Application of Steel Pipe Demagnetization Technology in Construction

The residual magnetism of steel pipes affects the quality of subsequent deep processing. This article provides a detailed introduction to the principles, advantages and disadvantages of three commonly used methods, namely DC demagnetization, AC demagnetization, heating demagnetization, and composite demagnetization. It summarizes the practical experience of demagnetization and provides principles and methods for construction sites.

0. Introduction

To ensure quality, steel pipe manufacturers use magnetic flux leakage and eddy current methods to inspect the pipe body and magnetic particle methods to inspect the pipe ends. The processes of magnetic flux leakage, eddy current, and magnetic particle inspection are all magnetization, inspection, and demagnetization. If the parameters are adjusted properly, it will prevent the residual magnetism at the pipe end from exceeding the API standard average by 3mT. The excess magnetism will accelerate the wear of the machining tool, affect the accuracy of the pointer, affect the quality of the inner and outer surfaces of the pipe material, affect the surface quality of the thread, and affect the welding quality of the pipeline pipe. As the usage environment of pipeline pipes gradually deteriorates and the requirements for steel grade and performance gradually increase, demagnetization of pipe materials is becoming increasingly important. Therefore, this article will conduct a detailed analysis of the principles and methods of demagnetization of steel pipes, which is of great significance for on-site construction guidance.

1. Demagnetization principle

Ferromagnetic materials differ from other materials in that they contain magnetic domains, and the magnetic moments of atoms or molecules in local regions are arranged in parallel. When a material is unmagnetized, the orientation of magnetic domains is random, and the sum of their respective magnetic induction is equal to zero. When the material is in a magnetized field strength H, the magnetic domains align with the applied magnetic field and increase the applied magnetic field. When the magnetization source H is removed, some magnetic domains maintain a new direction without returning to their original random direction, which is manifested as remanence to the outside world. The method of reducing remanence is demagnetization.
The method of demagnetization is to apply a magnetizing magnetic field to the workpiece, then continuously change the direction of the magnetic field and gradually reduce the external magnetization intensity H to zero. The demagnetization principle is to apply a high magnetization intensity sufficient to overcome the initial coercive force and place the workpiece in a magnetic field with alternating directions over time, generating a hysteresis loop (as shown in Figure 1). As the magnetic field intensity gradually decreases to zero, the area surrounded by the loop decreases, and the residual magnetism in the workpiece also decreases, eventually reaching zero.

2. Demagnetization method

At present, the commonly used demagnetization methods mainly include: DC demagnetization, AC demagnetization, heating demagnetization, and composite demagnetization.

2.1 DC demagnetization

There are two commonly used methods for DC demagnetization: the commutation DC contact coil method and the commutation cable winding method.
The commutation DC contact coil method adopts a high ampere DC and a device with reverse current memory and gradually reduces the amplitude to zero. The demagnetization cycle is generally automatically controlled and can be completed in about 30 seconds. When using a coil, the workpiece should remain stationary within the coil until the demagnetization cycle ends. When using the contact method, contact should be maintained until the demagnetization cycle ends. The direct current method penetrates deeply and has been successfully used for workpieces that are difficult to demagnetize. Due to economic considerations, commutation DC demagnetization is usually used in the design of relatively large fixed magnetization devices.
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Figure.1 Hysteresis Loop
The reverse cable winding method involves winding multiple turns of high-current flexible cables around the surface of the workpiece that needs demagnetization. In contrast, the cables are connected to a fixed DC magnetization power supply (as shown in Figure 2). Alternating the direction of the current and reducing the amplitude to zero through multi-level adjustment is usually achieved by the automatic circuit installed in the magnetization circuit. However, for construction sites where there is no automatic demagnetization cycle power supply device, the cable joint can be manually changed and the current controller can be manually operated for demagnetization. This method is applicable to the construction site of pipeline circumferential welding, using a DC welding power source with a maximum current of 200A and a diameter of Φ 10-20mm welding cable is wound around the tube end for 10-15 turns, gradually reducing the current value, replacing the electrode, and instantaneously energizing to achieve demagnetization. However, this method has a relatively long demagnetization cycle (as shown in Figure 3).
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Figure.2 DC Magnetization Power Supply

