Search

The martensite-austenite constituents are known to deteriorate the impact toughness of alloyed steel. In order to reduce this adverse effect, the pre-tempering process at low temperatures was introduced before the tempering of intercritical heat treatment process of a MnNiMo bainitic steel. The effects of pre-tempering temperature on the decomposition of martensite-austenite constituents and impact toughness properties of the pre-tempered specimens were investigated. The results of instrumented Charpy V-notch impact testing show that the impact toughness of the materials can be significantly improved by the pre-tempering process, especially for the condition of pre-tempering at 400 °C. After pre-tempering at 400 °C, more martensite-austenite constituents are decomposed into ferrite and small carbides, resulting in the reduction of microcracks nucleation sites. The residual martensite-austenite constituents become mainly the island-like, which can effectively hinder the propagation of secondary cracks. In addition, the pre-tempering at 400 °C increases the fraction of high-angle grain boundaries and refines the size of blocks, which can effectively prevent or deflect the propagation of microcracks. All of those microstructure changes are beneficial to the crack propagation energy. The results suggest that the application of the pre-tempering process is a feasible method to enhance the impact toughness of the studied alloyed steel. • During the pre-tempering, more martensite-austenite constituents were decomposed. • The impact toughness of materials was significantly improved by the pre-tempering process. • The interactive effects among pre-tempering temperature, M-A constituents, and impact toughness were investigated. [ABSTRACT FROM AUTHOR]

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 °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 °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. Unlabelled Image • Developed a new heat treatment process to improve the impact toughness of SA508 Gr.3 steel with wall thickness of 500 mm. • Evolution of M-A constituents in conventional and new heat treatment processes was studied. • Effect of M-A constituents and carbides on crack initiation and propagation was investigated. [ABSTRACT FROM AUTHOR]