Effect of microstructure evolution on the creep properties of a polycrystalline 316H austenitic stainless steel

Creep deformation and failure is one of the most critical life limiting factors of structural components used at elevated temperatures, such as in nuclear power plants. Understanding of the mechanisms of creep in nuclear power plant steels, such as Type 316H austenitic stainless steels, is still incomplete. It has been observed that long-term creep curves of initially solution-treated (ST) 316H stainless steels exhibit multiple secondary stages at the operational temperature and stress range. This paper probes the internal mechanisms for this complex phenomenon by correlating and quantifying the evolution of microstructural state (dislocations, precipitation and solid solution elements) and its mechanistic influence on the material's creep properties. This is examined for the first time by a multi-scale self-consistent crystal plasticity framework combined with a simple classical phase transformation model and thermal solute strengthening model. The novel integrated model is capable of describing a broad range of physical processes, including dislocation multiplication (hardening) and climb-controlled recovery, precipitation nucleation, growth and coarsening (Ostwald Ripening) and thermal solute dragging. The mechanisms responsible for the observed multiple secondary stages in the creep curves of initially solution-treated 316H stainless steels are explained through the strengthening and softening effects associated with these processes.