Deformation mechanism-based true-stress creep model for SA508 Gr.3 steel over the temperature range of 450–750 °C

Abstract

Stress and temperature effects on the creep behaviors and mechanisms of a typical nuclear reactor pressure vessel material, SA508 Gr.3 steel, are investigated, over the temperature range of 450–750 °C and stress range of 10–400 MPa. Because of the importance of creep life prediction for nuclear reactor failure prevention, three creep models are assessed: Orr-Sherby-Dorn (OSD) and Larson-Miller (LM) parameter methods, and deformation-mechanism based true-stress (DMTS) model. The OSD model employs a single activation energy Q and stress exponent n, which shows a large discrepancy between the experimental and predicted time-to-strain (3% and 5%) data with a coefficient of determination (R2) less than 0.33 over the temperature range of 450–750 °C. Both the OSD and LM methods are effective in correlating the time-to-rupture with R2 ∼0.84 over a narrow temperature range of 650–750 °C. The DMTS creep model, on the other hand, characterizes the creep behavior in three normalized stress regions: low, intermediate and high, as dominated by grain boundary sliding (GBS), intragranular dislocation climb (IDC) and dislocation glide (IDG), respectively. The microstructural characteristics and creep damage mechanisms of SA508 Gr.3 steel are also examined using scanning/transmission electron microscopy to confirm the predominance of the aforementioned creep deformation mechanisms. The DMTS model provides a fully consistent description of the strain-time curves, the minimum creep rates (MCRs) and the time-to-strain/rupture as well. They are all in good agreement with the experimental observations, particularly with R2 ∼0.94, 0.995, and 0.79 for time to 3% strain, time to 5% strain, and time to rupture, respectively. These analyses demonstrate that the DMTS model is an effective tool in assessing creep properties of SA508 Gr.3 steel for in-vessel retention.