In the welding operation, occasionally the phenomenon of magnetic deflection blowing will alter the welding process. The formation of magnetic deflections is the result of the presence of remanence in the pipe metal.
Normally, remanent magnetism is split into induction magnetism and process magnetism. Induction magnetism is often produced in the link of the factory pipe, such as: metal smelting, the use of electromagnetic crane for loading and unloading, steel pipe in the strong magnetic field stop, magnetization method to complete nondestructive inspection, steel pipe close to the strong power supply line placement and so on.
Process magnetism is often produced in the assembly and welding operation and the use of magnetic gripper, fixture and DC welding pipeline, such as: long time contact with the DC power supply connected to the electrical wire, wire exposed section or short circuit between the welding pliers and the pipe.
When Metal Welding Service pipe with magnetic service, there are usually problems such as arc ignition trouble, arc combustion stability destruction, arc deviation in the magnetic field, liquid metal and slag splashing out from the welding pool. In order to stabilize the welding process and enhance the quality of welded joints, the magnetized steel pipe should be demagnetized before welding.
It is usually hard to achieve full demagnetization of welded steel tubes. Consequently, when the remanence is not sufficient to alter the quality of welding, welding is permitted.
The demagnetization process at the joint of a single steel pipe and the steel pipe is normally embraced as follows:
(1) Calculate the size and direction of the remaining magnetic field of the steel pipe;
(2) select demagnetization method and technological means;
(3) with the selected method of degaussing the steel pipe or welding joints;
(4) Check the amount of remanence after degaussing to see whether it meets the requirements.
Commonly used demagnetization method is divided into: with direct current or alternating current, with the help of electromagnet or permanent magnet established magnetic field method.
Here we mainly analyze the DC degaussing process.
The principle of demagnetization is to form a magnetic field around the degaussed pipe in the opposite direction to the remaining magnetic field of the steel pipe, and regulate the strength of the magnetic field formed by controlling the size of the current to reach the purpose of demagnetization.
Configuration on the steel pipe section of 35 ~ 50 mm flexible welding wire coil, form a closed loop, the end of two welding wire separately by direct current welding power source is negative, the other side were received on the degaussing tube fixed on the metal plate of the formation of the magnetic field direction, in contrast to the steel pipe residual magnetic field direction,
At the beginning of demagnetization, the current can be adjusted to 😯 ~ 1OO A. As the current increases, the results of the degaussing magnetic field are monitored periodically on the steel pipe (calculated when the power is switched on). If NECESSARY, THE CURRENT SHOULD BE REGULATED (GENERALLY there should be a current regulating device) or its direction should be altered (the method used to change the welding wire on the DC welding power supply).
After the demagnetization is complete, the current should be slowly decreased for Lmin until it reaches zero, and then the power is cut off in order to reduce the flux smoothly.
When it is a single steel pipe demagnetization, you can first round the coil of 8 ~ 12 turns along the outer circle at one end of the steel pipe, with the maximum magnetic field value to demagnetization, and then the other end of the steel pipe in the same way.
When a single steel pipe is demagnetized to the joint of the steel pipes, the distance between the two pipes is not less than 300 mm, and the coil of 18 ~ 20 turns is wound 80 ~ 100 mm away from the end face of each pipe, and then the demagnetization is carried out according to the above method.
In some cases, it is approved to employ the degaussing scheme of welding tongs and metal plates connected to the electrical system. The electrode loaded in the electrode holder is short-circuited with the metal plate for about 10 s at A current of 300 A. Then disconnect. After each short circuit and break cycle, check the magnetic strength of the pipeline and repeat the above demagnetization process if necessary.
When the assembled joint is demagnetized, a welded wire with a cross-section of 35 ~ 50 mm is wound on the end of the butt steel pipe to form a common coil of two steel pipes, as shown in the figure above. The coils can be overlapped (clockwise or counterclockwise) with a total of 16 to 22 turns. At this time, the number of turns should be on the steel pipe with deeper remanence. This demagnetization process is usually optimal.
Utilizing alternating current, with the help of electromagnet or permanent magnet magnetic field method and DC degaussing principle is essentially the same.