30. Why does spiral welded pipe need demagnetization? What is demagnetization?

Demagnetization, as the name suggests, refers to eliminating or weakening magnetic properties in materials. The fundamental principle involves altering the internal arrangement of magnetic domains within magnetic materials through external forces such as reverse magnetic fields, high temperatures, or impacts, thereby reducing or eliminating magnetism. Magnetic materials contain numerous microscopic magnetic domains whose initial aligned magnetic moments create overall magnetism. When external forces disrupt this order, the material's magnetism diminishes. The primary reasons for demagnetizing spiral welded pipes include: (1) Enhancing Welding Quality During production, especially during welding processes using direct current (DC), strong magnetic fields are generated at the weld joint and wire areas due to high currents. These fields align with the pipe's magnetic moments. After welding, as the magnetic field gradually weakens, residual flux density (commonly called "剩磁") remains in the pipe due to magnetic hysteresis. This residual flux negatively impacts subsequent welding by affecting arc stability and reducing quality. Demagnetization effectively eliminates or weakens this residual flux, ensuring optimal welding performance. (2) Ensuring Inspection Accuracy Residual flux also affects inspection accuracy. For instance, in X-ray industrial imaging systems, residual flux causes electron beam deflection in image intensifiers, resulting in "S" -shaped distortion. This distortion compromises detection of natural defects like porosity and slag inclusion, particularly significantly reducing the detection rate of linear defects such as incomplete penetration and cracks. Therefore, to ensure the accuracy of inspection results, demagnetization treatment is required for spiral welded steel pipes. (3) Meeting Usage Requirements Spiral welded steel pipes are widely used in various industrial fields such as oil and gas pipelines, building structural supports, etc. In these applications, the performance and stability of steel pipes are crucial. The presence of residual magnetism may affect the service performance of steel pipes, such as reducing their corrosion resistance and fatigue resistance. Therefore, to meet usage requirements and ensure long-term safety and reliability of steel pipes, demagnetization treatment is necessary. (4) Eliminating Safety Hazards In certain special environments, such as strong magnetic field areas or high-precision measurement scenarios, residual magnetism may cause safety hazards or measurement errors. For example, in areas densely populated with electronic devices, residual magnetism may interfere with their normal operation; in precision measurement situations, residual magnetism may lead to measurement deviations. Therefore, to eliminate these safety hazards and measurement errors, demagnetization treatment is required for spiral welded steel pipes. In summary, the main reasons for requiring demagnetization of spiral welded steel pipes include improving welding quality, ensuring inspection accuracy, meeting usage requirements, and eliminating safety hazards. Demagnetization treatment is an indispensable process in the production of spiral welded steel pipes, playing a significant role in ensuring the performance and stability of steel pipes.