Influence of phase transformation on the creep deformation mechanism of SA508 Gr.3 steel for nuclear reactor pressure vessels

Abstract

This study analyzed the effect of phase transformation on the creep behavior and deformation mechanism of SA508 Gr.3 steel used for nuclear reactor pressure vessels (RPVs). The creep tests were conducted on the steel at the temperatures from 650 to 800 °C at a stress ranging from 15 to 80 MPa. Detailed microstructural examination and theoretical analysis were conducted on the specimens to investigate the creep mechanism in each testing condition. The results showed that phase transformation induced an increase in the minimum creep rate and a decline in creep life. Based on the calculations of stress exponents, activation energies, Larson-Miller and Orr-Sherby-Dorn parameters, the creep deformation and fracture behavior were deemed to be different before and after phase transformation. Before the phase transformation, the predominant creep-deformation mechanism was dislocation gliding whereas it was converted to dislocation climbing after the phase transformation. During the phase transformation, the dislocation gliding coupled with grain-boundary sliding became the rate-controlling creep mechanism. This study provides a comprehensive insight into understanding the creep deformation of the RPV steel, giving a direct guideline to optimize the parameters for in-vessel retention strategies.