What is the welding process for straight-seam pipes?

　　 Straight-seam pipe What is the welding process like? Let’s take a look!

　　1. Control the welding gap

　　Steel is fed to the welded pipe‑making unit, where it passes through a series of rollers that progressively coil the steel strip into a circular pipe with an open seam. The pressure of the squeeze rolls is adjusted to maintain the weld gap within 1–3 mm, ensuring that the two ends of the weld are flush. If the gap is too large, the joint‑forming effect diminishes, resulting in insufficient heat and an imperfect weld fit, which may lead to lack of fusion or cracking. Conversely, if the gap is too small, the adjacent‑area effect intensifies, generating excessive heat that can cause burn‑through or, after squeezing and rolling, create deep pits that degrade the weld surface quality.

　　2. Welding Temperature Control for Straight-Seam Pipes

　　The temperature of welded straight-seam pipes is primarily influenced by the high-frequency eddy-current heating power. According to Equation (2), this heating power is mainly governed by the current frequency, with the eddy-current heating power being proportional to the square of the excitation frequency. The excitation frequency, in turn, depends on the excitation voltage, the current, and the values of the capacitor and inductor. The relevant frequency expression is as follows: F=1/[2(CL)1/2].(1)

　　In the format: F – excitation frequency (Hz); C – capacitance of the excitation circuit (F), where capacitance = power supply / voltage; L – inductance of the excitation circuit, where inductance = magnetic flux / current.

　　As shown above, the excitation frequency is inversely proportional to the square root of the capacitor and inductor values in the excitation circuit, or directly proportional to the square roots of the voltage and current. By adjusting the circuit’s capacitance, inductance, or the voltage and current, the welding temperature can be controlled by varying the excitation frequency. For low‑carbon steel, maintaining the welding temperature within the range of 1250–1460°C ensures adequate penetration for pipe wall thicknesses of 3–5 mm. Additionally, the welding temperature can also be regulated by adjusting the welding speed.

　　If the heat input is insufficient, the heated weld edge will fail to reach the welding temperature, leaving the metal microstructure in the solid state and resulting in lack of fusion or incomplete penetration. Conversely, when the heat input is excessive, the heated weld edge may exceed the welding temperature, leading to overheating or drooling and consequently causing molten‑pool holes in the weld.

　　3. Pressure Control of Longitudinal Seam Pipes

　　After the two edges of the straight-seam pipe are heated to the welding temperature, they are pressed together by the squeeze rolls, causing the metal particles at the interface to interpenetrate and crystallize, ultimately forming a strong weld. If the squeezing pressure is too low, insufficient metallic bonds form, resulting in poor weld strength and susceptibility to cracking under load. Conversely, if the squeezing pressure is excessive, molten metal may extrude from the weld, reducing its strength, generating substantial internal and external burrs, and potentially leading to defects such as weld overlap.

　　4. High-frequency induction circular positioning adjustment

　　The high-frequency induction coil should be positioned as close as possible to the extrusion roller. When the induction coil is located far from the extrusion roller, the effective heating time is prolonged, the heat-affected zone becomes extensive, and the weld strength is reduced. Conversely, insufficient heating of the weld edges can lead to poor forming after extrusion.

　　5. The impedance is a magnetic rod specifically designed for welded pipes. Its cross-sectional area should generally be at least 70% of the pipe’s internal cross-sectional area. This configuration, together with the inductive coil, the pipe‑blank weld edge, and the magnetic rod, forms an electromagnetic induction loop that generates a proximity effect. Vortex currents are concentrated near the weld edge of the pipe blank, heating the straight‑seam pipe blank to the welding temperature. The impedance is drawn into the pipe as a conductor, with its central position required to remain relatively fixed near the center of the extrusion roll. During startup, due to the rapid motion of the pipe blank, the impedance experiences significant frictional losses against the inner pipe wall, necessitating frequent replacement.