The Generation and Elimination of Residual Magnetism in Steel Pipe Welds
After years of on-site practice and observation, we have found that there have been multiple problems in pipeline construction where magnetic interference at the pipe mouth seriously affects welding operations and welding quality. In view of this, this paper summarizes a set of effective construction methods for demagnetization of steel pipe welds based on the author ‘s relevant work experience, and makes a brief discussion, hoping to be helpful to everyone.
The author once discovered at the construction site of a water supply pipeline in a certain power plant that the DN800 spiral pipe used in the project had a magnetic problem at the pipe mouth during the welding process, which caused the welding rod to adsorb onto the pipe mouth, affecting the normal welding techniques of the welder. Sometimes, there may be phenomena such as magnetic bias blowing, arc breaking, and splashing in the solution pool, directly affecting the welding quality. We summarize the construction methods for demagnetizing welded joints, hoping to summarize the experience and promote its application in future construction in order to improve the quality level of construction projects further.
2. Residual Magnetism Hazards at Pipe Orifices and Analysis of Their Causes
(1) The impact of pipeline residual magnetism on welding work is relatively significant, specifically manifested in the following aspects:
- 1) Arc ignition is relatively difficult;
- 2) Poor stability after arc ignition;
- 3) Arc blowing may occur in a magnetic field;
- 4) Liquid metal and slag melt will splash out from the welding pool, affecting the welding quality and appearance.
The above aspects may have a slight impact on welding quality, including weld porosity, slag inclusion, and a decrease in weld appearance quality level. In severe cases, it may cause weld undercutting, cracks, and difficulty in non-destructive testing, which may have a significant or minor impact on project quality, delay project progress, reduce enterprise efficiency, and even affect the social reputation of the enterprise due to construction quality issues, resulting in negative social impacts. Therefore, it is necessary to analyze the causes of residual magnetism and take corresponding demagnetization measures to solve the problem of residual magnetism at pipeline welded joints.
(2) Analyze the causes of residual magnetism, mainly including two categories: process residual magnetism and induced residual magnetism:
- 1) Process remanence: During assembly and welding operations, magnetic grippers are used and direct current is used for welding operations, causing the pipeline to come into contact with the wires connected to the direct current power source for a long time to generate remanence. The construction process causes this remanence and is therefore called process remanence.
- 2) Induced remanence: It is caused by the use of an electromagnetic crane for loading and unloading during the process of pipe manufacturing and transportation in the factory, the stagnation of steel pipes in strong magnetic fields, or the proximity of steel pipes to strong electric cables, which results in magnetic induction of steel pipes under magnetic field induction.
3. Summarize the methods for eliminating residual magnetism in steel pipes
From the classification of residual magnetism, it is relatively easy to avoid process residual magnetism because nonmagnetic clamps can be selected during the welding process. It is also possible to avoid direct contact between the wires connected to the DC power supply and the welded pipeline when using DC welding and maintain a certain distance. This requirement can be clearly defined in the welding process evaluation or special construction plan and emphasized in the technical disclosure process. Thus, it can be completely avoided in the selection of construction methods.
From the source of its generation, induced remanence mainly refers to the induced magnetism of other magnetic devices or materials on pipes. Since it is induced magnetism, it has a polarity that can be offset or weakened by a magnetic field that is opposite to its polarity. Based on the experience of on-site construction, I propose the following methods to eliminate pipeline residual magnetism:
- Method 1: For pipelines with smaller diameters and lighter weights, the simplest and most practical method can be used for demagnetization under on-site construction conditions. This involves turning the magnetized steel pipe around and adjusting the direction of the magnetic pole to change the original welding port with the same polarity into the opposite polarity, thereby offsetting their respective magnetism, reducing magnetic flux, and ensuring normal welding. Suppose the diameter of the pipeline is large. In that case, the weight is heavy, or it cannot be adjusted when encountering fixed welded joints; this method is very limited, and the second method of demagnetization should be considered.
- Method 2: The main principle is to use a DC welding machine to wind the secondary wire used for welding on the welding pipeline. When the welding machine is powered on and starts welding, a set amount of current will be generated on the secondary wire. Adjust the current direction to make the winding coil generate a magnetic field in the opposite direction of the pipeline residual magnetism, thereby eliminating the pipeline residual magnetism. This method is also the most commonly used and effective.
4. Main construction steps and precautions to solve the problem of residual magnetism at welding junctions
(1) Construction preparation:
Check whether the butt joint of the pipeline meets the construction specification requirements. When assembling the pipe mouth, the inner wall should be flush, and the allowable deviation of the inner wall misalignment should be 0.1 times the wall thickness and should not exceed 2mm. Only after the steel pipe butt joint is qualified can “spot welding” be carried out; After spot welding is fixed, remove the brackets and corresponding angle iron within 1.5 meters of both ends of the welded joint so that there are no other accessories that affect the demagnetization construction on the pipeline within 1.5 meters of the pipe mouth.
