Modern degaussing measures before Russian pipeline welding

The modern degaussing method before Russian pipeline welding is compiled by Xu Guizhi (Nanjing Aviation Accessories Factory 210002). The reason for the remanence in steel pipe and its influence on welding quality are analyzed. The demagnetization process and demagnetization methods of Russian pipelines before welding are introduced. Production experience proves that this method is practical and effective. Subject headings: Pipeline steel pipe welding residual magnetism demagnetization process 1 Causes of residual magnetism and its influence on welding quality During the construction and repair of gas pipelines for welding operations, sometimes the phenomenon of magnetic bias blowing affects the welding process. The formation of magnetic bias is the result of residual magnetism in the tube metal. Generally, remanence is divided into induction magnetism and process magnetism. Inductive magnetism is often produced in the process of pipe making in factories, such as: metal smelting, electromagnetic cranes for loading and unloading, steel pipes parked in a strong magnetic field, non-destructive inspections completed by magnetization, steel pipes placed close to strong power supply lines, and so on. Process magnetism often occurs during assembly and welding operations, magnetic clamps, clamps, and direct current welding of pipes, such as long-term contact with electrical wires connected to a direct current power source, exposed sections of wires or short circuits between electric welding tongs and pipes. When welding magnetic steel pipes, we often see the difficulty of arc ignition, the destruction of arc combustion stability, the deviation of the arc in the magnetic field, and the splashing of liquid metal and slag melt from the welding pool. In order to stabilize the welding process and improve the quality of the welded joint, the magnetized steel pipe must be demagnetized before welding. It should be pointed out that it is difficult to achieve complete demagnetization of welded steel pipes. Therefore, when the residual magnetism is not enough to affect the welding quality, welding is allowed. 2 Degaussing method for pipeline welding in Russia When welding and repairing pipelines in field conditions or semi-finished products bases, degaussing is particularly necessary. The relevant Russian authorities have formulated corresponding pipeline degaussing process documents. The document contains advanced domestic and foreign experience in similar operations. 2.1 Degaussing process For the degaussing before welding, a degaussing process for the joint between a single steel pipe and the steel pipe is formulated, including the following: ①Determine the size and direction of the residual magnetic field of the steel pipe; ②Select the demagnetization method, system diagram and technical means ⑧ Demagnetize the steel pipe or welded butt with the selected demagnetization method; ④ Check the residual magnetization after demagnetization to see if it meets the requirements. 2.2 Degaussing methods The following degaussing methods are specified in the established process documents: using direct current or alternating current, and the magnetic field method established by means of electromagnets or permanent magnets. Analyze the remanence parameters (see Table 1), combined with the specific construction site Conditions (for example, given equipment, etc.), selection of degaussing methods and systems. Table 1 Residual magnetism grade and welding conditions Residual magnetism grade Residual magnetism induction intensity (×10 T) Welding condition is weak <20 100=””>100 Degaussing uses the electromagnetic coil composed of the welding wire with the cross-section 35～50 mm to complete the direct current and alternating current Degauss. The wire is wound on a steel pipe or two butt-connected steel pipes, and is wound into coils with different turns according to the remanence of the steel pipe. When using direct current to demagnetize, a welding rectifier or converter with a current of 5o0 to 1 000 A must be used, including multi-station ones. When degaussing with alternating current, a welding transformer with a current of 500 to 1 000 A is used. All used power supplies should have remote control and current adjustment devices, allowing the use of stabilizing rheostats. When welding transformers are used for demagnetization, it is recommended to use portable current measuring card meters LI-4505, LI-4501, etc. to measure the demagnetization current. With the help of a dedicated electromagnet for degaussing, a welding rectifier or a transformer should be used as the power source, as shown in Figure 1(a). When using permanent magnets to demagnetize, no power supply is needed, see Figure 1(b) and (c). The demagnetization of steel pipes is divided into three grades, as shown in Table 1. Weipu Information http://www.cqvip.com Welded Pipe September 2002 Figure 1 Electromagnet (a), C-shaped permanent magnet (b) and cylindrical permanent magnet (c) are used to degauss the pipe end. Degaussing steel pipe 2 Electromagnet 3 a welding wire 4 DC welding power source 5 a C-shaped permanent magnet 6 When the cylindrical permanent magnet is demagnetized, the magnetic field should