GMA root welding of pearlitic rails using magnetic arc deflection

Abstract

In this work, optimization research for automated GMA root welds of pearlitic rails of grade R260 is presented. The important novelty of the used approach consisted of weld arc deflection via an externally applied magnetic field to increase the lateral penetration at the root of a narrow gap weld. During welding tests in the laboratory, an optimization of parameters was carried out, which comprised the welding current, voltage, and speed; filler wire diameter; and the strength of the external magnetic field. Results were evaluated through geometrical aspects of the weld, such as the maximum achievable lateral and diagonal root penetration, and the microstructure and the hardness were evaluated. Additionally, the behavior of the process under the influence of the external magnetic field was studied using a high-speed camera. The beneficial influence of the external magnetic field could be illustrated. It was found that the best results can be obtained in high welding current spray arc mode (380400 A) with the 1.6 mm wire at high welding speed (65 cm/min). A high and constant magnetic flux density close to the weld arc of at least 30 mT and increased welding voltage (3031 V) for a longer weld arc was found to be beneficial. Applying this approach, it was possible to weld the root layer of the used pearlitic rail foot samples with a continuously penetrated root. A smooth transition at the lower edges could be achieved. The resulting microstructure inside the heat-affected zone was fully pearlitic and, in comparison to the currently predominant aluminothermic (AT) rail welding, as a result of lower heat input, of finer morphology. Thus, hardness drops at the transition from HAZ to BM could be avoided. Furthermore, the size of the HAZ was reduced by more than 75% in comparison to an AT rail weld.