Analysis and Solution of Residual Magnetism in Seamless Steel Pipe
In the first level underground pipeline project of Zhong’an United Coal Chemical Co., Ltd., the seamless steel pipe supplied by Changjiang Steel Pipe Co., Ltd. was unable to be welded due to magnetic bias due to residual magnetism. The construction unit believes that the residual magnetism of the steel pipe is a defect that the manufacturer did not handle properly during the production process, increasing in construction time and labor costs. They want to claim compensation from the supplier. The manufacturer believes that a strong magnetic field causes the residual magnetism of the steel pipe during transportation, which causes the steel pipe to become magnetized, not due to production defects. Both parties are entangled in the issue of whether the production quality of the steel pipe with magnetism is questionable.
1. Analysis of the causes of residual magnetism in steel pipes
In steel pipe manufacturing plants, there are various reasons why steel pipes become magnetic. The main reasons for residual magnetism in steel pipes are: process magnetism and induction magnetism. Induction magnetism often occurs in the pipe manufacturing process of the factory. For example, electromagnetic cranes are often used for loading and unloading metal melting, steel pipes are stopped in a strong magnetic field, and non-destructive testing is completed using the magnetization method (using abnormal magnetic phenomena in steel pipes for non-destructive testing before testing, magnetizing the steel pipes), steel pipes are placed near strong power supply lines, etc. Among them, magnetization non-destructive testing mainly refers to eddy current testing. The eddy current testing equipment is easy to operate and master. If used properly, it can detect most defects in steel pipes. It is an indispensable non-destructive testing method in several steel pipe flaw detection tests such as visual inspection, ultrasonic testing, and hydraulic testing. Process magnetism often occurs during assembly and welding operations, as well as when using magnetic clamps, fixtures, and welding pipes with direct current, such as prolonged contact with wires connected to direct current power sources, exposed sections of wires, or short circuits between welding tongs and pipes.
Changjiang Steel Pipe Co., Ltd. believes that its steel pipes are magnetic because they pass through high-voltage wires with high magnetic fields during transportation.
2. Hazards of residual magnetism in steel pipes
When welding magnetic steel pipes, unstable arc combustion, even difficulty in arc ignition, deviation of the arc in the magnetic field, and splashing of liquid metal and slag melt from the welding pool are often seen. In order to stabilize the welding process and improve the quality of welded joints, the magnetized steel pipe must be demagnetized before welding. It is difficult for the welded steel pipe to achieve complete demagnetization, so welding is allowed when more than the residual magnetism is needed to affect the welding quality.
3. Demagnetization process method
For demagnetization before welding, a demagnetization process has been developed for the joint between a single steel pipe and the steel pipe, including the following contents: determining the size and direction of residual magnetism in the steel pipe; Selecting the method and technical means of demagnetization; Use the selected demagnetization method to demagnetize the steel pipe or welded joint; Check whether the residual magnetism after demagnetization meets the requirements.
4. Research on demagnetization methods for steel pipes in pipe factories
The commonly used methods to solve the problem of residual magnetism at present include heating the workpiece to the Curie point and disrupting the orderly arrangement of the magnetic chips inside the workpiece; Adding a DC demagnetization method with opposite polarity and equal numerical value magnetic field to the workpiece with residual magnetism; The alternating current demagnetization method and the alternating current demagnetization method that combine the two methods to gradually move the workpiece with residual magnetism away from the alternating magnetic field at a uniform speed.
4.1 Heating demagnetization method
Heating the material above the Curie point, disrupting the orderly arrangement of its internal magnetic chips, and then cooling it in an environment without external magnetic fields to eliminate residual magnetism is the most effective method. However, heating is equivalent to a heat treatment process on the workpiece, which can change the mechanical properties and internal metallographic structure of the workpiece. It is also difficult to achieve in engineering. Therefore, there are better methods than this method for pipe factories.
4.2 DC demagnetization method
The DC demagnetization method of eliminating residual magnetism by adding a reverse magnetic field to a workpiece with residual magnetism can be achieved as long as the polarity of the residual magnetic field is opposite to that of the demagnetized magnetic field. The magnetic field strength. However, there needs to be a solution to how to match the two. For workpieces of different specifications and materials, different magnetization states are required, and the corresponding demagnetization magnetic field strength also varies. The demagnetization current needs to be repeatedly adjusted to match the two. Moreover, for the same batch of workpieces, there are differences in the size of residual magnetism, and using the same demagnetization current cannot eliminate residual magnetism.
4.3 AC demagnetization method
The method of eliminating residual magnetism by gradually moving the residual magnetic workpiece away from the alternating magnetic field at a uniform speed. When it is moved away from the alternating magnetic field of the demagnetization coil, the magnetic field amplitude at a certain point of the workpiece gradually decreases. When the magnetic field decreases to zero, the remaining magnetic field of the workpiece also approaches zero. However, due to the “skin effect” of the alternating magnetic field, the demagnetized magnetic field is the strongest on the surface of the steel pipe and gradually decreases to zero as the wall thickness increases, so residual magnetism cannot be eliminated for thick-walled pipes. In addition, there is a “velocity effect” in AC demagnetization, where the magnetism of ferromagnetic metals undergoes periodic magnetic field changes from strong to weak, requiring at least 10 consecutive reversals to achieve a good demagnetization effect. However, when using power frequency demagnetization, due to the fast transmission of the steel pipe, it may not be possible to complete 10 reversals in time and achieve a good demagnetization effect.
