SA508 Gr.3 Mn-Mo-Ni계 저합금강의 두께방향 깊이에 따른 탄화물의 석출거동과 천이영역 파괴인성의 변화 (Characterization of Precipitation Behavior and Fracture Toughness along Thickness Direction in SA508 Gr.3 Mn-Mo-Ni low alloy steels)

서지정보

· 발행기관 : 대한금속·재료학회
· 수록지 정보 : 대한금속·재료학회지 / 53권 / 10호 / 700 ~ 710페이지
· 저자명 : 송재민, 김민철, 홍석민, 이봉상

초록

SA 508 Gr.3 Mn-Mo-Ni low alloy steel forgings thicker than 200 mm are used for reactor pressure vessels in nuclear power plants. The cooling rate difference along the thickness direction during the quenching process causes variation in the microstructure and mechanical properties. The microstructural variation and its influence on the fracture toughness of RPV steels were investigated in this study. The cleavage fracture toughness in the transition region were evaluated with the ASTM E1921 master curve method for samples at different locations from the inner surface to the center thickness of the RPV steel. The microstructural features, such as the area fraction, and the size and distribution of precipitates were quantitatively evaluated at each sampling position. Microstructure observations showed that at near the surface position, bainite laths are finer, and furthermore, the carbides are smaller and homogeneously distributed. The fracture toughness at the surface was better than those at deeper positions. The reference temperature T0 showed a linear relationship with the area fraction of the carbides bigger than a certain critical size. It is concluded that the size of the precipitates caused by the cooling rate gradient may have a dominant role in controlling the cleavage fracture toughness variation along the thickness direction for a very thick RPV steel.

영어초록

SA 508 Gr.3 Mn-Mo-Ni low alloy steel forgings thicker than 200 mm are used for reactor pressure vessels in nuclear power plants. The cooling rate difference along the thickness direction during the quenching process causes variation in the microstructure and mechanical properties. The microstructural variation and its influence on the fracture toughness of RPV steels were investigated in this study. The cleavage fracture toughness in the transition region were evaluated with the ASTM E1921 master curve method for samples at different locations from the inner surface to the center thickness of the RPV steel. The microstructural features, such as the area fraction, and the size and distribution of precipitates were quantitatively evaluated at each sampling position. Microstructure observations showed that at near the surface position, bainite laths are finer, and furthermore, the carbides are smaller and homogeneously distributed. The fracture toughness at the surface was better than those at deeper positions. The reference temperature T0 showed a linear relationship with the area fraction of the carbides bigger than a certain critical size. It is concluded that the size of the precipitates caused by the cooling rate gradient may have a dominant role in controlling the cleavage fracture toughness variation along the thickness direction for a very thick RPV steel.

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