Detailed Explanation of Welding Process for Large-Diameter Straight Seam Steel Pipes

Detailed Explanation of Welding Process for Large-Diameter Straight Seam Steel Pipes Double-wire automatic welding of straight seam steel pipes is a welding technology developed in recent years. Besides possessing the characteristics of semi-automatic single-wire welding, it also features concentrated energy and high deposition efficiency. The main and auxiliary wires are supplied with welding current by separate ordinary welding power sources, which are independently adjustable, allowing for optimal configuration of welding process parameters. The consistent spacing and welding angle between the two wires effectively control electromagnetic interference between the two arcs, exhibiting excellent static and dynamic characteristics. The two separate power sources work together in coordination through welding software to supply power to the main and auxiliary wires. Simultaneously, the main and auxiliary wires melt and transfer metal into the weld, forming a stable molten pool, ensuring the strength of the welded joint. It not only allows welding using conventional consumable electrode welding power sources but also reduces equipment costs, highly concentrates welding heat, accelerates deposition, increases welding efficiency, minimizes post-weld deformation, and reduces labor intensity. It effectively improves the microstructure and properties of the straight seam steel pipe weld, especially for welding high thermal conductivity materials, where the concentrated energy effect is particularly prominent. 1. Weld Gap Control: The strip steel is fed into the welding pipe unit and rolled by multiple rollers, gradually forming a circular tube blank with an open gap. The pressure of the extrusion rollers is adjusted to control the weld gap within 1-3 mm, ensuring the weld ends are flush. If the gap is too large, the proximity effect is reduced, resulting in insufficient eddy current heat and poor intergranular bonding, leading to incomplete fusion or cracking. If the gap is too small, the proximity effect is increased, resulting in excessive welding heat and weld burnout; or, after extrusion and rolling, deep pits may form in the weld, affecting the weld surface. 2. Welding Temperature Control: When the input heat is insufficient, the heated weld edges do not reach the welding temperature, and the metal structure remains solid, resulting in incomplete fusion or penetration. When the input heat is insufficient, the heated weld edges exceed the welding temperature, causing overheating or molten droplets, leading to weld cavities. 3. High-Frequency Induction Coil Position Adjustment: The high-frequency induction coil should be positioned as close as possible to the extrusion rollers. If the induction coil is far from the extrusion rollers, the effective heating time is longer, the heat-affected zone is wider, and the weld strength decreases; conversely, insufficient heating of the weld edge results in poor forming after extrusion. 4. Extrusion Pressure Control: After the two edges of the billet of a large-diameter straight seam steel pipe are heated to the welding temperature, under the extrusion of the extrusion rollers, common metal grains interpenetrate and crystallize, ultimately forming a strong weld. If the extrusion pressure is too low, the number of common crystals formed will be small, the weld metal strength will decrease, and cracking will occur under stress; if the extrusion pressure is too high, the molten metal will be squeezed out of the welded steel pipe, not only reducing the weld strength but also producing a large number of internal and external burrs, and even causing defects such as weld overlap. 5. The impedance device is one or a group of special magnetic rods for welded steel pipes. The cross-sectional area of ​​the impedance device should generally not be less than 70% of the cross-sectional area of ​​the inner diameter of the steel pipe. Its function is to form an electromagnetic induction circuit between the induction coil, the weld edge of the billet, and the magnetic rod, generating a proximity effect. Eddy current heat is concentrated near the weld edge of the billet, heating the edge of the billet to the welding temperature. The impedance device is dragged inside the tube blank by a steel wire, and its center position should be relatively fixed, close to the center of the extrusion roller. During startup, due to the rapid movement of the tube blank, the impedance device experiences significant wear due to friction against the inner wall of the tube blank, requiring frequent replacement. 6. Weld scars will form after welding and extrusion, which need to be addressed. The method is to fix a tool on the frame and use the rapid movement of the welded steel pipe to scrape off the weld scars. Burrs inside the welded steel pipe are generally not a problem. Post time: Dec-04-2025