원전 내부구조물 오스테나이트 스테인리스강 용접부의 열시효 거동 및 열취화 활성화에너지 분석 (Analysis of Thermal Aging Behavior and Activation Energy of Austenitic Stainless Steel Weld for Reactor Vessel Internal)

서지정보

· 발행기관 : 대한금속·재료학회
· 수록지 정보 : 대한금속·재료학회지 / 63권 / 5호 / 330 ~ 343페이지
· 저자명 : 김유진, 김지수, 서주원, 김종민, 이수열, 홍석민

초록

This study examines the thermal aging behavior and associated changes in mechanicalproperties of austenitic stainless steel welds (ASSW) used within reactor vessel internals (RVI). ASSWspecimens were fabricated employing the Gas Tungsten Arc Welding (GTAW) technique, adhering to thewelding procedure specifications for RVI, and were thermally aged at 340°C, 400°C, and 450°C for durationsup to 15,000 hours. Assessments included strength, impact toughness, and delta-ferrite hardness, whilethermal embrittlement activation energy was estimated using nano-indentation combined with the Cahn-Hilliard model. Microstructural analysis revealed a delta-ferrite content of approximately 7%, whichappeared as fine dendritic structures (thickness <2 μm) within the austenite matrix. Spinodaldecomposition in the delta-ferrite led to the formation of Cr-rich α′ phases at all thermal agingtemperatures, with a noted increase in Cr concentration over time. Upon aging at 340°C for 14,000 hours,Ni-rich clusters were observed, indicating that extended aging could be necessary for G-phase precipitationat lower temperatures. Conversely, Ni-rich precipitates identified as a face-centered cubic (FCC) G-phaseemerged in delta-ferrite aged at 400°C and 450°C. The absence of Cr carbide precipitation supports aconsistent aging mechanism across the 340–450°C range. Aging resulted in a slight increase in tensilestrength, a decrease in elongation, and a significant reduction in impact toughness. Nano-indentationconfirmed an increase in delta-ferrite hardness, which was normalized using the austenite-to-delta-ferritehardness ratio. Activation energy for thermal embrittlement was calculated to be 100 kJ/mol for the 340–400°C range and 77 kJ/mol for the 340–450°C range.

영어초록

This study examines the thermal aging behavior and associated changes in mechanicalproperties of austenitic stainless steel welds (ASSW) used within reactor vessel internals (RVI). ASSWspecimens were fabricated employing the Gas Tungsten Arc Welding (GTAW) technique, adhering to thewelding procedure specifications for RVI, and were thermally aged at 340°C, 400°C, and 450°C for durationsup to 15,000 hours. Assessments included strength, impact toughness, and delta-ferrite hardness, whilethermal embrittlement activation energy was estimated using nano-indentation combined with the Cahn-Hilliard model. Microstructural analysis revealed a delta-ferrite content of approximately 7%, whichappeared as fine dendritic structures (thickness <2 μm) within the austenite matrix. Spinodaldecomposition in the delta-ferrite led to the formation of Cr-rich α′ phases at all thermal agingtemperatures, with a noted increase in Cr concentration over time. Upon aging at 340°C for 14,000 hours,Ni-rich clusters were observed, indicating that extended aging could be necessary for G-phase precipitationat lower temperatures. Conversely, Ni-rich precipitates identified as a face-centered cubic (FCC) G-phaseemerged in delta-ferrite aged at 400°C and 450°C. The absence of Cr carbide precipitation supports aconsistent aging mechanism across the 340–450°C range. Aging resulted in a slight increase in tensilestrength, a decrease in elongation, and a significant reduction in impact toughness. Nano-indentationconfirmed an increase in delta-ferrite hardness, which was normalized using the austenite-to-delta-ferritehardness ratio. Activation energy for thermal embrittlement was calculated to be 100 kJ/mol for the 340–400°C range and 77 kJ/mol for the 340–450°C range.

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