What Welded Steel Pipe Is
Welded steel pipe is a tube with a longitudinal or helical seam, formed by bending steel strip or plate into a round, square, or other shape and closing the edges by welding. The raw material is strip or plate rather than a solid billet, which is why welded pipe is cheaper to produce and is made in a much wider range of diameters than seamless pipe. The weld seam is the defining feature: it is made without a filler in electric-resistance welding, or with a filler in arc welding, and the quality of the seam is verified by non-destructive testing. Welded pipe is generally used for circulating cooling-water lines, pressure drainage, rainwater pipes, structural members, and a large share of line pipe, while seamless pipe is specified where the highest pressure integrity is required.
Welded vs Seamless
The fundamental difference between welded and seamless pipe is the seam. Seamless pipe is pierced from a solid billet and has no longitudinal weld, so its structure is uniform around the whole circumference and it can withstand the highest pressures and the most severe cyclic service. Welded pipe is formed from strip and welded, which is faster and cheaper, but the seam is a discontinuity that must be made and inspected to a high standard. For the majority of water, gas, structural, and low-to-medium-pressure service, a properly made and examined welded pipe is fully adequate, and its economy makes it the standard product.
Classification by Welding Method
Welded pipe is classified by the welding process. Electric-resistance-welded (ERW) pipe is made by high-frequency resistance welding without filler metal; it is used for oil-drilling casing, line pipe, structural tubing, and general service. Submerged-arc-welded (SAW) pipe is made with the arc submerged under a granular flux, giving deep penetration, and it is used for large-diameter high-integrity lines. Gas-welded and furnace-welded pipe are the older processes, with the furnace-butt-welded type still produced to some general standards. The process is chosen from the diameter, the wall thickness, and the service requirements.
Classification by Seam Shape
By the shape of the seam, welded pipe is divided into straight-seam pipe and spiral (helical) pipe. Straight-seam pipe has a longitudinal weld and a simple production process with high efficiency and low cost, and it dominates the smaller diameters. Spiral-welded pipe has a helical seam formed from strip at an angle; its strength is generally considered equal to or higher than straight-seam pipe for the same wall because the spiral seam is not aligned with the hoop stress, and it can produce very large diameters from a narrow strip, with different diameters from the same strip width. The trade-offs are that the weld length is 30 to 100% longer than for the same length of straight-seam pipe, and the production speed is lower, so small diameters use straight-seam welding and large diameters use spiral welding.
Applications
The applications follow the process family. Electric-welded pipe is used for oil drilling, machinery manufacturing, line pipe, and structural tubing. Furnace-welded pipe serves water and gas service. Large-diameter straight-seam welded pipe is used for high-pressure oil and gas transportation and structural columns, and spiral-welded pipe is used for oil and gas transportation, pipe piles, bridge piers, and large water mains. The standards, such as A53, API 5L, EN 10217, and GB/T 3091, define the requirements for each product, and the correct standard is selected by the service.
Frequently Asked Questions
Q: What is welded steel pipe?
A tube formed from steel strip or plate and closed by welding, with a longitudinal or spiral seam, produced at lower cost than seamless pipe.
Q: What is the difference between welded and seamless pipe?
Seamless pipe has no weld and withstands higher pressure; welded pipe is cheaper, faster to produce, and available in larger diameters.
Q: What are the main welding methods?
Electric resistance welding (ERW), submerged arc welding (SAW), gas welding, and furnace welding, selected by the size and service.
Q: What is the difference between straight-seam and spiral-welded pipe?
Straight-seam pipe has a longitudinal weld and is efficient for small diameters; spiral-welded pipe has a helical seam, suits very large diameters, and has a longer weld per length.
Q: What is spiral-welded pipe used for?
Oil and gas transportation, water mains, pipe piles, and bridge piers, where large diameters are required.
Q: Is welded pipe as strong as seamless?
At the same grade and with a sound weld, welded pipe meets the same specified strength; seamless remains the choice where the absence of a seam is essential.
