Effect of two-step tempering treatment on microstructure and impact toughness of bainitic steel for heavy wall thickness reactor pressure vessels

SA508 Gr.3 steel is widely applied for the fabrication of reactor pressure vessels, which require high impact toughness to ensure the safety of nuclear power plants. In this work, a two-step tempering treatment is proposed to manufacture the SA508 Gr.3 steel with high toughness. A pre-tempering at 400 degrees C was added before the ultimate tempering of intercritical heat treatment process. The ductile-brittle transition temperature of the steel processed with the new heat treatment is significantly decreased by 26 degrees C. The scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy characterizations show that the decomposition of M-A constituents into dispersed fine carbide particles and the increase of high-angle grain boundaries contribute to the decrease in ductile-brittle transition temperature. The results suggest that the modified two-step tempering treatment is effective to enhance the impact toughness of SA508 Gr.3 steel.