육상용 LNG 저장탱크 건설용 고Mn강 용접부 개선형상에 따른 파괴안전성 향상에 관한 연구

Abstract

Evaluation of fracture safety of high manganese austenitic steel welded joint according to groove shape for onshore LNG storage tanks Wonjun Jo Advisor : Prof. Gyubaek An, Ph.D. Department of Naval Architecture & Ocean Engineering, Graduate School of Chosun University Recently, the severity of global warming has increased around the world. Many countries are doing their best to reduce greenhouse gase emissions, which are the main cause of global warming. Hence, clean energy sources (such as hydrogen, ammonia) have become important solutions to address global warming caused by the excessive use of fossil fuels. However, the use of clean energy still has technical limitations, because of which LNG is drawing attention as an intermediate step to clean energy. As the demand for LNG increases rapidly, LNG storage and fuel tank construction technology are being actively developed in the shipbuilding and ocean industries. Additionally, in the energy industry, the construction of onshore LNG storage tanks are increasing. With the development of construction technologies, onshore LNG tanks are becoming larger and the applied steel thickness is increasing. LNG in storage tanks is stored at cryogenic temperatures, and hence, sufficient safety of the tanks at these temperatures must be ensured. Therefore, for LNG storage tanks, the materials suitable for use in cryogenic conditions include aluminum alloys, stainless steel, and 9% Ni steel. These existing materials have advantages and disadvantages with respect to the productivity and weldability. Recently, the economic feasibility of increasing the manufacturing cost of steel has become an important criterion for the selection of applied steel. therefore, a novel cryogenic steel has been developed that ensures safety at cryogenic temperatures by controlling the content of manganese (high manganese austenitic steel). Its applicability has been verified through the construction performance of onshore LNG stroage tanks and fuel tanks in shipbuilding. However, in the application of high manganese austenitic steel, there are disadvantages in productivity if the same weld groove shape is used as in conventional cryogenic steel. This is because the conventional weld groove shape has a relatively large groove angle. To improve productivity by reducing the shape used in conventional 2G and 3G welding processes, confirming that the strength and fracture safety of the reduced groove shape are equal to or higher than that of the conventional groove shape is necessary. In this study, the aim is to derive an optimal groove shape that satisfies the requirements of joint strength and fracture safety by reducing the groove angle of the weld joint used in the onshore LNG storage tanks. First, the characteristics of high manganese steel base metals and weld joints are reviewed. The review examines chemical compositions, mechanical properties, and microstructures. Second, the optimal groove shape from the point of view of the strength was derived through FE-analysis, and the optimal groove shape was verified by tensile tests. Finally, the derived optimal and conventional groove shapes were compared with respect to the welding residual stress. Subsequently, fracture toughness tests were conducted to evaluate the possibility of brittle fracture in high manganese austenitic steel weld joints. In addition, the possibility of unstable ductile fracture was evaluated using the δ–R curve test. From the results of the fracture safety of high manganese steel weld joints at cryogenic temperature, there is an insignificant difference in fracture toughness with change of groove shape. We infer no possibility of unstable ductile fracture.