised austenite grains and its carbide-decorated boundaries, and induces the continuous formation and refinement of ferrite grains, and the development of intergranular cracks that evolve further into the bulk structure. These results emphasise the necessity to account for environmental-creep interactions at elevated temperatures in structural assessment procedures, in the current gas-cooled fission reactor fleet and beyond into (very) high-temperature Gen IV and advanced modular reactor designs.