2.2 AC demagnetization

There are two commonly used methods for AC demagnetization: the AC through coil method and the AC fixed coil method.
The AC through coil method is commonly used for demagnetization, which uses a coil powered by an alternating power source and operates with a fixed amplitude. Due to the periodic alternating current, a continuous commutating magnetic field is generated, and the workpiece is transmitted through the coil, causing it to experience the strongest magnetic field within the coil range. The workpiece comes out of the coil and reaches outside the magnetic field influence zone of the coil, and the magnetic field strength gradually decreases to zero. The coil power cannot be cut off when the workpiece reaches a distance of 3 or 4 times the diameter of the coil or between 1 meter away.
The fixed coil method of alternating current (as shown in Figure 4) also uses an alternating power supply to supply the coil, but the coil operates with varying amplitudes. The workpiece is placed inside the demagnetization coil, and as the current decreases, the magnetic field strength gradually decreases to zero. The workpiece can be removed.
Due to the skin effect of AC, the depth of magnetic field penetration into the workpiece is insufficient, which has a poor effect on eliminating residual magnetization caused by DC magnetization.
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Figure.3 Schematic diagram of demagnetization using reversing cable winding method

2.3 Heating demagnetization

The heating demagnetization method is to heat ferromagnetic materials to the Curie point, and the internal thermal disturbance of the material disrupts the parallel arrangement of atomic magnetic moments, thereby achieving the goal of reducing residual magnetism. During the heating demagnetization process, the workpiece that needs demagnetization can be simultaneously struck to achieve an accelerated demagnetization effect. The Curie point of industrial pure iron is 770 ℃. This method is neither economical nor practical.

2.4 Composite demagnetization

The composite demagnetization method is a demagnetization system that combines DC demagnetization and AC demagnetization. The general process sequence is DC magnetization, DC demagnetization, and AC demagnetization. The demagnetization system is fixed, and the demagnetized workpiece passes through the demagnetization coil at a uniform speed to achieve the goal.

3. Demagnetization detection

The residual magnetism during the demagnetization process and the final effect of demagnetization need to be tested, especially when using the reverse cable winding method for demagnetization at the steel pipe welding construction site; the positive and negative contacts of the power electrode need to be determined based on the polarity of the residual magnetism. The residual magnetic detection methods generally include magnetic field indicators and Hall effect Gauss meters.
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Figure.4 Schematic diagram of fixed coil method for AC
A magnetic field indicator is a handheld device used to measure the relative strength of a magnetic field. When using it, approach it to the workpiece and read the deflection value of the pointer to measure the relative strength of the external magnetic field. When measuring, the edge of the fulcrum end of the magnetic field indicator pointer should be close to the tested workpiece. Due to the ease of operation, convenience, and affordability of magnetic field indicators, monitoring the effectiveness of demagnetization processes is most commonly used in industry.
The Hall effect Gaussian meter is a commonly used instrument for quantitatively measuring magnetic field intensity. The sensing element is connected to the Gauss meter body through a flexible multi-contact cable inside the remote handheld probe. This can measure the magnetic field close to the surface of the workpiece. This instrument can accurately display numerical values and magnetic field polarity through automatic zero calibration and is also widely used in factories.
In the absence of magnetic field indicators and Hall effect Gauss meters at the steel pipe construction site, a string of paper clips or welding rods can be used near the pipe end to determine the magnetic field strength based on its attraction to it. However, this method can only provide a rough estimate of the magnitude of residual magnetism and cannot accurately evaluate its value and polarity.

4. Conclusion

The processes of steel pipe inspection, processing, coating, and transportation all have an impact on the residual magnetism of the steel pipe, so it is necessary to understand the commonly used demagnetization methods. Based on on-site practical experience, the following conclusions can be drawn:

  • (1) Before demagnetization, understand the source of residual magnetism to facilitate the selection of demagnetization methods. For the residual magnetism generated by AC, it is recommended to use AC demagnetization; Residual magnetism generated by direct current is recommended to be demagnetized using direct current.
  • (2) Under the premise of on-site conditions, use a magnetic field indicator and a Hall effect Gaussian meter to determine the polarity of the magnetic field at the tube end before demagnetization in order to improve the demagnetization effect.
  • (3) When adjusting demagnetization parameters, use residual magnetism detection equipment or other simple methods to monitor the demagnetization effect.

Author: Hou Qiang

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