The synchronized tandem wire welding process applied to the welding of structural plates*

Abstract

As part of a European research project, partially funded by the Steel Research Fund (RFS-CR-03049), the potential benefits derived from the use of synchronized tandem wire welding (STW) technology applied to structural plates, in place of the currently used arc techniques, have been evaluated. In particular, two welded joint types have been investigated: T joints and butt joints in S355 grade structural plate, 6 and 12 mm thick. Considering the variables that characterize the process, also depending on the type of tandem torch, i.e. variable or fixed electrode configuration, the influence of the distance between the electrodes, stick-out, the type of arc and weld speed on the process characteristics have been examined. The joints obtained have been compared with those produced using traditional technology in terms of overall joint deformation, structural integrity, microstructural analysis and mechanical performance (toughness and fatigue strength). The results have demonstrated that the STW process is an effective technology for increasing both the weld speed, at the same penetration and the quantity of metal deposited, by about 50% compared to GMAW welding, with only slightly higher but relatively modest heat input (1 kJ/mm). These operating conditions have allowed the preservation of a level of joint quality (EN ISO 5817 quality level B) and structural integrity, comparable with those of GMAW and SAW welded joints, in addition to significant reductions in distortion. Furthermore, the fatigue strength of STW joints has also been in compliance with the reference class (butt joint, FAT class 100). The objective of the project (RFS-CR-03049), partially supported by the Research Fund for Coal and Steel, was to explore and quantify the benefits of STW applied to structural steels as an alternative technology to established arc welding technologies. T joints and butt joints of 6 and 12 mm steel grade S355 plates have been investigated using two different types of welding gun configuration, i.e. fixed electrodes and variable electrodes configuration. The influence of electrodes distance, stick-out, arc types and welding speed have been examined. The optimized joints have been characterized in terms of structural integrity, microstructure, mechanical performance (impact fracture toughness and fatigue resistance), total deformation and compared to traditional arc welding joints. STW technology allowed an increase of about 50% in both the welding speed, at the same penetration and deposited metal compared to GMAW with a not much greater heat input (1 kJ/mm) maintaining the same level of joint quality (EN ISO 5817 quality level B) and structural integrity with a reduction of joint distortion. In addition, fatigue resistance of STW joints was in compliance with the requirement (FAT Class 100 MPa).