What are the technical specifications of a straight-seam pipe?

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2022-03-14

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Straight-seam pipe is a type of welded steel pipe, referring to steel pipes whose weld seam runs perpendicular and parallel to the pipe’s longitudinal axis. Based on wall thickness, it is classified into thin-walled straight-seam pipes and thick-walled straight-seam pipes. According to its shape, it can be categorized into round pipes, square pipes, or specially shaped pipes. By application, it includes general welded pipes, galvanized pipes, oxygen‑service pipes, electrical conduit pipes, metric‑size pipes, roller‑track pipes, deep‑well pump pipes, automotive pipes, transformer pipes, film‑processing pipes, formed pipes, and spiral‑welded pipes.

　　 Straight-seam pipe It is a type of welded steel pipe, referring to steel pipes whose joints are aligned perpendicular to the pipe’s longitudinal axis. Based on wall thickness, it is classified into thin-walled straight-seam pipes and thick-walled straight-seam pipes. According to its shape, it can be categorized into circular pipes, square channels, or specially shaped channels. By application, it is further divided into general‑purpose channels, galvanized channels, oxygen‑gas channels, electrical conduit pipes, metric‑standard channels, roller‑track pipes, deep‑well pump pipes, automotive channels, transformer channels, film‑processing channels, formed‑section channels, and spiral channels.

　　What are the technical specifications of a straight-seam pipe?

　　1. Welding

　　Straight pipes typically employ high-frequency welding, which offers several advantages: it requires no welding filler metal, produces minimal spatter, creates a narrow heat-affected zone, yields aesthetically pleasing welds, and delivers excellent mechanical properties—making it widely used in steel pipe manufacturing. Based on the principles of electromagnetic induction and the skin effect, proximity effect, and eddy‑current heating in conductors, the steel edges to be welded are locally heated to the molten state; then, under roller pressure, the butt joint is consolidated into a crystalline structure, achieving the desired weld. After cooling, this results in a strong, straight weld.

　　2. Welding Gap

　　Steel is supplied to the welded pipe forming unit, where it is rolled by multiple rollers, gradually coiling into a circular passage with an open gap. The pressure of the squeeze rolls is adjusted to maintain the gap within 1–3 mm, ensuring that the two ends at the entrance are flush. If the gap is too large, the overlap will be insufficient, resulting in inadequate vortex energy and poor alignment of the joint, which can lead to lack of fusion or cracking. Conversely, if the gap is too small, adjacent‑layer effects intensify, generating excessive joint energy and causing burn‑through; alternatively, after extrusion and roll‑forming, deep pits may form, compromising the quality of the weld surface.

　　3. Welding temperature

　　Using low-carbon steel, the temperature is maintained within the range of 1250 to 1460°C to meet the requirements for penetration in joints with wall thicknesses of 3 to 5 mm. Temperature is primarily controlled by adjusting the high-frequency eddy‑current heating power and the welding speed. If the heat input is insufficient, the heated joint edges fail to reach the required joining temperature, leaving the metal microstructure in a solid state and resulting in incomplete fusion or lack of proper weld penetration. Conversely, excessive heat input causes the heated weld edges to exceed the joining temperature, leading to overheating or droplet formation and ultimately to the development of molten holes in the weld.

　　What are the requirements and inspection procedures for straight-seam pipes?

　　According to the GB/T 3091 standard “Steel Pipes for Low-Pressure Fluid Transport,” the nominal diameters of welded pipes range from 6 to 150 mm, with nominal wall thicknesses from 2.0 to 6.0 mm. The typical pipe lengths are 4 to 10 m, and they may be delivered in foot‑based or multiple‑foot lengths. The pipe surface shall be smooth, with no defects such as folds, cracks, delamination, or laps. Minor surface imperfections—such as runs, scratches, or weld‑related irregularities—may be present, provided they do not exceed the negative deviation of the wall thickness. Additionally, wall‑thickness thickening and internal seam‑weld ribs may occur.

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