be greater than the residual magnetic field: H-(1.2～1.5)H! In the formula, H is the intensity of degaussing magnetic field; H. Remanence magnetic field strength. The intensity of the degaussing magnetic field is determined according to the formula: H — I · N 7I In the formula, a coil is energized with current. A; N is the number of turns of the degaussing coil; L is the length of the winding, m. To measure magnetism. Recommend to use lIMI1 97 X magnetometer. The magnetometer is a portable instrument used to evaluate the magnetic induction intensity of the pulsating magnetic field and the leakage magnetic field in the air gap of the magnetic system. The meter is composed of measuring converter, electronic device and charging device. The power supply of the meter is a 9V built-in battery pack. The technical characteristics of the magnetometer are shown in Table 2. (1) Degaussing with direct current The process of degaussing with direct current is as follows: ① Determine the size and square of the remanent magnetic field of the steel pipe with the help of a magnetometer. 2 The technical characteristics of the magnetometer are measured. ×10 T) 1 Adjusting the working specification time (s) 30 Continuous working time after charging the power supply (h) 8 Dimensions of the electronic device 1 70×60×35 (mm×mm×mm) The charging device part 70×70×30 Quality electronics The device 0.35 (kg) measures the 0.35 direction of the converter; ② Configure a coil composed of a flexible welding wire with a cross-section of 35-50 mm on the steel pipe, and connect it to one or two welding converters connected in sequence, The direction of the magnetic field formed by it is opposite to the direction of the residual magnetic field of the steel pipe, as shown in Figure 2; ③ In the demagnetization, Figure 2 Single steel pipe (middle part) with DC degaussing system Figure 1 Degaussed steel pipe 2 Welding wire 3 DC welding power source starts When the current is 80～100 A. ④ During the degaussing process, the results of the degaussing magnetic field must be checked periodically on the steel pipe with a magnetometer (measured when the power is turned on). If necessary, control the current or change its direction (used in the method of changing wires on the welding converter). ⑤ After the demagnetization, in order to reduce the magnetic flux smoothly, the current should be gradually reduced within 1 min until it reaches zero, and then the power supply should be cut off. Degaussing with direct current can be done in several ways. To demagnetize a single steel tube, first wind a coil of 8-12 turns around the outer circle of one end of the steel tube, and demagnetize it with the largest magnetic field value; then use the same Weipu Information http://www.cqvip. com Volume 25, Issue 5 Xu Guizhi compiled: The modern degaussing method before Russian pipeline welding is to degauss the other end of the steel pipe. When a single steel pipe is demagnetized to the butt joint of the steel pipes, the distance between the two steel pipes is not less than 300 mm, and a coil of 18-20 turns is wound at a distance of 80～100 mm from the end face of each pipe, and according to Figure 3 ( a) The method completes the degaussing. Figure 3 Diagram of the system of demagnetization with direct current before steel pipe butt assembly: l-Degaussed steel pipe 2-welding wire 3 DC welding power source 4 welding tongs with welding rod 5-metal plate In some cases, it is recommended to use welding tongs and metal plate The scheme of demagnetization in the access electrical system is shown in Figure 3(b). The welding rod loaded in the welding tongs is short-circuited with the metal plate for 10 s at a current of 300 A. Then disconnect. After each short-circuit-break cycle, check the magnetism with a magnetometer and repeat the demagnetization process if necessary. When the assembled butt joint is demagnetized, a welding wire with a cross-section of 35-50 mm is wound on the end of the butt-joined steel pipe to form a common coil of two steel pipes, as shown in Figure 4(a). The coils can be overlapped (clockwise or counterclockwise), and the total number of turns is 16-22. At this time, the larger number of turns should be on the steel pipe with larger remanence. This degaussing process is often the best. When the measured residual magnetism level is less than 2O×10 T, complete the welding of the root of the weld. At this time, it is recommended to perform supplementary demagnetization at a small current of 10 to 20 A. (2) Alternating current degaussing. Alternating current degaussing can be applied to the end of a single steel pipe before the single steel pipe is assembled, and the butt end of the assembled steel pipe with a wall thickness of 25 mm. At this time, in addition to the above-mentioned diagram 4, use the common welding wire to degauss the end of the butt pipe. The diagram (a) Degauss with direct current (b) Degauss with alternating current. 