Recent developments of laser beam submerged arc hybrid welding in the thick sheet range

Abstract The economic joining of large pipes and pipelines with large sheet thicknesses is becoming increasingly important in wind turbine and pipeline construction. Currently, only conventional arc welding processes such as the submerged arc welding (SAW) are used for joining these large sheet thicknesses. Process-induced, a large number of weld passes are required to fill the large weld volume. This results in considerable costs, which are composed of the energy requirements, weld filler metal and production time, among other things. By combining the laser beam process with the conventional SAW process in a common process zone, it was possible to develop a high-performance welding process at the Welding and Joining Institute at RWTH Aachen University that enables more efficient joining in the thick sheet area. With the laser beam submerged arc hybrid welding process (LUPuS Hybrid), it has already been possible to join structural steel of up to 50 mm using the layer-counterlayer technique in just two weld passes. The use of high-strength steels enables a significant reduction in sheet thickness and thus more cost- and resource-efficient production in wind turbine and pipeline construction. In the scope of this work, the LUPuS Hybrid process is therefore qualified for the single-sided and double-sided joining of thick unalloyed structural steels and high-strength pipeline steels. In detail, first various concepts for root-side weld bath support (e.g., ceramic or copper backings) for single-sided joining and layer/counterlayer welding for double-sided welding of sheet thicknesses of up to 30 mm are presented using a structural steel of grade S355. The knowledge gained is then transferred to the joining of high-strength steels of grade API X65. Since increased hydrogen content, which is mainly introduced by the welding consumables (flux and wire) in the LUPuS Hybrid process, can lead to hydrogen-induced cold cracking (HIC), the welding results generated are analysed with regard to the hydrogen input introduced by the LUPuS Hybrid process and possible measures for hydrogen effusion are discussed. As a result, single and double-sided welds were generated on S355 and API X65 steels. The diffusible hydrogen contents of the LUPuS and SAW reference samples—quantified according to DIN EN ISO 3690—were generally at a similar level. By additionally oscillating the laser beam, a reduction in the diffusible hydrogen content was achieved with LUPuS welding compared to the SAW reference samples (LUPuS 5.83 ml/100 g; SAW 6.64 ml/100 g).