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Korean Journal of Metals and Materials > Volume 55(11); 2017 > Article
Korean Journal of Metals and Materials 2017;55(11): 752-759. doi: https://doi.org/10.3365/KJMM.2017.55.11.752
SA508 Gr.3 상용 원자로용기강의 두께방향 미세조직 변화가 인장 및 충격특성에 미치는 영향
홍석민1, 이초롱1,2, 김민철1, 이봉상1
1한국원자력연구원 원자력재료연구부
2과학기술연합대학원 양자에너지화학공학
Effects of Microstructure Variation on Tensile and Charpy Impact Properties in Heavy-Section SA508 Gr.3 Low Alloy Steels for Commercial Reactor Pressure Vessel
Seokmin Hong1, Cholong Lee1,2, Min-Chul Kim1, Bong-Sang Lee1
1Nuclear Materials Safety Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea
2Quantum Energy Chemical Engineering, University of Science & technology, Daejeon 34113, Republic of Korea
Correspondence  Min-Chul Kim ,Tel: +82-42-868-2761, Email: mckim@kaeri.re.kr
Received: 12 June 2017;  Accepted: 7 July 2017.  Published online: 31 October 2017.
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 AlN precipitates at high temperature, and suppressed the grain growth of austenite.
Keywords: reactor pressure vessels, Mn-Mo-Ni low alloy steels, SA508 Gr.3 Cl.1, microstructure, toughness
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