Characteristics of Welding Residual Stress Distribution in Dissimilar Weld Joints

Gyubaek An,Jeongung Park,Ilwook Han,Woongtaek Lim,Hongkyu Park

doi:10.3390/met12030405

Abstract

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.

Highlights

The problem of environmental pollution has risen worldwide, and concerns about environmental pollution at sea have increased
Efforts to apply ammonia and hydrogen are in progress, and liquefied natural gas (LNG) fuel is applicable to shipbuilding [6,7]
The materials used in the experimental examination were conventional cryogenic steels, i.e., 9% Ni steel, STS, and high-manganese austenitic steel, and they were welded with an STS pipe

Summary

Introduction

The problem of environmental pollution has risen worldwide, and concerns about environmental pollution at sea have increased. High-manganese austenitic steel was recently developed and applied to an LNG storage tank. When the LNG storage tank is manufactured, most STS pipes are connected to it with dissimilar welding. The welding residual stress distribution was evaluated for dissimilar weld joints Cryogenic steels such as high-manganese austenitic steel and STS have different thermal coefficients; they exhibit complex welding residual stress distribution in dissimilar joints [25]. The objective of this study was to ensure safety through the evaluation of the stress-distribution characteristics generated by the temperature difference during the loading–unloading operations that can occur in the storage tank during the life of the carrier. The materials used in the experimental examination were conventional cryogenic steels, i.e., 9% Ni steel, STS, and high-manganese austenitic steel, and they were welded with an STS pipe.

Measurement of Stress Change via Thermal Cycles

Welding Residual Stress

Welding Residual Stress Distribution in Dissimilar Weld Joints