P 1315 – Control of the heat input during MSG thick-wire welding using the example of fine-grained structural steels
Thick-wire GMAW with wire diameters up to d = 4.0 mm is a promising highperformance welding process, which has not yet enabled reproducible and economical joint welding of long weld seams. The applicability of the high-performance welding process in the area of temperature-sensitive, high-strength fine-grained structural steels was also unknown until now. In the research project carried out, the novel welding process was analyzed and validated with regard to its suitability for the material-specific processing of fine-grained structural steels in the thick plate range (t ˃ 10 mm).
The base material S690Q (1.8931) was used as a reference material for the group of high-strength fine-grained structural steels within the process qualification. In preliminary tests, a thermophysical material characterization was carried out on a dilatometer, whereby the critical limit ranges of the cooling times t8/5 were defined and confirmed (5 s ≤ t8/5 ≤ 15 s).
Within the project, systematic welding tests were undertaken to analyze the effect of different wire diameters on the welding behavior and the welding result at the technical process upper limit in thick-wire GMAW. Solid wires of type G69 with diameters of d = 2.4; 3.0; 3.2 and 4.0 mm were used for this purpose. The approaches to controlling the energy input into the base metal were aimed at extending the determined technical process limit in the upper power range. For example, upstream wire heating, which was achieved by increasing the stick-out (s = 30 mm), reduced the absolute welding power by up to 20 %. With the solid wire, deposition rates of approximately MR = 20 kg/h could thus be achieved. This corresponded to an increase in deposition rate of approx. 30 %. The use of a basic cored wire with a diameter of d = 3.0 mm also had a positive effect on the welding behavior and allowed deposition rates of up to approx. MR = 30 kg/h to be achieved. A separate cold wire feed into the melt during the welding process made it possible to increase the deposition rate of the thick wire process up to approx. 60 %. With the aid of a targeted process adaptation, significantly reduced cooling times t8/5 were achievable:
- use of cored wire – 35 %
- increased stick-out – 25 %
- cold wire feeding – 10 %
In addition to the direct control of the energy and heat input into the base metal, this made it possible to increase profitability by increasing the deposition rate in thick-wire GMAW. With the process combination of cored wire, increased stick-out and cold wire feed, maximum deposition rates of approx. MR = 45 kg/h were achieved in the project, which represents the current maximum value of thick-wire GMAW. The pulsed and alternating current technologies were not considered to be effective in terms of reducing the energy input for the thick-wire GMAW process.
In comparison with submerged arc welding, potentials of thick-wire GMAW were determined with regard to welding parameters and economic parameters, as well as mechanical-technological quality values on joint welds of fine-grained structural steels. Here, t = 20 mm thick plates of grade S690Q were welded together with equal quality in only 70 % of the time and with 60 % of the total costs of the submerged arc welding process when using thick-wire GMAW, with savings in powder costs and powder handling. In addition, thick-wire GMAW allows joint welds up to t = 25 mm with only two weld beads. From the process qualification (tensile tests, hardness measurements, notched bar impact tests and bending tests), no negative impairment of the mechanicaltechnological quality values of the high-strength fine-grained structural steel used (S690Q) regarding the cooling times t8/5 and processing by thick-wire GMAW was found in the case under consideration. It was reproducibly and economically possible to place the failure location of the welded high-strength component outside the weld seam and the heat-affected zone.
Prof. Dr.-Ing. J. Hensel, Dr.-Ing. M. Kusch, Dr.-Ing. A. Hälsig, M. Sc. J. Kimme