Spiral welded pipes generally have higher strength than straight seam welded pipes. They can produce larger-diameter pipes using narrower billets, and pipes of different diameters can be produced using billets of the same width. However, compared with straight seam pipes of the same length, the weld seam length increases by 30~100%, and the production speed is lower. Therefore, straight seam welding is mostly used for smaller-diameter pipes, while spiral welding is preferred for larger-diameter pipes. For large-diameter spiral steel pipe manufacturers, after the two edges of the spiral steel pipe billet are heated to the welding temperature, under the pressure of the extrusion rollers, common metal grains penetrate and crystallize each other, ultimately forming a strong weld seam.
If the extrusion force is too small, the number of common crystals formed will be small, resulting in decreased strength of the weld metal and cracking under stress. Conversely, excessive extrusion pressure can squeeze molten metal out of the weld seam, not only reducing weld strength but also generating numerous internal and external burrs and even causing defects such as overlapping seams.
The impedance device is a special magnetic bar or a set of magnetic bars for spiral steel pipes. Its cross-sectional area should generally be no less than 70% of the inner diameter cross-sectional area of the steel pipe. Its role is to form an electromagnetic induction loop with the induction coil, the weld seam edge of the pipe billet, and the magnetic bar, generating a proximity effect that concentrates eddy current heat near the weld seam edge of the pipe billet, heating it to the welding temperature. The impedance device is dragged within the pipe billet by a steel wire, with its central position relatively fixed close to the center of the extrusion rollers. During startup, due to the movement of the pipe billet, the impedance device experiences significant wear from friction against the inner wall of the pipe billet.
Large-diameter spiral steel pipes are frequently produced from steel strip coils as raw materials, which are formed through extrusion at room temperature and welded using the automatic double-wire double-sided submerged arc welding process to create spiral seam steel pipes. The main production process is as follows:
(1) Raw materials, namely steel strip coils, welding wires, and fluxes, undergo rigorous physical and chemical inspections before use.
(2) The head and tail of the steel strip are butt-welded using single-wire or double-wire submerged arc welding, and automatic submerged arc welding is used for repair welding after the steel strip is rolled into a pipe.
(3) Before forming, the steel strip undergoes leveling, trimming, edge planing, surface cleaning, conveying, and pre-bending.
(4) An electric contact pressure gauge controls the pressure of the hydraulic cylinders on both sides of the conveyor to ensure smooth transportation of the steel strip.
(5) External or internal control roller forming is employed.
(6) A weld gap control device is used to ensure that the weld gap meets welding requirements, with strict control over pipe diameter, alignment deviation, and weld gap.
(7) Both internal and external welding use American Lincoln electric welding machines for single-wire or double-wire submerged arc welding, achieving stable welding specifications.
(8) The completed weld seams are inspected using online continuous ultrasonic automatic flaw detectors, ensuring 100% non-destructive testing coverage of spiral weld seams. If defects are detected, automatic alarms are triggered, and markings are sprayed. Production workers can then adjust process parameters promptly to eliminate defects.
Advantages of Thick-Walled Spiral Steel Pipes: The sturdy oxide film on the surface of thick-walled steel pipes enables stainless steel to exhibit excellent corrosion resistance in all water qualities, including soft water. Even when buried underground, it demonstrates superior corrosion resistance and good erosion resistance.
