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Harm of magnetic field in double-sided submerged arc welding of spiral welded pipe
In the process of double-sided submerged arc welding of spiral pipe, direct current welding is used in the internal welding. Because of the high current, strong magnetic field is generated in the internal welding line and the welding head. Under the action of the magnetic field, the magnetic moment of the pipe body can be obtained or tends to be arranged in the same direction as the external magnetic field. When the welding is over and the magnetic field decreases gradually until it disappears, the residual magnetic flux density still exists on the tube due to hysteresis, which is what we usually call the residual magnetic phenomenon.
The existence of remanence deflects the electron beam direction of image intensifier, and the image of X-ray industrial television system produces "S" distortion, which affects the detection rate of natural defects such as blowhole and slag inclusion, especially the linear natural defects such as impermeability and crack. With the improvement of pipeline steel grade and the change of image intensifier in X-ray TV system, the influence of remanence is particularly prominent. The existence of remanence will cause arc deviation in circumferential butt welding of pipelines, which will affect the welding quality. The existence of remanence seriously affects the construction quality.
In order to eliminate the magnetic field, it is necessary to circle the secondary output cable of DC arc welding machine around the groove edge of the pipeline pointing to one side in a certain direction for 3-5 cycles, and the cable is short to form a circuit. The adjusting welding current knob is changed from small to large, and then from big to small instantaneously to do degaussing treatment. Several welding ground wire joints are reserved for the swing table to adjust the welding current balance and the uniform distribution of magnetic field force lines, reduce the disturbance of magnetic bias blowing, ensure the consistency of arc stiffness and shape, and make the weld of spiral welded pipe uniform and beautiful.
