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This study performed root pass welding with a V-groove using a pulsed gas metal arc welding process for mild steel. Welding process parameters such as weaving, root gap size, and travel speed were major factors in the formation of the back-bead. A high-speed camera and a synchronized data acquisition system captured dynamic molten pool images and welding signals (current and voltage) simultaneously. The back-bead shape differed depending on the root gap opening. Without a root gap, the accumulated molten pool created a cushion effect in the arc center which reduced the momentum of downward flow. In contrast, the back-bead depth could be formed with a root gap opening (1 mm, 2 mm), but the back-bead shapes were different from each other at different travel speeds. The back-bead shapes also varied depending on the weaving conditions. Despite the same amount of heat input, the microstructures also varied depending on the weaving. The microstructures in the heat-affected zone with weaving were mixed with ferrite and pearlite. However, the microstructures in the heat-affected zone without weaving mainly consisted of coarse bainite. Acicular ferrite with some amount of grain boundary ferrite dominated in the weld metal in all cases. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to investigate the influence of welding residual stress (WRS) of the liquefied natural gas (LNG) fuel tank. In general, WRS and distortions are caused by non-uniform temperature distribution by the welding heat source. This will not only cause a brittle fracture, local buckling and corrosion damage, but also adversely affect the fatigue strength of the welded structures. Since LNG is treated at cryogenic temperatures of −162 ∘ C, leakage of LNG from the fuel tank to the outside may cause cracks in the hull and tank support system and cause severe damage. Therefore, it is necessary to predict the thermal behavior and WRS before welding is carried out. In this study, the WRS is calculated by thermal stress analysis based on the temperature distributions over time obtained from the transient thermal analysis. The results of this study can be used as a fundamental research for WRS analysis of plasma arc welding applied to LNG fuel tanks for coastal vessels. [ABSTRACT FROM AUTHOR]

This study performed welding on various welding positions in the flat, overhead, and vertical down positions using a pulsed gas metal arc welding (P-GMAW) process. Despite the same amount of heat input on various welding positions, the welding characteristics varied depending on the welding positions. The effect of gravity on the welding process determined the different formation of the weld bead, and the influence of molten pool behavior on various welding positions changed the microstructure in the weld metal. The current and voltage signals were synchronized with a high-speed camera using a data acquisition (DAQ) system. To induce the one pulse one drop (OPOD) process of metal transfer, the shielding gas was used 95% Ar+5% CO2. The microstructure of the weld metal was analyzed in relation to molten pool behavior. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the applicability of automatic plasma arc welding (PAW) to cryogenic materials used in liquefied natural gas (LNG) fuel storage tanks based on experimental data. The mechanical properties of the materials were tested at room and cryogenic temperatures to investigate the fatigue and fracture performances of weld joints made by PAW. In addition, the influence of welding parameters on the welded joints such as material types and temperature were considered in this experimental study. Based on the results obtained by this experimental study, it was observed that the experimental results of all materials at room and cryogenic temperatures satisfied all the requirements of each mechanical test. Finally, we propose the experimental results of PAW that can be used in the structural design of LNG fuel storage tank applications. [ABSTRACT FROM AUTHOR]