Position Control of High-Frequency Induction Coil for Straight Seam Steel Pipes: The excitation frequency of a straight seam steel pipe is inversely proportional to the square root of the capacitance and inductance in the excitation circuit, or directly proportional to the square root of the voltage and current. Changing the capacitance, inductance, voltage, or current in the circuit alters the excitation frequency, thereby controlling the welding temperature. For low-carbon steel, a welding temperature of 1250~1460℃ can meet the penetration requirements for straight seam steel pipes with a wall thickness of 3~5mm. Alternatively, the welding temperature can also be adjusted by regulating the welding speed.
The high-frequency induction coil should be placed as close as possible to the extrusion rollers. If the induction coil is too 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 at the weld edge results in poor forming after extrusion. An impedance device is one or more magnetic rods specifically designed for welding steel pipes. The cross-sectional area of the impedance device should typically be no less than 70% of the inner diameter cross-sectional area of the straight seam steel pipe. Its function is to create an electromagnetic induction loop between the induction coil, the weld edge of the pipe blank, and the magnetic rod, generating a proximity effect. Eddy current heat is concentrated near the weld edge of the pipe blank, heating the edge to welding temperature. The impedance device is dragged inside the pipe blank by a steel wire, and its center position should be relatively fixed near the center of the extrusion rollers. During startup, due to the rapid movement of the pipe blank, the impedance device suffers significant wear due to friction against the inner wall of the pipe blank, requiring frequent replacement.
After the two edges of the pipe blank are heated to the welding temperature, the oil casing, under the pressure of the extrusion rollers, forms common metal grains that interpenetrate and crystallize, ultimately forming a strong weld. If the extrusion pressure is too low, the number of common crystals formed will be small, resulting in decreased weld metal strength and potential cracking under stress. Weld scars will also form after welding and extrusion, requiring scraping. The method involves fixing a cutter on the frame, and the rapid movement of the welded steel pipe smooths out the weld scars. Burrs inside welded steel pipes are generally not present. However, excessive extrusion pressure can cause molten metal to be squeezed out of the weld, reducing weld strength and producing numerous internal and external burrs, even leading to weld overlap and other defects.
When the heat input is insufficient, the heated weld edge does not reach the welding temperature, and the metal structure remains solid, resulting in incomplete fusion or incomplete penetration. Conversely, insufficient heat input causes the heated weld edge to exceed the welding temperature, leading to overheating or molten droplets and the formation of weld cavities. Welding temperature is primarily affected by the high-frequency eddy current heat power. According to relevant formulas, the high-frequency eddy current heat power is mainly influenced by the current frequency, which is proportional to the square of the current excitation frequency. The current excitation frequency is also affected by the excitation voltage, current, capacitance, and inductance.
Straight seam steel pipes have a simple production process, high production efficiency, low cost, and rapid development. Welded steel pipes generally have higher strength than straight seam steel pipes, and can be produced from narrower blanks to create larger diameter welded steel pipes, or from blanks of the same width to create welded steel pipes of different diameters. However, compared to straight seam pipes of the same length, the weld length increases by 30-100%, and the production speed is lower. Therefore, smaller diameter welded steel pipes mostly use straight seam welding, while larger diameter welded steel pipes mostly use welded joints.
Welded steel pipes are widely used in water supply projects, the petrochemical industry, chemical industry, power industry, agricultural irrigation, and urban construction, and are one of the twenty key products developed in my country. They are used for liquid transportation: water supply and drainage; gas transportation: coal gas, steam, liquefied petroleum gas; and structural applications: piling pipes, bridge construction; and pipes for docks, roads, and building structures.
Flattening cracking of high-frequency steel pipes is caused by weld microcracks, hard and brittle phase inclusions, and coarse granular structures. To better control the weld, the concept of a weld inclusion crack index has been proposed. This is mainly caused by insufficient weld strength, shape, or ductility. When small inclusions affecting impact toughness are present in a seam weld, weld cracking is only possible when the two opposite straight seam pipe walls are flattened to near the size of an iron box. To reduce weld cracking, improve weld toughness, and reduce weld inclusions, how can we reduce weld inclusions?
First, improve the purity of the raw materials, reducing phosphorus (P) and sulfur (S) content to decrease inclusion content. Second, check the edges of the steel strip for damage, rust, and contaminants, as these hinder molten metal discharge and easily lead to weld inclusions. Third, uneven wall thickness, burrs, and bulges can cause fluctuations in welding current, affecting the weld.
Post time: Jan-29-2026