DC welding
Relevance
At high-frequency welding of stainless pipes with diameter up to 16 mm technological difficulties can arise. For example, the shunt current along the perimeter of the pipe increases, but due to the reduction of the length, the perimeter current concentration on the edges decreases. Parameters of the inductor and welding stand rollers do not allow to bring them close enough to significantly decrease the active resistance of the edges, thus reducing the current on the circular path and increasing its concentration on the edges. And reducing the size of the magnetic core can cause overheating and breaks.
Welding Technology
In the production of stainless welded tubes of 12X18n10t grade Ø 5-32 mm, direct current welding by the resistance method is used. The method consists in heating of edges with the help of high current through the joints of joined edges. Due to the rough surface they have a high resistance. Current is supplied to the edges by contact rotating electrodes. The edges are heated and welded while the workpiece is moving through the welding gauge. The current supply scheme is such that it is suitable for the production of pipes Ø> 12 mm. With smaller diameters, the shunt current increases considerably, which reduces the welding voltage, and the increase in temperature on the circumference of the tube makes it impossible to squeeze the weld with sufficient force to obtain a firm connection.
Advantages
DC welding produces a very even internal bevel of 0.15-0.25 mm, which cannot be achieved by high-frequency welding. By calibrating the cut washers and forming rolls it is possible to create conditions, when the internal burr does not exceed 0.15 mm.
Tube welding
Direct current welding is most commonly used for pipes of Ø 10-20 mm with a wall thickness of 0.7-1 mm. Tubes are made from cold-rolled strip. The welding speed is up to 1.2 metres per second. Pipes of smaller diameters are produced by cold reduction and total crimping up to 50%. For example, TESA 5-16 is equipped with a reducing and stretching mill with 9 driven and 8 non-driven alternating two-roller stands. Finished tubes, Ø 5-12 mm, are coiled into coils, and tubes with larger diameters are cut to finished lengths.
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