Choose a DC welding machine that is suitable for pipeline welding. For this construction, it is recommended to use an inverter welding machine or other DC welding machines to place the welding machine in the welding shed near the welding joint. Connect the primary power and secondary wires of the welding machine, and check whether the cross-section of the primary power and secondary welding wire can meet the welding requirements and whether there is any damage. Check whether the control box and secondary outlet socket of the welding machine are intact and undamaged. Is there any obstacle at the construction site from the welding machine to the welding junction? Is it convenient for the laying of secondary cables, and can the rated current of the welding machine meet the welding requirements of the pipeline?
A welding process qualification suitable for this project has been prepared and approved. The welding rods have been baked according to the welding process requirements, and the welders have qualified welding projects that meet the welding process qualification requirements.
The temperature, humidity, and wind speed at the construction site meet the requirements of welding specifications. The ambient temperature should be>0 ℃, the relative humidity should not exceed 90%, and the ambient wind speed should not exceed 8m/s (manual arc welding is considered for welding in this case).
(2) Check whether the construction environment on site is safe, whether various safety facilities are in place, whether the number of fire extinguishers meets the requirements, whether the fire extinguishers are qualified and effective, whether there are flammable and explosive materials around the construction site (or effectively isolated from them), whether safety monitoring personnel are in place, and whether various safety procedures have been completed and are complete and effective.
(3) Adjust the welding current according to the welding process qualification requirements, select the welding rod and corresponding welding process, fix the grounding clamp of the secondary wire of the welding machine at the far end of the welding seam, lay the positive wire of the secondary wire of the welding machine along the shortest path to the welding seam, and then wind it in a clockwise (or counterclockwise) direction for three to five turns within 1m of one end of the welding seam. Connect the welding clamp at the end, connect the power supply, and clamp the welding rod for welding.
(4) Magnetic adjustment: During the welding process, check whether the welding seam is magnetic. If the welding seam is still magnetic and the magnetic force does not decrease, replace the positive and negative pole plugs of the secondary wire on the welding machine, change the current direction, perform welding again, and recheck the weakening of the magnetic force at the welding seam. If the welding seam still has magnetic force after changing the positive and negative pole direction, adjust the magnitude of the induced magnetic flux by increasing or reducing the number of winding circles of the secondary wire on the pipeline Until the residual magnetism at the welding junction weakens to almost no magnetism.
(5) Normal welding: Welding can only be carried out after multiple adjustments have confirmed that the residual magnetism at the welding junction has disappeared. To ensure that the magnetic field at the welding junction does not change during the normal welding process, the winding coil should be fixed to the pipeline with hemp rope after the demagnetization adjustment is completed to prevent the winding coil from loosening.
(6) If on-site conditions permit, a magnetometer should be used to determine the direction and magnitude of residual magnetism in order to reduce blindness in the magnetic field adjustment process and improve the speed and quality of demagnetization construction.
After repeated on-site practice, the author believes that the “welding wire winding demagnetization method” should be given priority and is the simplest and easiest method to eliminate residual magnetism of pipelines on construction sites. The characteristics of this construction method are:
- (1) strong practicality, it can be applied to pipelines of various specifications and metal materials, as well as various construction environments, including overhead pipelines, buried pipelines, pipelines laid in trenches, and of course, it can also be applied to pipelines of various media.
- (2) This method is easy to operate, easy to learn, and easy to understand. Any worker who has attended junior high school physics and the principle of electromagnetic induction can master the principle of this method, and any certified welder can learn the operating techniques of this demagnetization process.
- (3) Easy to promote, the most widely used welding machine in construction is the DC welding machine, so it is easy to meet the needs of DC power usage. Moreover, the secondary wire outlet socket on the welding machine is connected in a socket type, which is convenient for adjusting the current direction and the magnetic pole direction of the demagnetization coil. The demagnetization current can also be adjusted by adjusting the number of winding coils of the secondary wire.
Of course, there have been various demagnetization methods in many references, such as directly using magnetic materials for demagnetization, heating for demagnetization, and knocking for demagnetization. However, I still recommend using the most simple and effective “welding wire winding demagnetization method”.
5. Should the steel pipe manufacturer carry out the demagnetization process?
Demagnetization is an important process used to remove residual magnetism from steel. This is to prevent magnetic fields from attracting impurities and reduce the strength of the steel. Usually, demagnetization is performed by the manufacturer during the manufacturing process. The demagnetization process requires professional equipment and technology to ensure the effective removal of magnetism. This ensures that the steel has the required mechanical properties and durability during use.
For welding procedures, the demagnetization process is particularly important. During the welding process, if the steel still has magnetism, it can lead to a decrease in welding quality and may cause welding defects. These defects may lead to insufficient structural strength or other issues during use. Therefore, a demagnetization process must be performed before welding to ensure welding quality and the reliability of the final product.
Overall, the demagnetization process should be carried out by the steel pipe manufacturer or specially trained professionals. If the steel pipe manufacturer has already carried out the demagnetization process, many potential problems can be avoided during the welding process. However, the welding process itself also needs to follow a series of strict procedures and standards to ensure the quality of welding and the reliability of the connecting parts.
Author: Wang Xiaocheng