Spiral welded steel pipe (SSAW pipe) will naturally become magnetic due to long-term friction during the production process. If the magnetism is too large, it will cause natural defects such as pores and slag inclusions in the spiral steel pipe, especially the detection rate of linear natural defects such as incomplete penetration and cracks. With the increase of pipeline steel grade, the influence of residual magnetism is particularly prominent. The existence of residual magnetism will produce arc deflection phenomenon when pipeline girth welding is butt joint, which will affect the welding quality. The existence of residual magnetism seriously affects the construction quality, so degaussing is an inevitable process for spiral welded steel pipes before leaving the factory.
Degaussing method of spiral steel pipe
1. Wrap the coil outside the steel pipe, connect the coil with alternating current, and gradually reduce the current of the alternating current until it reaches zero. Degaussing effect: The physical properties of the spiral steel pipe will not be changed, but once it encounters an external magnetic field again, the spiral steel pipe will be magnetized.
2. After the heat treatment process, it can be degaussed more thoroughly. However, after degaussing, the hardness and rigidity of the spiral steel pipe will change. When encountering an external magnetic field in the future, after the external magnetic field disappears, the steel pipe will basically have no residual magnetism.
3. The process of heat treatment is roughly: heating to a certain temperature without oxygen, and then gradually cooling down to normal temperature in 72 hours.
The reason why the spiral steel pipe is magnetic
During the welding process, the internal welding adopts the DC welding method, and the large internal welding wire and the welding head part generate a strong magnetic field due to the large current. Under the effect of the magnetic field, the magnetic moment of the steel pipe body is or tends to be placed in the same direction as the external magnetic field. When the welding is completed and the magnetic field gradually decreases until it disappears, due to the hysteresis phenomenon, there is still a residual magnetic flux density on the tube body, which is the remanence phenomenon.
Degaussing method of spiral steel pipe
1. Wrap the coil outside the steel pipe, connect the coil with alternating current, and gradually reduce the current of the alternating current until it reaches zero. Degaussing effect: The physical properties of the spiral steel pipe will not be changed, but once it encounters an external magnetic field again, the spiral steel pipe will be magnetized.
2. After the heat treatment process, it can be degaussed more thoroughly. However, after degaussing, the hardness and rigidity of the spiral steel pipe will change. When encountering an external magnetic field in the future, after the external magnetic field disappears, the steel pipe will basically have no residual magnetism.
3. The process of heat treatment is roughly: heating to a certain temperature without oxygen, and then gradually cooling down to normal temperature in 72 hours.
The reason why the spiral steel pipe is magnetic
During the welding process, the internal welding adopts the DC welding method, and the large internal welding wire and the welding head part generate a strong magnetic field due to the large current. Under the effect of the magnetic field, the magnetic moment of the steel pipe body is or tends to be placed in the same direction as the external magnetic field. When the welding is completed and the magnetic field gradually decreases until it disappears, due to the hysteresis phenomenon, there is still a residual magnetic flux density on the tube body, which is the remanence phenomenon.
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