Development of Welding Parameters of Automatic GTAW Process for Pipes with Variant edge preparation

摘要

Couples of forces, namely, the impact force given by a burning arc where thegravity force which tends to drop the molten metal and the surface tensionforce acting on the surface of weld puddle, can vitally influence weld beadcontour during puddle solidification process. Especially in the root passwelding of a butt joint in a piping system, how to control the bead contourcan really affect the service life of pipe line. The interactions of theseforces play a very essential role to build a good "bridging" in everyspecific root pass welding. A poor "bridging" in the first pass can not onlydowngrade the deposition of subsequent weld passes but also introduce localturbulent flows at the inner surface to induce erosion/corrosion phenomenonin inner weld locals. The quality of a root pass weld often depends on theso-called edge preparation factors. Even some sizing suggestions can befound in some industry Codes or relevant handbooks, however most of them donot meet the more strict requirement in automatic welding process. As aresult, the welding defects could be found from time to time in an automaticwelding process.In this research, a method of adjusting the backing gas pressure was used inthe automatic GTAW process to weld Φ 8" Sch 40 SUS 316 and Φ 6" Sch 10 SUS316 stainless steel pipes. Besides micro-structure evaluation and microhardnessanalysis, the ultrasonic evaluation was also performed and theydemonstrated that the "dross dropping" could be improved by at least 80%withthis method.