Due to their excellent mechanical properties combined with high corrosion resistance, duplex steels are among the high-performance steels that have proven themselves for applications under extreme conditions. They are therefore used in shipbuilding, offshore platforms and pipelines, as well as for gas, oil or food containers. Duplex steels owe their properties to a microstructure ratio of 60 percent austenite and 40 percent delta ferrite. For corresponding welded joints, international regulations require an austenite content of at least 30 percent. In order to achieve this specification, controlled heat input is required during welding in order to define the cooling time. A cooling time that is too long can lead to precipitation (nitrides and intermetallic phases) as well as to a brittle coarse grain zone. A cooling time that is too short, on the other hand, is detrimental to austenite formation. Both would result in a reduction of the mechanical properties and corrosion resistance of the weld.

Laser welding as an alternative for industrial welding production

The currently preferred filler metal-based arc welding process, such as submerged arc welding and shielded metal arc welding, is able to positively influence the welded joints and the heat input by the filler metal. On the other hand, however, productivity is low due to low welding speeds, multi-layer welding and complex seam preparation. Beam welding processes, on the other hand, have a high productivity due to high welding depths and welding speeds, but cannot fulfil the requirements with regard to the microstructure proportions to be achieved.

Researchers at the Laser Zentrum Hannover e. V. (LZH) and the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) have therefore developed a new process as part of the DupLUH project on behalf of the Forschungsvereinigung Stahlanwendung e. V. (FOSTA), which combines the advantages of established submerged arc welding with the high productivity of a beam welding process. The result has the potential to establish laser submerged arc hybrid welding in the industrial welding production of thick duplex steels.

Excellent joining properties with high productivity

The scientists involved were able to produce single-layer welded joints on 16-millimetre-thick duplex steel 1.4462 at a welding speed of one metre per minute without time-consuming seam preparation. The seams achieved the required minimum austenite content of 30 per cent and, in addition to excellent strength properties, also exhibited high notched impact strengths of 120 joules at a test temperature of minus 40 degrees Celsius. "The results achieved impressively demonstrate the high productivity of laser submerged-arc hybrid welding in the joining of welded joints that meet the requirements for this challenging class of materials," says Dr.-Ing. Rabi Lahdo, research associate in the Joining and Cutting of Metals group at the LZH.

It can also be a little thicker

For 20-millimetre-thick duplex steels, the process was expanded to a two-layer welding process in the layer-counterlayer technique without complex seam preparation. For the first layer, the process just described is used with a welding speed of 60 centimetres per minute. Conventional submerged-arc welding was used for the opposite layer. These welded joints also exceed the requirements in terms of mechanical properties and corrosion resistance. Even with sheet thicknesses of up to 30 millimetres, laser beam submerged-arc hybrid welding in the layer-counterlayer technique is convincing. Of course, as sheet thickness increases, so do the demands on the welding process for producing high-quality welded joints. But at least up to a sheet thickness of 30 millimetres, the high requirements could be achieved or even surpassed.