Analyzed the causes of residual magnetism in steel pipes and its effects on welding quality, and introduced the demagnetization process and pre-welding methods in the second phase of the central heating project in Shengli oil field. The technical team of the Shengli Oil Field Project Department reviewed materials and proposed the method of using direct or alternating current and electromagnets or permanent magnets to build a magnetic field. After the exercise on site, the problem was resolved, ensuring the smooth progress of the project and laying the foundations for future works.
1. Causes of residual magnetism and its influence on welding quality
During welding work for the construction and repair of gas pipelines, magnetic polarization can sometimes occur, which affects the welding process. Magnetic polarizations are the result of residual magnetism in the tube metal.
Residual magnetism is generally divided into two types: induced magnetism and process magnetism. Inductive magnetism often arises during pipe manufacturing in factories, such as metal melting, using electromagnetic cranes for loading and unloading, stopping steel pipes in strong magnetic fields, performing non-destructive testing using magnetization, and laying steel pipes near strong magnetic fields. lines. Process magnetism frequently occurs in assembly and welding operations, as well as in the use of magnetic clamps, jigs, and welding of DC piping, such as prolonged contact with lead wires connected to DC sources, sections of exposed wire, or short circuits between welding guns. welding and piping.
When welding magnetic steel pipes, difficulties in arc ignition, damage to the stability of arc combustion, arc deviation in the magnetic field, and splashes of liquid metal and molten slag from the weld pool often occur. To stabilize the welding process and improve the quality of the welded joints, the magnetized steel pipe is demagnetized before welding. It should be noted that it is difficult to completely demagnetize welded steel pipes. If residual magnetism is not sufficient to affect weld quality, welding is permitted.
2. Demagnetization method for welding in the second phase of central heating project in Shengli oil field
Demagnetization is particularly necessary when welding and repairing external piping or semi-finished bases.
2.1 Demagnetization process
For demagnetization before welding, a demagnetization method for the connection between a single steel pipe and the steel pipe has been developed, which includes the following contents: determining the magnitude and direction of the residual magnetic field of the steel pipe; Selection of demagnetization methods, system diagrams and technical means; Using the selected demagnetization method to demagnetize the steel pipe or welding joint; Check whether the residual magnetism after demagnetization meets the requirements.
2.2 Demagnetization method
An electromagnetic coil made of welding wires with a cross section of 35-50 mm 2 is used to demagnetize direct and alternating current. The wires are wound around a steel tube or two connected steel tubes, and the coils are wound into different turns depending on the residual magnetism of the steel tube. When using direct current for demagnetization, a welding rectifier or converter with a current of 500-1000 A must be used, including multiple workstations. When using alternating current for demagnetization, a welding transformer with a current of 500-1000 A is used. All power sources used must have remote control and current regulation devices that allow the use of stable rheostats. When using a welding transformer for degaussing, use portable current measurement cards such as u-4505 and u-4501 to measure the degaussing current. With the help of special electromagnets for demagnetization, a rectifier or welding transformer must be used as a power source. To measure magnetism use ИM∏-97X Magnetometer. A magnetometer is a portable device that can be used to measure the magnetic induction intensity of pulsating magnetic fields and stray magnetic fields in the air gap of a magnetic system. The device consists of a measurement converter, an electronic device and a charger. The device is powered by an internal 9V battery.
The technical characteristics of a magnetometer are as follows: The measured range of magnetic induction intensity is 1-1999 × 10 -4 T; The lower limit of sensitivity is 10 -4 T; the default working time is 30 seconds; continuous working time after charging the power supply is 8 seconds; the size of the electronic device is 170mm × 60mm × 35mm; charger size is 70mm × 70mm × 30mm; the mass of the electronic device is 0.35 kg; the measured mass of the inverter is 0.35 kg
2.2.1 DC degaussing process
Use a magnetometer to determine the strength and direction of the steel pipe's residual magnetic field. Set up a coil of flexible welding wires with a cross section of 35-50 mm on the steel pipe and connect it to one or two welding converters connected in sequence so that the magnetic field it creates acts in the opposite direction to the residual magnetic field of the steel pipe. At the beginning of demagnetization, the current is 80-100 A. During the demagnetization process, it is necessary to periodically check the results of the demagnetization magnetic field on the steel pipe with a magnetometer (measured when the power is turned on) and, if necessary , control the current or its change in direction (used when replacing wires in welding converters). After demagnetization, to smoothly reduce the magnetic flux, the current should be gradually reduced to zero within 1 minute, and then the power supply should be turned off.
2.2.2 AC demagnetization process
AC demagnetization can be applied to the end of a single steel pipe before assembly, as well as to the end of an assembled steel pipe with a wall thickness of up to 25 mm. At this point, in addition to demagnetization according to the above method, there are also the following additions: a coil of 1 welded wire is used and a steel wire 0.5-1.0 m long and 1.5-3 m long diameter 0.0 m is connected to the circuit. This steel wire is placed on a pad of insulating and non-flammable material (such as asbestos brick), and the steel wire can gently change the magnitude of the current, thereby changing the magnitude of the demagnetizing magnetic field. After being turned on, the steel wire is heated and burned within a certain time depending on the diameter, length and current of the steel wire. After burning the steel wire, the residual magnetism is checked with a magnetometer. If the demagnetization effect is not enough, it needs to be repeated (sometimes 4-5 times).
2.2.3 Demagnetization of electromagnets and permanent magnets
This method is mainly used on individual sections 100 to 200 mm long in already connected steel pipes, especially near magnetic fields where the signal changes. At this point, after the individual sections have been demagnetized, the root welding must be completed and then the next section must be demagnetized. An electromagnet with a special structure was selected for demagnetization. The electromagnet is installed at the connection of the steel tube so that the N pole of the electromagnet is placed with the magnetic pole S on the edge of the steel tube, and the magnetic pole S is connected to the magnetic N pole of the tube. During the demagnetization process, a magnetometer should be used to periodically measure the direction and strength of residual magnetism in the steel pipe (when the power is on). The magnitude of the demagnetized magnetic field is adjusted by changing the current strength, and the direction of the magnetic field is adjusted by changing the current direction, that is, by converting the positive and negative poles of the power supply.
The C-shaped or cylindrical permanent magnet IOHÄ KT5 An alloy with permanent magnets was selected for demagnetization. When the magnet is installed correctly, the magnetic pole should face the magnetic pole of the magnetized steel tube. You can check whether the magnet is installed correctly using a magnetometer. To increase the demagnetization effect, magnets can be connected together (more than two or three) and their effects are the same. After demagnetizing the fitting section, root welding must be completed. The magnet must then be moved to the next degaussing section. To strengthen the demagnetizing magnetic field, the magnet must be close to the demagnetizing field, otherwise it may move away. When the magnet is moved along the surface of the steel pipe, the remanence in the welded joint can be reduced to the minimum value. To change the direction of the demagnetizing magnetic flux, the C-shaped magnet needs to be rotated 180° in the horizontal plane, and the cylindrical magnet mounted on the edge of the docking station must be replaced or rotated 180° in the vertical plane. After each demagnetization process, the size of the remanence must be checked with a magnetometer.
