Advances in Gas Metal Arc Welding and Application to Corrosion Resistant Alloy Pipes

Abstract

According to recent estimations, the construction of pipelines will continue to increase during the next thirty years, in particular as a result of oil and gas discoveries in remote locations. Significant advances in welding technology during the last ten years have potential to provide improvements in productivity, quality and structural integrity of pipe girth welds. In this thesis, several new processes Lincoln STT, Lincoln RapidArc, Fronius CMT, Fronius CMT-P and Kemppi FastROOT have been compared the first time to the GMAW-P to understand how these new waveforms operate for pipe welding. The process setting parameters have been analysed to understand their effect on metal transfer and arc stability control, and on bead shape characteristics. Although all waveforms present similar burn-off ratios, individual waveforms differ considerably, and especially the arc voltage waveform. This leads to considerable differences in the mechanism of metal transfer and the stability of the processes under similar experimental conditions. Understanding of these new waveforms in terms of the effect of setting parameters in the mechanism of metal transfer, process stability and melting phenomena provides a basis for assessing the potential of these processes for a range of applications, and in particular application to CRA pipe root welding Since the arc energy is the overall energy delivered from the power source at the contact tip of the torch, and part of that energy is not absorbed by the workpiece, research was performed to measure the process efficiency associated with some of these waveforms and process setting conditions. The study led to a better understanding of the potential errors in calculating process efficiency. The results obtained show that all the short-circuiting waveforms analysed (i.e. CMT, STT and FastRoot) had a similar process efficiency of 90±3%, while pulse spray waveforms (GMAW-P, CMT-P and RapidArc) are characterized by lower process efficiency, approximately 78±3%. The application of these waveforms to the welding a narrow groove pipe with a “J” groove design was investigated. These analyses were focused on the variation of bead shape characteristics and welding quality performance based on the analysis of the conditions that result in lack of penetration and top bead defects, such as lack of side wall fusion or undercutting. It was observed that RapidArc and CMT-P are able to satisfy the quality requirements, i.e. full penetration and absence of defects for the specific conditions described in this thesis. High welding speeds (up to 1m/min) were achieved with these processes, four times the typical speed 0.25m/min. Finally, the shielding gas plays an important role in terms of quality and weld bead performance. This led to an optimization of the shielding gas composition used, based on mixtures of carbon dioxide, argon and helium. Statistical modelling was undertaken to optimize the shielding gas mixtures using RapidArc and CMT-P waveforms. In parallel, a new purging shielding gas device was designed to achieve a weld root free of oxidation.