A brief discussion on the analysis and solutions of magnetic deflection during pipeline welding

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For the chemical industry, the use of high-temperature and high-pressure pipes and equipment in plants is now a very common practice. However, when installing and maintaining certain large-diameter, thick-walled pipes and equipment, a common problem that arises is magnetic deflection of the welds. For example, on July 23, 2010, during the renovation of the air return pipeline for Unit 1 in the compressor building of the olefin air separation plant, severe magnetic deflection occurred after the flange connection ﹝DN900, PN2.0, 20R﹞ and the pipe end ﹝∮910×10﹞ were aligned with each other, making it impossible to carry out proper welding. The main manifestations of magnetic deflection are as follows: during the first pass of welding at the root, the welding arc is deflected toward one side of the metal of the tube, or the arc emits a buzzing sound and fails to ignite properly. When TIG welding is used, the arc does not burn toward the metal to be welded at the tip of the tungsten electrode, but rather toward the argon gas nozzle. Analysis of common demagnetization methods and their principles. Magnetic deflection is a major challenge in on-site welding operations: in mild cases, it affects the stable combustion of the arc, leading to strip-like under-welding at the root of the weld seam, interlayer pores, and other issues that severely impact the quality of the weld and require rework ； In severe cases, the arc cannot be sustained, preventing the welding process from proceeding. And the case of us replacing the air separation air return pipeline falls under the latter category. The fundamental cause of the magnetic deflection phenomenon is the presence of a magnetic field in the welded workpiece itself or around the welding site. There are various theories regarding its causes. But one thing is clear: most instances of magnetic arc blow occur because the metal of the workpiece itself has a certain degree of magnetism, which leads to magnetic arc blow during welding. In addressing the magnetic deflection issue in the air return pipeline of Unit 1 in the compressor building of the air separation plant, we attempted several welding methods, including electrode welding with magnetic induction, adding high-conductivity materials for demagnetization, and heating-based demagnetization. However, none of these methods yielded significant results. To avoid delays in the construction schedule and ensure the smooth startup of the facility, with the support of plant management at all levels and under the guidance of experts, we ultimately adopted the physical reverse-winding demagnetization method. The following section introduces and analyzes the principles of these common demagnetization methods: 2.1 Demagnetization by winding direct-current cables in the opposite direction to the arc, utilizing physical principles. This method makes use of the principle of a demagnetizer, and on-site, simple demagnetizers are constructed using limited and readily available materials. Through actual observations during welding, it can be seen that when using TIG welding for root pass on weldments whose base material possesses magnetism, the magnetic field of the base material causes the arc to shift in a regular pattern. And this regular offset is the basis we use to determine the winding direction. In manual tungsten inert gas welding, a direct current with straight polarity is used; that is, the welding torch is connected to the negative pole of the power source, while the workpiece is connected to the positive pole. In other words, the direction of flow of the welding current is from the welding torch to the workpiece. If the welding arc is considered to be a wire, and according to the right-hand rule, if the four fingers of the right hand are directed in the direction of the arc’s deflection, then the direction pointed to by the thumb represents the \"N\" pole of the magnetic field generated by the base material itself. Similarly, the welding cable is directed in the opposite direction to the arc offset, and several turns are wound around the welding joint on the magnetic tube (the number of turns is determined through experimentation depending on the situation), thereby creating a reverse magnetic field to eliminate the arc offset. As the number of winding turns increases, the phenomenon of arc deviation gradually diminishes until the arc combustion returns to normal, indicating that the magnetic induction intensity has completely disappeared. When the welding wire is wound, root welding is performed; demagnetization takes place simultaneously with welding, which is both simple and fast. Moreover, practice has shown that when wire winding is used in the welding process, the welding current remains unchanged; there is no need to adjust it, and the arc burns stably. When it is found that the welded pipe is magnetic, first ignite the arc inside the groove, carefully observe the direction and angle of the arc’s deviation, and pay close attention to any changes in the sound of the arc, in order to determine the direction of the magnetic field present in the pipe and estimate the magnitude of the magnetic induction. Then the arc is extinguished, and the welding wire is wound around the magnetic tube, at a distance of 30 mm from the weld joint, in the direction opposite to that of the arc’s displacement; the turns should be arranged closely and neatly without any overlap. The arc is ignited again, and the burning condition of the arc is observed. If the arc still shifts in its original direction, the number of turns of the winding wire should be increased until the arc burns properly; if the arc shifts in