Tandem Flux-Cored Arc Welding for High Strength Line Pipe

The benefits of mechanized welding for pipeline construction are well known, as reflected by the high industrial acceptance and usage of its variations. However, the engineering and qualification costs associated with the preparation of alternative acceptance criteria for typical pulsed and short-circuit MIG (GMAW-P and GMAW-S) girth welds can make the implementation of mechanization too costly and/or time consuming for small projects. A multi-wire welding technology, employing a high-deposition consumable that possesses excellent positional capability, along with paired digitally controlled asynchronous inverter power sources, is presented. Trials were performed on CSA Z245.1 914 mm (NPS 36) OD × 20.4 mm WT Grade 483 heavy wall high strength line pipe. One variant used an 8-head internal welding machine for the root pass, and a conventional single torch short-circuit GMAW hot pass in a compound narrow-groove configuration. A second variant utilized an externally applied controlled short-circuit GMAW-S process for the root pass in a factory-style pipe bevel configuration. Both variants employed fill and cap passes using tandem pulsed gas-shielded flux-cored arc welding (T-FCAW-G/P), using rutile consumables, with the “bug and band” MOW II mechanized welding system. Basic mechanical testing was performed on the first weld variant, along with single-edge notched bend (SENB) crack tip opening displacement (CTOD) tests, and results are presented. A productivity comparison is then shown, using weld data from the second weld variant against alternative processes, showing considerably lower fill and cap pass arc time using the T-FCAW-G/P process. Given the process’s low tendency for the formation of planar discontinuities, the process is appealing for the use of “workmanship” acceptance criteria. With further procedure development and fine-tuning of the process, tandem flux-cored arc welding may prove viable, particularly for “short” pipelines, where the costs of comprehensive engineering critical assessment/fitness-for-purpose weld procedure qualification and associated engineering work aren’t justified; as a higher productivity alternative to single wire flux-cored arc welding for mechanized tie-in welding; as a much higher productivity alternative to SMAW for tie-ins; or with a narrow groove design, mainline applications for longer-distance projects.