Perfection in laser welding
Problems are there to be solved
In order to permit continuous processing of the coils in the successive phases of the production process, the coils must be welded to each other. A buffer installation on the processing lines gives us precisely enough time to weld each incoming coil to the preceding one, resulting in a single, endless steel strip. This takes place for example on the pickling line and in the cold rolling mill, where the hot rolled coils are welded together. A few years ago, we switched from conventional butt welding to laser welding for this purpose. Laser welding and cutting now form part of everyday operations.
The high cool-down rate of the laser beam during welding and the complex chemical composition of the new generation of materials together produce a very hard and brittle weld seam. This is particularly the case when ultra-high-strength steels such as TRIP are welded.
This in turn gives rise to a number of problems:
- Cracks in the weld seam
- Heterogeneity in the microstructure (internal grains)
- Very poor mouldability and malleability
- Risk of fracture of the coils in the weld seam
These problems can be solved by adaptation of the welding process: we are able to combine laser welding, using a focussed laser beam, in-line with inductive heat treatment of the weld seam. This solution enables the thermal process of laser welding to be positioned and to be controlled with far greater accuracy. The electrical energy from the inductor is imparted to the surface by means of a magnetic field. (fig. 1)
Figure 1: schematic sketch
This technique allows control of the peak temperature, the heating time and the penetration depth. A typical cycle involves preheating, welding and postheating. The cycle parameters are determined by the hardness index and the chemical composition of the incoming material. The diagram below shows an example of such a cycle. (fig. 2)
Figure 2: cycle
Among the production stages using this technique is the ArcelorMittal Gent Tandem II cold-rolling line. We employ a 6 kW CO2 laser for the actual welding process, and two 30 kW induction generators for preweld and postweld heating. Figure 3 shows a welding machine of this type; figure 4 shows a detail image of the laser system actually in use, together with the two inductors.
Figure 3: welding machine
Area of application
Figure 4: detail image of the laser system
Not all steel types require laser welding in combination with induction. Figure 5 clearly shows the steel types (marked in red) that require induction support during welding.
An outstanding technique
Figure 5: area of application
With this innovative laser technique, we have obtained excellent results in the production departments of the rolling mills. In addition, it also assures our customers a greater scope for the processing of UHSS types.
- Optimization of the process for the laser welding of high-strength steels by induction-assisted heat treatment
- Crack- and fracture-free weld seams (Fig. 6)
Figure 6: weld seams
- Improved mouldability and malleability of the weld seams. Figure 7 shows typical Bulgetest results for the TRIP 700 steel type, with and without induction treatment
Figure 7: Bulge test
- Reduction of the hardness from 600 to 350 HV. Figure 8 shows a typical measurement of the microhardness on a conventionally laser-welded strip (black line in the figure) and on a coil laser-welded with induction treatment (in this case, also TRIP 700 steel type).
Innovation, research and development
Figure 8: measurement of the microhardness
Studies of this kind are performed at ArcelorMittal Global Research and Development Gent. The staff there has access to all the necessary equipment, besides knowledge of the modelling, testing and characterization of materials under the most extreme conditions.