Characteristics of Welding Residual Stress Distribution in Dissimilar Weld Joints

For the construction of ecofriendly ships, fuels such as liquefied natural gas (LNG), ammonia, and hydrogen are being discussed as alternatives. LNG fuel has recently been applied to shipbuilding. The most important aspect of an LNG propulsion ship is the LNG storage tank, because LNG is stored at a high pressure and low temperature. Cryogenic steels are needed to evaluate safety in weld joints, especially the LNG storage tank which has a dissimilar weld joint with a STS pipe. The dissimilar weld joint has a complex welding residual stress distribution. It is necessary to evaluate the effects of temperature changes that occur during the loading-unloading process of LNG. In this study, the residual stress distribution characteristics of heterogeneous welding parts welded to STS pipes using 9% Ni steel, STS, and high-manganese austenitic steel in an LNG storage tank were investigated through experimental and analytical methods. The thermal stress due to the difference in thermal expansion coefficient between cryogenic steel and the STS pipe occurred with a small amount in loading-unloading of LNG. When high-manganese austenitic steels and the STS pipe were joined, tensile stress was generated at the dissimilar weld joint owing to the temperature difference generated during the LNG loading-unloading process. STS has a homogenous weld joint and identical thermal expansion coefficients; therefore, the shrinkage and expansion were not affected by the temperature change. The welding residual stress at the dissimilar weld joints was measured via an experimental cutting method, and the results indicated that the tensile residual stress had distribution similar to the yield stress of the material. The stress generated by the temperature change and the welding residual stress overlapped and occurred during the loading-unloading process of the LNG tank; however, the final tensile stress below the tensile stress was distributed in the storage tank.