Effects of Microstructure Variation on Tensile and Charpy Impact Properties in Heavy-Section SA508 Gr.3 Low Alloy Steels for Commercial Reactor Pressure Vessel

摘要

In this study, the effects of microstructural variations in heavy-section reactor pressure vessel (RPV) steels on tensile and Charpy impact properties were investigated. Two PRV blocks, OPR1000 and APR1400 (ORV, ARV) were taken from the archive materials used in Korea standard nuclear power plants. Test specimens were sampled from five different positions at intervals of 1/4 thickness from the inner surface to the outer surface. The chemical contents in the ORV and ARV were homogenous, and macro-segregation problems were not observed. In the ORV, the microstructure was fully composed of bainite structure. The bainite lath was fine at the surface but it coarsened toward the center. In the ARV, coarse-lath upper bainite was formed at the surfaces but polygonal ferrite was observed toward the center. Variations in cooling rate due to the material's thickness caused changes in microstructure along the thickness during the water quenching heat treatment. The larger-sized ARV did not cool down quickly enough to create a fully bainitic microstructure. Tensile strength and Charpy impact properties tended to improve toward the surfaces. Compared to the ORV, the ARV showed higher strength and lower energy transition temperature (ETT). These variations in mechanical properties were related to the microstructure variations. Surface regions with the fine-lath bainite structure and small carbides showed better properties than the center region. The ARV, having smaller grain size and homogeneous carbide distribution, also showed better properties than the ORV. This is because the ARV was fabricated using an advanced steelmaking process, the VCD + Si + Al methods. The addition of Al resulted in the formation of AIN precipitates at high temperature, and suppressed the grain growth of austenite.