Determination of the capability and mechanical properties of small diameter plasma MMA electrodes

Previous work characterizing large diameter electrode plasma MMA welding highlighted potential benefits of the process, however, their manipulation during the making of real joints was never fully verified. Small, realistic commercial sized electrodes were tested for their ability to make realistic joints using a commercial butt joint preparation in 12.5 mm steel. It was found that initial problems of slag control could easily be overcome by a trained welder and that an excellent, slag free deposit was readily made. Increased weld deposition rates offered by this process were re-confirmed when small diameter tubular electrodes were employed. Weld joint manufacture was undertaken, using a commercial butt joint preparation and found to be little different from welding with commercial solid rod electrodes. Impact testing of the resultant weld beads found that the small diameter plasma MMA deposit was tougher, having a microstructure with finer side plate ferrite and less pro-eutectoid ferrite than the deposit from a commercial solid rod MMA electrode. Both oxygen and nitrogen contents of the plasma MMA deposit were also noticeably lower than in the conventional MMA deposit. It has been speculated that these lower gas contents are responsible for the increased toughness via their effect on the weld microstructure. Even using tubular electrodes of a more usual commercial size showed that the deposition rate increase was such that realistic time savings were industrially available at almost no extra cost.