German researchers are addressing the difficult of welding high-strength steel materials, which require joints of suitable strength and depth, and yet frequently exhibit inconsistencies and quality-related defects. 

Laser-gas metal arc (GMA) hybrid welding is a fast, effective approach for completing deep welding, and can be used to bridge large gaps, but for joining thicker, high-strength, fine-grain structural steels the process is known to result in seams with hot cracks and undesired hardness levels can occur in the welding seam.

An interdisciplinary project involving scientists at the Laser Zentrum Hannover e.V. (LZH) and the Leibniz Universität Hannover (LUH) have developed a hybrid welding system with inductive preheating. The process is reliable for high-quality, single-layer welding of high-strength, fine-grain structural steels up to a thickness of 20 mm, they report.

In order to increase the weld-seam quality for laser-GMA hybrid welding, the LZH Joining and Cutting of Metals Group expanded the process to include integrated, inductive preheating. Just prior to welding, an induction coil is used to introduce energy to a targeted area of the welding seam. In this way, the steel is preheated to a desired temperature without direct contact.

After the subsequent welding step, the resulting seams are less brittle, fracture strain values are better, and hot cracks can be avoided, the researchers report. With this process, steel sheets can be welded with a single-layer, and time and material can be significantly saved, they add.

Developed for Practical Application — The process was developed for high-strength, fine-grain structural steels with yield strengths from 460 to 690 N/mm². Among the HSS grades used in the research — S700MC, X70 and S690QL — applications can be found in crane, pump, pipeline and bridge construction, and in offshore structures and systems. With the newly developed process, depending on the sheet thickness, feed rates of 0.75 m/min to 2.5 m/min can be reached, the researchers discovered.

The basis for the project was cooperation between the material and process technicians from the LZH and the engineers from the civil engineering and electrical engineering fields at the LUH. The former engineers at the LZH experimented with welding processes, with the latter team at the Institute for Electrotechnology and the Institute for Steel Construction (both LUH) made simulations of the preheating process and the welding process, respectively, and evaluated the welding seams.

The DOVOR project – “High-performance welding of high-strength, fine-grained steels with high process reliability, using a hybrid welding process with integrated preheating” – was financially supported by the German Federation of Industrial Research Associations "Otto von Guericke" e.V. (AiF) and the Research Association for Steel Application (FOSTA).