Detailed explanation of online annealing process for steel pipe welds and equipment selection
[ I. Purpose and Function of Weld Annealing ] ()
During high-frequency welding, the edges of the steel pipe are rapidly heated to a molten state and pressed together, followed by rapid cooling. This process results in the formation of a hardened structure in the weld seam and heat affected zone, leading to concentration of internal stress. If annealing treatment is not carried out, the weld seam is prone to cracking during subsequent bending, expansion, flattening and other processes.
The main functions of online annealing include: eliminating welding residual stress, reducing weld hardness, improving plasticity and toughness; Refine the grain size of the weld seam and improve the metallographic structure; Improve the corrosion resistance of welds and extend the service life of pipes. For high-end products such as pressure pipes, structural pipes, and oil and gas transmission pipes, weld annealing is an essential process.
[ II. Technical Advantages of Medium Frequency Induction Annealing ] ()
At present, the online annealing of welds generally adopts the medium frequency induction heating method, which has the following significant advantages compared to gas annealing furnaces:
**Fast heating speed: Induction heating can heat the weld seam to the desired temperature within seconds, perfectly matching the high-speed operation of the welded pipe production line without affecting the production cycle.
**Accurate and controllable heating: By adjusting the power and operating speed, the annealing temperature and holding time can be precisely controlled to ensure stable and consistent annealing quality.
*Local heating energy-saving: * The heat is concentrated in the weld area, without the need to heat the entire steel pipe, resulting in much lower energy consumption than the overall annealing furnace and lower operating costs.
*Environmentally friendly and clean: * No open flames, no smoke, no exhaust emissions, improving the working environment of the workshop and meeting environmental requirements.
*Easy to automate: * The equipment can be linked with the speed of the production line, achieving unmanned operation and reducing manual intervention.
[ III. Equipment Composition and Workflow ] ()
The intermediate frequency equipment for online annealing of steel pipe welds produced by Baoding Jinfan mainly consists of four parts: power cabinet, lifting adjustment mechanism, dedicated sensor, and water cooling circulation device. According to the pipe diameter specifications and production line speed, single or multiple machines can be arranged in series.
During operation, the welded steel pipe passes through the annular sensor, and the alternating magnetic field generated by the intermediate frequency power supply induces eddy currents at the weld seam, rapidly heating the weld seam to above the recrystallization temperature (usually 700-900 ℃). Then, the steel pipe is slowly cooled in air to complete the recrystallization annealing process. By adjusting the power supply to match the production line speed, the annealing temperature and cooling rate can be precisely controlled.
[ IV. Key Points for Equipment Selection ] ()
*Power selection: * Calculate the required power based on pipe diameter, wall thickness, and production line speed. The larger the pipe diameter, thicker the wall thickness, and faster the line speed, the higher the required power. Common configurations range from 100kW to 500kW.
*Frequency selection: * Welding seam annealing generally uses a 1-8kHz intermediate frequency power supply. The lower the frequency, the greater the heating depth, which is suitable for thick walled pipes; The higher the frequency, the more concentrated the heat, making it suitable for thin-walled tubes.
**Sensor design: The shape and size of the sensor directly affect the uniformity of heating and need to be customized according to the specific tube type. Baoding Jinfan can design specialized high-efficiency sensors based on user pipe specifications to improve heating efficiency.
*Multi machine configuration: * For high-speed large-diameter welded pipe production lines, when the power of a single device is insufficient, 2-3 intermediate frequency annealing machines can be arranged in series and heated in sections to ensure sufficient annealing of the weld seam.
**Cooling system: Both the intermediate frequency power supply and the sensor require reliable water cooling. It is recommended to equip them with independent water coolers or closed-loop water systems to ensure long-term stable operation of the equipment.
[ V. Features of Jinfan Products ] ()
Baoding Jinfan High Frequency Equipment Co., Ltd. has rich supporting experience in the welded pipe industry. Its welding seam intermediate frequency annealing equipment adopts IGBT solid-state intermediate frequency power supply, which has high energy efficiency and stable operation. It is equipped with a dedicated lifting adjustment mechanism, making it easy to adjust when changing specifications. The equipment can be equipped with infrared temperature measurement devices to achieve temperature closed-loop control, ensuring better annealing quality. It has been stably operated in multiple welded pipe enterprises in China.
