The Wuppermann group showcases annealed stainless steel tubes for the first time at this year's Tube trade show. A new rig for in-line annealing at Wuppermann Rohrtechnik GmbH in Wilnsdorf has been producing these tubes for several months. The plant specializes in the production of stainless steel tubes in special profiles and the new process step now allows it to meet increasingly exacting customer specifications. The annealing process restores the original structural state of the material which has changed as a result of the loads applied during pipe cold forming, allowing such tubes to be used much more widely than non-annealed tubes. The annealed stainless steel tubes are characterized by metallurgical purity, excellent forming properties and positive corrosion behaviour. Wuppermann has integrated this recrystallization annealing in-line into its tube production line – a much more process-reliable method than the post-annealing of the finished product. With the new plant, Wuppermann fulfils all the requirements for the AD2000-W2/ASTM A-249 certificate and can now service new markets. This certificate is an approval for pressure vessel, condenser, superheater and heat exchanger tubes made of stainless steel. Annealed stainless steel tubes are required by power station builders, as well as by major players in the machine-building and plant construction sector, and by the chemical industry.
"We have set-up our roll-forming tube production operations to keep the material loads during the forming process very low. For quality reasons we use only the WIG/TIG welding process for heat exchanger tubes or condenser tubes. The new annealing process now enhances our quality even further. The design of the annealing rig allows us to determine the temperature much more precisely than in conventional in-line annealing processes. Because of its accuracy this plant is excellently suited not only for austenitic stainless steel tubes, also for ferrites and special materials that are difficult to anneal. The process temperatures are monitored permanently and can be documented for each tube. Our customers have provided very positive feedback on our first batches of annealed stainless steel tubes", says Manfred Hartmann, plant manager at Wuppermann Rohrtechnik GmbH in Wilnsdorf. "With our patented forming process and the fully automated plant control system we can bright anneal any conceivable shape of special section tube in-line."
While the strip is being transformed cold rolled into tube, the forming process changes the structure of the material. In the case of austenitic grades, for example, the structure changes from cubic face centred to cubic body centred. A phenomenon that is referred to as lattice shearing or transformation. When the structure changes the tube has other material properties than the original strip and is, for instance, magnetisable and less corrosion resistant. The original structure can be restored using recrystallization annealing. The high temperatures ensure that the structure recomposes and reorganizes, restoring the original crystalline lattice structure of the basic material.
This process is carried out at Wuppermann Rohrtechnik GmbH in Wilnsdorf with a bright annealing plant in an oxygen-free environment using, for example, the material AISI 304/1.4301 in an all-hydrogen environment throughout the entire annealing and cooling line. Multiple induction coils perform the annealing, heating the tube to around 1080 °C. A muffle then maintains the temperature for a defined period to allow the heat to penetrate the complete tube thickness. After the annealing process, graphite jaws are used to cool the tubes to room temperature.
"The successful start-up of the annealing plant at our factory in Wilnsdorf was an important step toward the expansion of our Stainless Steel division. The Stainless Steel division is continuing to grow in all areas: tube, tube components and sheet metal components," explains Dr. Arnd Schaff, head of Wuppermann’s Technical Products division.
At the Tube trade show, Wuppermann is showcasing its entire range of standard, special form and perforated tubes made of black, pickled, galvanized and stainless steel. We look forward to welcoming you at our stand 3B30 in Hall 03.