Non-Conservatism of ASME BPVC Section III Division 5 Isochronous Stress-Strain Curves for 316H Stainless Steel at Low Stresses

Future Gen IV high-temperature reactors are expected to operate above 450C where creep effects are significant in safety-related structures, e.g., reactor vessels. ASME BPVC Section III Division 5 provides the rules and methodologies for design of such high-temperature components. Of relevance to the Designer are the isochronous stress-strain curves (ISSCs) part of the rules for deformation limits in the Code. The ISSCs are an important method to estimate accumulated inelastic strains at a given stress and duration at elevated temperatures. In this study, the ISSCs for 316H steel in the current edition of ASME BPVC Section III Division 5 have been re-evaluated between 593-750C by adopting a physics-informed minimum creep rate model to re-construct them. It is demonstrated that the current ASME Section III Division 5 minimum creep rate model underpredicts creep rates compared to experimental data at low stresses (e.g., 650C, 40 MPa). By employing a physics-informed minimum creep rate model capturing both diffusive- and dislocation glide/climb-controlled creep regimes, this deficiency is addressed. The ASME ISSCs for 316H stainless steel are then reconstructed by adopting this modified minimum creep rate model. It was found that the ASME ISSCs could underestimate total accumulated strains at ~S/Sy of 0.65 for durations of 1,000 hr by 10 times which could give rise to non-conservatism in inelastic strain. Experimental data at various temperatures confirm the findings. Potential approaches to address this non-conservatism are discussed.