High-Speed Tandem GMAW for Pipeline Welding

Tandem gas metal arc welding (GMAW) differs from conventional GMAW as two welding wires are passed through the same welding torch. A single torch with two contact tips is used to feed both wires into a single weld pool. Although the potential of the multi-wire GMAW process was first explored as early as the 1950’s, it has not become commercially viable until relatively recently due to performance limitations associated with the power source technology, that resulted in process instabilities. However, with the advent of modern microprocessor-controlled inverter power sources and an improved understanding of metal transfer characteristics, tandem GMAW is now being successfully applied in many industries. Over the last four years, Cranfield University’s Welding Engineering Research Centre and TransCanada Pipelines have developed tandem GMAW for pipeline welding. Cranfield have developed a tandem GMAW torch specifically for use with narrow gap weld preparations utilized in pipeline welding. The process has been proven capable of high deposition rates and welding speeds two to three times those of conventional mechanized pipeline welding. Based upon this earlier work, the Cranfield Automated Pipewelding System (CAPS) is now being developed. This uses two tandem torches on a single carriage (dual tandem welding). The high speed of tandem GMAW is retained and two passes are deposited simultaneously which further reduces welding times. This results in a significant reduction in the number of welding stations required to achieve a given number of weld’s per day and leads to major savings in labor and equipment costs. In comparing welding systems for a recent project estimate, CAPS resulted in a 26% saving in alignment, welding, NDT and coating costs when compared with conventional mechanized welding systems. A major benefit of CAPS is that it has evolved from existing technology. It is not a ‘one-shot’ process. The completed weld has a very similar profile to conventional mechanized pipeline welds so conventional radiography and automated ultrasonic testing can be used for weld inspection. The weld metal microstructure and metallurgical properties are also similar to conventional mechanized pipeline welds. CAPS is therefore suitable for use on all linepipe materials including X80 and X100 steels. This paper reviews the development of the process and equipment together with information on productivity and metallurgical properties.