the opposite direction to its original direction, the number of turns of the winding wire needs to be reduced until the arc burns stably. Finally, use an angle grinder to clean the groove thoroughly, and weld while the welding wire is wrapped around the tube. Under normal circumstances, the arc combustion remains stable throughout the entire grounding process, without affecting the welding operation or the quality of the weld. During the grounding process, if the arc occasionally begins to tilt in the opposite direction to its original offset, it is necessary to promptly reduce the number of turns of the welding wire coil in order to adjust the welding arc back to its normal state. After the root pass procedure is completed, all the wrapped welding wires are removed. When performing manual arc welding for the cover pass, neither the welding operation nor the welding quality is affected in any way. Throughout the entire process, attention should be paid to the following three points: (1) The direction of arc offset and the reverse direction of wire winding must not be mistaken, otherwise it will have the opposite effect; (2) The number of turns of the welding wire winding should be appropriate to ensure effective demagnetization; (3) The position where the wire is wound should not be too far from the edge of the groove, otherwise it will affect the demagnetization effect. This demagnetization method requires no additional equipment or materials, only the welder’s own tools, making it quite simple; demagnetization takes place simultaneously during welding, and it takes only 3 to 5 minutes from testing to actual welding. 2.2 Adding high-permeability materials and using the bridging current-conducting method: Before welding, insert a piece of high-permeability material (such as silicon steel sheets or Permalloy) into the gap between the parts to be welded. Since these materials have a much higher magnetic permeability than air, magnetic flux lines hardly pass through the air gap; instead, they travel through the high-permeability material. As a result, the magnetism in the area filled with this material decreases due to the redirection of the magnetic flux lines. By gradually moving this high-permeability material forward while welding, welding can be carried out successfully. However, since the filler material cannot be the same as the jointing element and allow magnetic flux lines to pass entirely through the highly magnetic material, the magnetic flux lines in the remaining air gaps still have an impact on welding. Therefore, the method of using highly magnetic materials to guide magnetic fields is only applicable to shielded metal arc welding, and such highly magnetic materials are not commonly available in our chemical industry, making it difficult to have them readily available when needed. 2.3 Above the magnetic transition point, welding should be performed while bypassing the magnetically active temperature range of the workpiece. Heat the area to be welded so that the workpiece on both sides of the weld becomes demagnetized, and then proceed with welding. However, this method has limitations when it comes to outdoor or large-scale welding. The magnetic transition point of pure iron is as high as 768°C; it is very difficult to raise and maintain such a temperature during outdoor or large-scale welding. For alloy steels, the magnetic transition point of welded joints generally approaches or exceeds the phase transformation temperature of the steel itself. This can easily degrade the properties of the base material, significantly worsen the working conditions for welders, make welding operations extremely difficult to control, and increase the rate of rework for welded joints. Therefore, the method of welding around the magnetic temperature zone of the welded parts is generally only applicable in situations with favorable welding and heat retention conditions; it is highly unsuitable for welding large-diameter pipe fittings in our factory, especially those made of CrMo alloy steel. 2.4 Method of deflecting the magnetic field by welding magnetic components using shielded metal arc welding
The welding of various pipe fittings during production and construction has led us to realize that, in terms of its ability to overcome magnetic arc blow, shielded metal arc welding is superior to gas tungsten arc welding. The magnetism is strongest at the edges of the pipe cuts, exerting the greatest influence on welding; this magnetism varies depending on the distance from the cut edge. Therefore, by using shielded metal arc welding to weld the magnetic conductors, it becomes feasible to induce a magnetic field 30 mm outside the notches of the two welded parts. Therefore, based on years of experience and a thorough analysis of magnetic arc blow scenarios, the construction workers came up with this bold idea: using shielded metal arc welding to weld magnetic components, thereby diverting the magnetic field and balancing its effect on the arc. Through numerous experiments carried out by experienced welders, a method was successfully found to divert the magnetic field: two steel strips were spot-welded at equal intervals above and below the weld joint, ensuring the symmetry of these four magnetic-deflecting strips. Thereafter, segmented magnetization, segmented desoldering, and short-arc welding methods are applied one by one to completely eliminate the impact of magnetic deflection on welding. Conclusion: In different situations, construction workers can choose the appropriate demagnetization method based on the specific circumstances. This simple and convenient method not only saves a great deal of manpower and resources, but more importantly, it also saves considerable time, thereby ensuring that the project is completed on schedule.