Figure.1 Magnetic field line conduction shielding sleeve
Figure.2 “Magnetic Shield Sleeve” at the Pipe Orifice
4.4 AC/DC demagnetization method
By combining the DC demagnetization method with the AC demagnetization method, the drawbacks of using one method alone can be avoided and the purpose of demagnetization can be achieved. As long as a current adjustable DC demagnetization coil and a fixed nonadjustable AC demagnetization coil (power frequency) are installed after the magnetic saturation device, demagnetization can be achieved. When the DC demagnetization field is too small or too large, causing a small amount of residual magnetism or reverse magnetization after demagnetization of the steel pipe, the AC demagnetization coil will finally demagnetize it to achieve the purpose of demagnetization. This method is easy to implement, easy to operate, and has good practical application results. Steel pipe manufacturers often use this method for demagnetization after eddy current testing.
Through investigation, it was found that the residual magnetism of the steel pipe after eddy current testing should be at most 30GS to meet the user’s requirements. Therefore, the target value we set is that the residual magnetism of the steel pipe after eddy current testing demagnetization should be less than 30GS. Due to the different magnetization current values when steel pipes of different steel grades and specifications reach a magnetic saturation state, the corresponding DC demagnetization current values are also different. Considering that AC demagnetization is fixed and not adjustable, simply adjusting the DC demagnetization current value can achieve the effect of matching the demagnetization magnetic field.
5. Research on Field Demagnetization Methods
If the manufacturer does not effectively demagnetize the steel pipe, then the steel pipe stored on site without demagnetization will affect the welding use.
5.1 Method of watering and changing the placement of steel pipes
Changjiang Steel Pipe Co., Ltd. believes that the steel pipe has undergone demagnetization during manufacturing, testing, and storage, and the residual magnetic induction strength of the steel pipe is less than 30GS, which meets the performance requirements. Regarding demagnetization measures, Changjiang Steel Pipe Factory suggests watering both ends of the steel pipe and changing the direction of placement. Through experiments, it has been proven that this demagnetization method could be better.
5.2 Steel pipe winding and welding wire method
China Construction Installation proposes to use the steel pipe winding and electric welding method for demagnetization. During the demagnetization process, the residual magnetism of the steel pipe must be measured before each demagnetization, and the required number of coil turns and current size must be calculated. Due to the limited number of turns of the energized coil, this method has high requirements for the capacity of the demagnetization power supply and the configuration of construction facilities. More importantly, it cannot complete the project progress within the limited contract period. Therefore, this method is not advisable in cases of tight schedule requirements.
5.3 Using a magnetic shielding sleeve for demagnetization
When welding pipelines, arc bias blowing is prone to occur near the groove due to the relative magnetic attraction between the opposite poles and the repulsion between the same poles. Only when the magnetic force between the two poles is balanced and there is no magnetic field force, welding will not produce magnetic bias blowing.
5.3.1 Working principle of “magnetic shielding sleeve”
The working principle of the “magnetic shielding sleeve” is to use the force of the same pole repelling and different poles attracting magnets at both poles. In contrast, the magnetic force inside the magnet is balanced without showing any magnetic force. The two magnets are connected with magnetic metal to form a whole. At this point, the position at the root of the nozzle weld is equivalent to the inside of the magnet, so there is no magnetic force present at this location. As shown in Figure 1, add “magnetic shielding sleeves” to both ends of the pipe mouth to connect them as a whole. The unbalanced magnetic force of the pipe mouth becomes balanced, and the pipe mouth becomes nonmagnetic, thereby eliminating or reducing the magnetic bias of the welding arc.
5.3.2 Procedures for demagnetizing the “magnetic shielding sleeve”
According to the diameter of the welding pipeline, make a “magnetic shielding sleeve” for the pipe opening, as shown in Figure 2. The material of the magnetic shielding sleeve is required to be ordinary carbon steel material that can be magnetized, and it is made of steel plates or pipes with a thickness of about 3mm.
The radian of the “magnetic shielding sleeve” should be consistent with the radian of the pipe mouth and should be able to adhere to the surroundings of the pipe mouth closely.
Cut 2-4 spot welding slots in the middle of the “magnetic shielding sleeve” for easy spot welding. If necessary, it can also be disassembled and moved repeatedly.
Weld four wing plates on the “magnetic shielding sleeve” and drill four circular holes to facilitate the bolted connection of the “magnetic shielding sleeve” at both ends.
Use a “magnetic shielding sleeve” to fasten the pipe openings together, exposing the spot-welding groove for easy welding and spot welding.
Perform spot welding from the slot of the spot welding and use a tight “magnetic shielding sleeve” to make the welding junction a ferromagnetic flux whole, which can constrain the magnetic field lines in a specific closed circuit. The distribution of the magnetic field lines is uniform, and the magnetic force significantly weakens the arc during welding. After symmetrically welding several segments from the groove, the bias magnetic field inside the groove almost disappears, and the magnetic shielding sleeve can be removed for normal welding.
If seamless pipes cannot effectively eliminate residual magnetism, it will inevitably have adverse effects on welding. Doing a good job of residual magnetism elimination and testing before leaving the factory is an indispensable step to ensure product quality and maintain product image. When seamless steel pipes are found to contain residual magnetism that affects welding during on-site installation, a simple, efficient, and time-saving method must be chosen to solve the residual magnetism effect. The magnetic shielding sleeve demagnetization method is a fast and effective method for eliminating residual magnetism in steel pipes obtained after actual inspection by the on-site installation and construction unit.
Author: Wu Yiwen