The Role and Application of Magnetic Bar in High-Frequency Welding

The Role and Application of Magnetic Bar in High-Frequency Welding

Understanding the functionality of magnetic bars in high-frequency welding and knowing how to select and use them correctly are crucial for improving welding tube technology. This article explores the significance of magnetic bars in high-frequency welding, their working principles, and the proper selection and usage techniques.

Working Principle of Magnetic Bars in High-Frequency Welding:

High-frequency welding machines consist of several components, including a high-frequency induction coil, the steel pipe to be welded, a powerful magnetic bar, squeeze rolls, and a water cooling system. When a high-frequency current passes through the induction coil, it generates a high-frequency magnetic flux within the coil. This magnetic flux induces eddy currents in the welded pipe, melting the weld seam and achieving the desired welding result. By using a magnetic bar, the magnetic flux within the induction coil is significantly increased, leading to a higher induced electromotive force in the welded pipe and an increase in welding power. Additionally, the placement of the magnetic bar allows for a concentrated distribution of magnetic flux in the contact area with the pipe, reducing the magnetic flux between the induction coil and the pipe and thereby improving welding efficiency.

Selection and Usage of Magnetic Bars:

To effectively enhance the magnetic flux within the induction coil, magnetic bars must possess high magnetic permeability. This requirement is particularly crucial within the operating range of the magnetic field. For welding machines with high power output, it is essential to choose magnetic bars with a higher saturation magnetic flux density (also known as Bs). The temperature of the magnetic bar increases during welding. It is important to select magnetic bars with high Curie temperatures (Tc) to maintain their magnetic properties at the elevated working temperatures (around 200°C to 250°C). The Curie temperature is a significant technical parameter that indicates the performance of a magnetic bar. Furthermore, magnetic bars experience energy consumption during the welding process due to hysteresis losses, eddy current losses, and residual losses. Although these losses result in energy consumption and an increase in the temperature of the magnetic bar, they have minimal impact on welding. Therefore, the energy consumption is usually not emphasized as a performance parameter.

Correct Selection and Usage of Magnetic Bars:

In recent years, there has been rapid development in the research and production of magnetic bars for high-frequency welding in China. Some specialized manufacturers have approached or reached international standards (such as those set by Japan’s TDK Corporation) in terms of technical performance, with slight gaps remaining in intrinsic magnetic properties. Achieving the same level of intrinsic magnetic properties as foreign products is technically feasible in China, but it requires significant technological transformations in equipment and the thorough resolution of material-related issues, such as activity and purity. These factors would inevitably lead to a substantial increase in the price of magnetic bars. Considering the current capacity of domestic welding tube manufacturers, it is impractical to pursue this path. However, by correctly selecting and using magnetic bars, it is possible to meet the requirements of various imported and domestically produced welding machines and achieve high-quality and high-speed welding. The following points address the proper selection and usage of magnetic bars:

  • The magnetic bar should have good straightness and meet the usage requirements.
  • The surface of the magnetic bar should be dense, with fine and uniform grain structure and no cracks. It should produce a clear sound when tapped.
  • The length of the magnetic bar should be determined based on the welding machine. The recommended length is typically 1.5 to 2 times the distance between the centerlines of the welding rolls and the induction coil.
  • The diameter of the magnetic bar is complex to select since it is constrained by two factors. From the perspective of increasing magnetic flux, a larger diameter is preferred. However, to enhance water flow, a larger gap between the outer diameter of the magnetic bar and the inner diameter of the welded pipe is desired. The correct selection method is to ensure sufficient water flow while maximizing the diameter of the magnetic bar. For o4in welded pipes, a 2mm gap between the magnetic bar and the pipe is recommended. For larger diameter pipes, the gap should be correspondingly increased.


Understanding the functionality and proper usage of magnetic bars is essential for individuals involved in high-frequency welding. This article has provided detailed insights into the working principles of magnetic bars, the requirements for their performance during the welding process, and the correct selection and usage techniques. By following these guidelines, welding professionals can make informed decisions and optimize the utilization of magnetic bars, thereby enhancing the efficiency and effectiveness of high-frequency welding operations.