Schoeller Werk, Rofin mark 25-year relationship for laser welding

Since Schoeller installed its first laser welding systems 25 years ago, there has been a close partnership with laser specialist Rofin.

The 19 manufacturing halls of longitudinally welded stainless steel tube manufacturer Schoeller Werk (Hellenthal, Germany) stretch over several kilometers along the small river Olef through Hellenthal, not far from the Belgian border. More than 1000 people work in the numerous production sites of the Schoeller plants. Since the installation of the first laser welding systems 25 years ago, there has been a close partnership between Schoeller and laser specialist Rofin (Hamburg, Germany).

Schoeller can look back at almost 200 years of history in which the company has continuously been in the family. The first TIG tube welding line was put into operation in 1959 for manufacturing longitudinally welded stainless steel tubes. The company quickly became established and sold its tubes to the chemical industry as well as apparatus and plant engineering, among others. Starting production of tubes made of ferritic and austenitic steels for car exhaust tubes in 1975, the company soon entered the automotive industry market, which presently accounts for around 50% of sales. Today, 57 years after starting up the first system, 109 welding stations produce more than 90 million meters of tubes per year, generating sales of around €220 million in 2015.

As cars continued to develop in the 1980s, the demands placed on installed tubes increased and the company adapted to the more-compact bodies through bending. Since TIG-welded tubes are not suitable, or are only conditionally suitable for bending, a new joining technology was needed: the laser.

Laser welding has many advantages: low heat input, low evaporation during melting, and minimum separation of alloy elements. The fine-grained structure of the narrow laser weld seam allows a high degree of deformation, often without the heat treatments required in TIG welding, and is also up to three times faster than conventional methods.

"We checked out all laser manufacturers and we chose Rofin because their cross-flow laser beam quality was beneficial, as our strip edge preparation at that time was simply inadequate for better beam qualities,” says Hubert Reder, manager of maintenance at Schoeller Werk. A 5kW CO2 laser with a beam guidance system for tube welding was delivered initially. This predecessor of today's Rofin Profile Welding System had been a proven tube welding system for about 10 years, at that time with patented mechanical seam tracking. The concept was developed together with Stefan Ziesemer, the Hamburg project manager from Rofin. "Actually, one could say that we learned from each other at the beginning," says Ziesemer, as he describes the situation 25 years ago. "Together, we tackled the task and the questions that emerged." Obviously it was successful, because the working relationship has resulted in a total of 14 welding stations at Schoeller that were equipped by Rofin.

The results of the 18 laser systems in total are convincing, as they produce the same tonnage of tubes per year as the conventional systems. So, as the strip edge preparation at Schoeller and hence the gap accuracy improved considerably, Rofin developed, among other things, the CO2 slab laser that was patented at that time. "Naturally, we test other technologies on our lab system such as fiber lasers. So far, however, we achieve better results and our requirements with CO2 lasers," says Reder, as he explains why only these lasers have been used to date.

The system that was just installed replaces the currently oldest laser, almost 17 year old, with a new 6kW laser and the latest Profile Welding System (PWS) with motorized axes, used for the first time. With this, Schoeller is pursuing the goal of storing all process parameters in a database to improve the reproducibility of the processes and simplify operation of the systems. In addition, seam inspection will be used downstream from the welding process, which will be analyzed directly at the welding station with all process parameters.

People at the company seem to be very satisfied with the new configuration of the welding station: "The simple, easy-to-operate design of the weld sensor, and the known evaluation of the weld seam by a seam inspector, pave the way for a quick start into production," Reder says. "To us, Rofin has been a very good partner for the past 25 years, who has developed solutions together with us and adapted the systems to our requirements."

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