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Seokmin Hong,Se‑Mi Hyun,Jongmin Kim,Yo‑Seob Lee,Maan‑Won Kim,Min‑Chul Kim 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.3
To apply the leak-before-break (LBB) design to main steam line (MSL) piping in nuclear power plants, higher strength andrelatively tough SA508 Gr.1A steel were developed by changing the alloy design and fabrication process. Two actual-sizeprototype types of MSL piping (M1 and M2) with different contents of C, Mo, and V were fabricated and their mechanicalproperties were evaluated and compared with those of commercial SA508 Gr.1A steel. In the alloy design, the content ofC was decreased to reduce the degree to which cementite deteriorated the toughness, and Mo and V were added to increasethe strength by forming bainite and VC precipitates. During the fabrication process, the thickness of the MSL piping wasminimized by rough machining before a heat treatment to increase the strength by increasing the cooling rate during quenching. Both prototype steels had better strength and toughness than commercial steel. In particular, the mechanical propertiesof M2 with lower alloy contents had a good combination of strength and toughness. The yield strength, USE, and JIcof M2were 407 MPa, 393 J and 680 kJ/m2 at 286 °C, respectively, which were increased by 167 MPa, 90 J, and 98 kJ/m2 comparedto the correspondingly parameters of commercial steel. The LBB margin of MSL piping using M2 steels was 1.49, whichwas a 30% higher than those using commercial steels.
Seokmin Hong,Se‑Mi Hyun,Jongmin Kim,Jongmin Kim,Yo‑Seob Lee,Maan‑Won Kim,Min‑Chul Kim 대한금속ᆞ재료학회 2023 METALS AND MATERIALS International Vol.29 No.2
To apply the leak-before-break (LBB) design to main steam line (MSL) piping in nuclear power plants, higher strength andrelatively tough SA508 Gr.1A steel were developed by changing the alloy design and fabrication process. Two actual-sizeprototype types of MSL piping (M1 and M2) with different contents of C, Mo, and V were fabricated and their mechanicalproperties were evaluated and compared with those of commercial SA508 Gr.1A steel. In the alloy design, the content ofC was decreased to reduce the degree to which cementite deteriorated the toughness, and Mo and V were added to increasethe strength by forming bainite and VC precipitates. During the fabrication process, the thickness of the MSL piping wasminimized by rough machining before a heat treatment to increase the strength by increasing the cooling rate during quenching. Both prototype steels had better strength and toughness than commercial steel. In particular, the mechanical propertiesof M2 with lower alloy contents had a good combination of strength and toughness. The yield strength, USE, and JIcof M2were 407 MPa, 393 J and 680 kJ/m2 at 286 °C, respectively, which were increased by 167 MPa, 90 J, and 98 kJ/m2 comparedto the correspondingly parameters of commercial steel. The LBB margin of MSL piping using M2 steels was 1.49, whichwas a 30% higher than those using commercial steels.
Effect of Intercritical Heat Treatment on J-R Fracture Resistance of SA508 Gr.1A Low-Alloy Steels
Se‑Mi Hyun,Seokmin Hong,Min‑Chul Kim,Jongmin Kim,Seok Su Sohn 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.12
SA508 Gr.1A low-alloy steel is used for main steam line piping in nuclear power plants. The yield strength and J-R fractureresistance of the piping material must be high in order to apply the leak-before-break concept. In this study, intercritical heattreatment (IHT) was applied to two SA508 Gr.1A low-alloy steels having different chemical compositions, namely, samplesC and P; the latter containing Mo and V. Their microstructures, tensile properties, impact properties, and J-R fractureresistances were evaluated, and the effects of IHT on the mechanical properties of the steels were analyzed. After IHT, finegrains formed at grain boundaries and coarse cementite decomposed, which greatly improved toughness without reducingthe strength. Further, a model to predict the J-R fracture resistance (JIc) of the SA508 Gr.1A low-alloy steel was developedby considering the microstructural and mechanical factors that affect the J-R fracture resistance. According to the JIc resultsof 12 kinds of SA508 Gr.1A low alloy steel, subjected to different heat treatments, the JIc was linearly proportional to thenumber of effective grains (N) contained in the plastic zone, which showed that N is the main factor affecting JIc. Furthermore,the yield and tensile strengths were considered to compensate for the strength difference owing to the different chemicalcompositions. The JIc prediction model was derived considering the effective grain size, plastic zone size, and tensile properties. The predicted JIc values agreed well with the test JIc values.