Discussion on pipeline welding degaussing process

【Summary Description】 In the welding operation of power engineering construction, the phenomenon of magnetic bias blowing affecting the welding process sometimes occurs. The formation of magnetic bias blow is the result of the presence of remanence in the pipe metal. In general, remanence is divided into two types: induction magnetism and process magnetism. Induction magnetism often occurs in the process of pipe making in factories, such as metal smelting, loading and unloading by electromagnetic crane, stopping steel pipe in strong magnetic field, completing non-destructive inspection by magnetization method, placing steel pipe close to strong power supply line, etc. Process magnetism often occurs during assembly and welding operations and the use of magnetic grippers, clamps and direct current welding pipelines, such as long-term contact with electric wires connected to direct current power supply, exposed sections of wires or short circuits between electric welding clamps and pipes. When welding steel pipes with magnetic properties, problems such as difficulty in arc ignition, destruction of arc combustion stability, arc deviation in a magnetic field, and splashing of liquid metal and slag from the welding pool are often caused. In order to stabilize the welding process and improve the quality of the welded joint, the magnetized steel pipe must be demagnetized before welding. In general, it is difficult to achieve complete demagnetization of welded steel pipes. Therefore, when the residual magnetism is insufficient to affect the welding quality, the welding is allowed. The degaussing process for the joint between a single steel pipe and a steel pipe is as follows: ① Determine the size and direction of the remanence field of the steel pipe; ② Select the degaussing method and technical means; ⑧ Use the selected degaussing method to degausse the steel pipe or welded joint; ④ Check the amount of remanence after degaussing to see if it meets the requirements. Commonly used degaussing methods are divided into: direct current or alternating current, with the aid of electromagnets or permanent magnets established magnetic field methods. Here we mainly analyze the DC degaussing process. As shown in the figure: The principle of degaussing is to form a magnetic field opposite to the direction of the remanence field of the steel pipe around the degaussing pipeline, and control the strength of the formed magnetic field by controlling the current size, so as to achieve the purpose of degaussing. A coil composed of flexible welding wires with a cross section of 35~50mm is arranged on the steel pipe to form a closed loop. One end of the two welding wires is respectively connected to the positive and negative poles of the DC welding power supply, and the other end is respectively connected to the metal plate fixed on the demagnetized pipeline, so that the magnetic field formed by the two welding wires acts in the opposite direction to the remanence field of the steel pipe. As the current was gradually increased, the result of the action of the degaussing magnetic field was periodically checked on the steel pipe (measurement was made when the power was turned on). When necessary, control the current (usually there should be a current adjustment device) or change its direction (the method of changing the welding wire on the DC welding power source). After the demagnetization, in order to smoothly reduce the magnetic flux, the current should be gradually reduced within 1 min until the zero value, and then cut off the power supply. When a single steel pipe is degaussed, a coil of 8 to 12 turns can be wound along the outer circle at one end of the steel pipe to degaussing at the maximum magnetic field value, and then the other end of the steel pipe is degaussed in the same way. When a single steel pipe is demagnetized to the butt joint of the steel pipes, the distance between the two steel pipes shall not be less than 300mm, and a coil of 18-20 turns shall be wound at a distance of 80-100mm from the end face of each pipe, and then demagnetized according to the above method. On individual occasions, it is recommend to use the scheme of degaussing the electrode holder and the metal plate into the electrical system. Short-circuit the electrode inserted in the electrode holder to the metal plate for approx. 10 s at 300 A current. Then disconnect. After each short circuit and disconnection cycle, check the magnetic strength of the pipeline, and repeat the above demagnetization process if necessary. When the assembled butt joint is demagnetized, a welding wire with a cross section of 35~50mm is wound on the end of the butt joint to form a common coil of two steel pipes, as shown in the above figure. The coils can be wound overlapping (clockwise or counterclockwise) for a total of 16 to 22 turns. At this time, the number of turns should be on the steel pipe with larger remanence. This degaussing process is often optimal. The magnetic field method established by alternating current, by means of electromagnets or permanent magnets, is basically the same as the principle of direct current demagnetization.

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