1-Degaussed steel pipe 2-Common welding wire 3 DC welding power source 4-Smooth Current reduction device (steel wire) 5 an insulating material backing plate 6-welding transformer method degaussing, there are also the following supplements: assembling according to the degaussing system diagram of Figure 4 (b), using a coil composed of a welding wire, in the loop A steel wire with a length of 0.5 to 1.0 m and a diameter of 1.5 to 3.0 mm is inserted in the middle. This steel wire is placed on a backing board made of insulating and non-combustible material (such as asbestos brick). The steel wire can smoothly change the size of the energized current, thereby changing the size of the degaussing magnetic field. When the power is turned on, the steel wire is heated and burns out within a certain period of time. The burning time depends on the wire diameter, length and current value. After the wire is burnt, use a magnetic The force gauge checks the size of the remanence. When the degaussing effect is insufficient, the degaussing must be repeated (sometimes 4 to 5 times). The degaussing system can be removed after the root weld is welded. It is recommended to remove it immediately after degaussing. For the degaussing of alternating current, an electrical regulator can also be used to smoothly change the magnitude of the current. (3) Degaussing with electromagnets and permanent magnets is mainly used in the individual sections of the butt-joined steel pipes with a length of 1O0～200 mm, especially near the magnetic field whose sign changes. At this time, after the demagnetization of individual sections, the welding of the root weld should be completed, and then the demagnetization of the next section should be carried out. In order to demagnetize, an electromagnet with a special structure is selected. The electromagnet is installed at the butt joint of the steel pipe, as shown in Figure 1(a), so that the N pole of the electromagnet is placed on the edge of the steel pipe with the magnetic S pole, and the S pole of the magnet is connected with the magnetic N pole of the tube. In the demagnetization process, the direction and size of the remanent magnetism of the steel pipe must be measured regularly with the help of a magnetometer (when the power is turned on). The size of the degaussing magnetic field is adjusted by changing the current to a large value. The direction of the magnetic field is adjusted by changing the direction of the current, that is, by changing the positive and negative poles of the power supply. Demagnetization with permanent magnets, C-shaped or cylindrical permanent magnets made of IoH KT5 alloy are selected, as shown in Figure 1(a) and (b). When the magnet is installed correctly, the magnetic pole should be opposite to that of the magnetized butt steel pipe. The correct installation of the magnet can be checked with a magnetometer. In order to enhance the effect of demagnetization, magnets can be connected to each other (two or more than three, their functions are the same). After the butt joint section is demagnetized, the welding of the root weld must be completed here. After that, the magnet should move to the next demagnetization zone. In order to increase the degaussing magnetic field, the magnet should be close to the degaussing place, otherwise the magnet can be removed. When moving the magnet along the surface of the steel pipe, the residual magnetism at the welding butt joint can be reduced to a minimum. In order to change the direction of the degaussing magnetic flux, the C-shaped magnet must be rotated 180 on the horizontal plane. , And the cylindrical magnet installed on the edge of the butt joint has to exchange positions or rotate 180 in the vertical plane. . After each degaussing process, a magnetometer must be used to check the size of the remanence. 3 Conclusion Russian degaussing experience shows that the degaussing method in the existing process documents is ten com · 60· Welded pipes were adjusted in September 2002, and the direction of the magnetic field was adjusted by changing the direction of the current, that is, by changing the positive and negative poles of the power supply. Demagnetization with permanent magnets, C-shaped or cylindrical permanent magnets made of IoH KT5 alloy are selected, as shown in Figure 1(a) and (b). When the magnet is installed correctly, the magnetic pole should be opposite to that of the magnetized butt steel pipe. The correct installation of the magnet can be checked with a magnetometer. In order to enhance the effect of demagnetization, magnets can be connected to each other (two or more than three, their functions are the same). After the butt joint section is demagnetized, the welding of the root weld must be completed here. After that, the magnet should move to the next demagnetization zone. In order to increase the degaussing magnetic field, the magnet should be close to the degaussing place, otherwise the magnet can be removed. When moving the magnet along the surface of the steel pipe, the residual magnetism at the welding butt joint can be reduced to a minimum. In order to change the direction of the degaussing magnetic flux, the C-shaped magnet must be rotated 180 on the horizontal plane. , And the cylindrical magnet installed on the edge of the butt joint has to exchange positions or rotate 180 in the vertical plane. . After each degaussing process, a magnetometer must be used to check the size of the remanence. 3 Conclusion Russian degaussing experience shows that the degaussing method in the existing process documents is ten