Buried gas pipeline is subjected to axial load due to liquefaction-induced ground flow during large earthquake. The deformability of pipeline depends on the strength of girth weld joint. The heat affected zone (HAZ) in girth weld joint may be locally softened, because the grain size of tempered steel becomes coarsened and softened due to the heat input of girth welding. When weld joint with local softened zone is subjected to tensile load in perpendicular direction to welding line, the tensile strength of the weld joint is higher than the tensile strength of HAZ itself by plastic constraint effect. As for girth weld joint of gas pipeline, in addition to tensile load in pipe axial direction, internal pressure also affects the stress and strain state of girth weld joint. As a results, it is assumed that the tensile strength of girth weld joint varies depending on internal pressure. Therefore, this paper investigates the effect of internal pressure on the tensile strength and deformability of girth weld joint with softened HAZ. The tensile strength of girth weld joint with softened HAZ under internal pressure is formulated by modifying the existing tensile strength evaluation formula of welded wide plate specimen with softened HAZ. In this modification, the effects of internal pressure on the apparent tensile strength of pipe material and shrinkage behavior of pipe material are focused on using FEM results. Furthermore, by using the proposed formula, the effect of internal pressure on the tensile strength and deformability of girth weld joint with softened HAZ is evaluated.

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A general procedure in dissimilar metal friction stir welding (FSW) of steel and aluminum involves slightly scraping the welding surface of the steel side with a tool to expose a new surface and pressing the plastically flowing aluminum against this new surface to allow for metallurgical bonding. In the previous study, it was investigated a joint method, which uses the anchor effect of the protrusions formed by an additive manufacturing method, to establish a new dissimilar metal friction stir welding technique without scraping with a tool. However, it was revealed that the steel and aluminum were not metallurgically bonded at the interface and the welding strength was low. In order to improve the welding strength, the purpose of this study was to clarify the effects of various metals coated to steels on the metallurgical bonding behavior. The welding surface of the steel side was coated with gold, silver, copper, nickel, nickel-phosphorus, and tin, and upon conducting FSW, metallurgical bonding was achieved only with the silver and nickel coatings. In the case of silver-coated steel, the coating layer peeled off and dispersed owing to the plastic flow during FSW, with metallurgical boding achieving through Fe-Al intermetallic compounds at the welding interface. Meanwhile, for the nickel-coated steel, the coating layer did not peel off or disperse even after FSW, with metallurgical bonding occurring via Ni-Al intermetallic compounds. In addition, the silver-coated steel weld exhibited cracks within the layer of intermetallic compounds formed on the side of the protrusion and at the weld interface. This was because Fe-Al intermetallic compounds are harder and more brittle than the Ni-Al intermetallic compounds formed upon welding nickel-coated steel. In the welding joints with additive manufacturing protrusions, therefore, the welding strength improved up to 1.3 times for the silver-coated steel and up to 1.5 times for the nickel-coated steel compared to that of the uncoated material.

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The microstructure and properties of welds of duplex stainless steels are affected by cooling rate, so it is important to control welding heat input, and welding is performed in accordance with the recommended welding heat input range. In recent years, the demand for thick plates has been increasing, and there is a need to improve the efficiency of thick plate welding (high heat input). However, the range of welding heat input for welding thick plates has not been studied. The purpose of this study was to examine whether high heat input welding is possible for duplex stainless steels, and to propose a new welding heat input range that contributes to higher efficiency in welding thick plates. No sigma phase precipitation or hot cracking was confirmed in the weld metal and HAZ of 20 mm-thick duplex stainless steel (SUS329J3L) welded with heat inputs of 5 kJ/mm and 7 kJ/mm, which exceed the recommended heat input range, and good mechanical properties and pitting corrosion resistance were confirmed. Based on the TTT diagram, we examined the conditions under which sigma phase does not precipitate in SUS329J3L. As a result, the effective heat input is 3 kJ/mm, which is the upper limit of the recommended heat input range, when the plate thickness is about 10 mm. However, as the plate thickness exceeds 10 mm, the effective heat input at which sigma phase does not precipitate increases with increasing plate thickness, and the upper limit of the weldable heat input range becomes considerably larger, and good properties of welds were obtained even at a heat input of 7 kJ/mm in this study. Therefore, the upper limit of heat input to obtain sound welds in duplex stainless steel should be determined by the plate thickness, which will allow high heat input welding for welding thick plates and improve the efficiency of welding.

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