Homopolar Welding: Results of Five Years of Research
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Homopolar welding (HPW) is a high speed electric resistance forge welding process being developed for single shot girth welds proposed for the J-Lay pipelaying technique in deepwater. The homopolar welding technique uses the high amperage direct current output from homopolar generators to produce localized heating at the pipe interface, completing a weld in under five seconds regardless of pipe diameter or wall thickness. During a recently completed five year welding research program, homopolar pipe welding technology made substantial advances in the process using a laboratory fixture to join small diameter HSLA steel linepipe (3-inch schedule 80). Specific improvements on two high strength linepipe steels (X60 & X65) include increased impact toughness to near base metal values, improved weld profile, near base metal microstructure at the weld, and reduced generator requirements. Based on these improvements, a new welding fixture was designed and built capable of joining 12-inch schedule 60 steel linepipe. Seven welds have been completed, all having base metal yield and tensile strengths and ductility. During the next major research program, adjustments to the operation of the 12-inch welding fixture will be implemented to match the performance of the 3-inch welding fixture and improve impact toughness in 12-inch welds. The paper will present the results of homopolar welding research for the past five years including specific modifications to the homopolar welding process.