Reasons for the magnetic field of spiral steel pipe
In the welding process, welding by DC welding, in welding line, and the position of the welding head due to the high current, resulting in a strong magnetic field, pipe body under the action of magnetic field, magnetic moment or tend to obtain and external magnetic field in the same direction are arranged. When the welding end, magnetic field gradually decreased till disappeared, due to the hysteresis phenomenon, in the tube body and residual magnetic flux density, this is what we say normally remanence.
The harm of spiral steel tube magnetic field
Existent remanent magnetization, the image intensifier and the electron beam deflection and X-ray industrial TV image system produced the “s” type distortion, influence porosity, slag and other natural defects, especially the detection rate of incomplete penetration, cracks and other linear natural defects.
With the change and improve X ray image intensifier TV system pipeline steel grade, the remanence was particularly prominent.
Remanence existence, ring butt in the pipeline, will produce the phenomenon of partial arc, welding quality.
Remanence have seriously affected the quality of construction
The AC degaussing method of spiral pipe is the most common method:
1, with the coil around the outside of the steel tube, to the coil through the alternating current, slowly reduce the current of alternating current, until reduced to zero. Degaussing results: don’t change the physical properties of spiral steel pipe, however, once again encountered the external magnetic field, spiral steel tube also is magnetized.
2, through the heat treatment process, can be more thoroughly the degaussing, however, after degaussing, hardness and rigidity of the spiral pipe will change, encountered again in the future the external magnetic field, plus the magnetic field is removed, the steel pipe can basically there is no residual magnetism.
3, heat treatment process, is probably: oxygen free heating to a certain temperature, and then, with 72 hours slowly cooling to room temperature.
Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)
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