When high-frequency welding of stainless pipes with a diameter of up to 16 mm, technological difficulties may arise. Thus, the shunting current along the entire perimeter of the pipe increases, but because of the decrease in length and perimeter, the current density at the edges decreases. The parameters of the inductor and the rolls of the welding stands do not allow them to be brought so close as to substantially lower the active resistance of the edges, thereby reducing the current on the annular path and increasing its concentration at the edges. A reduction in the size of the magnetic core can cause overheating and breakage.
In the production of stainless welded pipes of grade 12 x 18n10t Ø 5−32 mm, DC welding is used by resistance. The method consists in heating the edges with the help of heat from the transmission of a current of large force through the joints of the reduced edges. Because of the rough surface, they have a high resistance. The current is led to the edges by contact rotating electrodes. The edges are heated and welded during the advance of the workpiece through the welding caliber. The current supply circuit is such that it is suitable for the production of pipes Ø> 12 mm. With smaller diameters, the shunting current is significantly increased, which leads to a decrease in the welding voltage, and a rise in temperature along the entire perimeter of the pipe does not allow the seam to be pressed with sufficient force to obtain a strong connection.
DC welding gives a very smooth internal grate with a height of 0.15−0.25 mm, which can not be achieved by high-frequency welding. Calibration of split washers and forming rolls can create conditions when the inner grate will not exceed 0.15 mm. Due to this, such pipes in some cases can replace the more expensive seamless pipes produced by cold rolling and serve, for example, in pressure oil lines of machines.
Most often with the help of DC welding, pipes Ø 10−20 mm with a wall thickness of 0.7−1 mm are made. Pipes are made from cold rolled tape. The welding speed reaches 1.2 m / s. Pipes of smaller diameters produce a cold reduction and a total reduction of up to 50%. So at TESA 5−16 a reduction-stretching mill with 9 drive and 8 unsuitable alternating two-roll stands is installed. Ready pipes Ø 5−12 mm are wound into riots, and pipes with a large diameter are cut into lengths.
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