Large-diameter welded steel pipe

I. What is a straight-seam welded pipe?

A straight-seam welded pipe is a steel pipe made from steel plates or strips as raw materials. Using a straight-seam forming process, the weld seam is created as a straight line parallel to the pipe’s axis, and then the pipe is welded together using various welding methods.

As its name suggests, its core feature is that straight longitudinal weld seam.

II. Main Features

Straight-seam welded pipes combine the cost-effectiveness of welded pipes with reliability that is nearly equivalent to that of seamless pipes. Their key features include:

Excellent pressure-bearing capacity: Thanks to its short and straight weld seams, as well as the use of advanced welding and post-weld treatment processes, this product boasts exceptionally high pressure-bearing capability, making it particularly suitable for high-pressure operating conditions.

High dimensional accuracy and uniform wall thickness: Made from flat blanks, with precise wall-thickness control and excellent surface quality.

Weld seam quality is easy to control and inspect: Straight weld seams are conducive to automated welding and non-destructive testing methods such as ultrasonic inspection, ensuring stable and reliable quality.

High production efficiency and moderate costs: Compared to seamless pipes, the production cost is lower; compared to spiral welded pipes, it offers superior quality and efficiency for certain specifications.

Wide caliber range: We can produce pipes ranging from medium to large diameters.

III. Main Categories (by Manufacturing Process)

This is key to understanding straight-seam welded pipes. Depending on the welding process, they can be broadly categorized into two main types:

1. Straight-seam high-frequency resistance-welded pipe

English abbreviation: ERW

Manufacturing principle: After the steel strip is cold-formed into shape, the skin effect and proximity effect of high-frequency current are utilized to instantly heat the edges of the tube blank to a molten state. Under the action of the extrusion rollers, pressure welding is then carried out.

Features:

Wireless: The welding process does not involve adding any filler metal.

The weld seam is narrow, and the heat-affected zone is small.

Production efficiency is extremely high.

After online heat treatment, the weld performance can be comparable to that of the base metal.

Main applications:

Medium- and low-pressure fluid conveyance pipes (such as water, gas, and air).

Structural pipes (e.g., scaffolding, building structures).

Crude oil and natural gas gathering and transmission pipelines.

2. Straight-seam submerged arc welded pipe

English abbreviation: LSAW

Manufacturing principle: The process begins with steel plates as the raw material. After pre-bending the edges, the plates are formed using various shaping methods (see below), and then welded internally and externally using submerged arc welding technology.

Features:

Welding quality is exceptionally high: submerged arc welding offers deep penetration and stable quality.

Capable of producing thick-walled, large-diameter steel pipes.

The weld zone contains filler metal and exhibits excellent mechanical properties.

It is the mainstay of long-distance oil and gas pipelines.

Main applications:

High-pressure, large-diameter pipelines for the transportation of oil and natural gas.

Large structural components, port pile driving.

Pressure-bearing equipment, tubing strings.

LSAW further subdivided according to forming process:

1. UOE welded pipe:

Process: The steel plate is first stamped into a U-shape using a U-shaped press, then fed into an O-shaped press to be formed into a circular shape, and finally undergoes welding and mechanical expansion.

Advantages: High quality, high dimensional accuracy, low residual stress—making it the preferred choice for high-end pipeline pipes.

Disadvantages: The equipment investment is enormous.

2. JCOE welded pipe:

Process: The steel plate is formed through multi-step stamping into J-shaped, C-shaped, and O-shaped profiles, then welded and expanded in diameter.

Advantages: Low mold costs and high production flexibility, making it suitable for multi-specification, small-batch production.

Disadvantages: Production efficiency is slightly lower than that of UOE.

Roll forming: This process involves continuously bending sheet metal using multiple-roll forming machines, and is primarily used for medium-diameter pipelines.

IV. Major Application Areas

Straight-seam welded pipes, especially LSAW, are the “major arteries” of modern industry:

Energy transmission:

Long-distance oil and gas pipelines—such as the national-level “West-to-East Gas Pipeline” project—almost entirely use straight-seam submerged arc welded pipes.

Submarine pipelines: Highly demanding in terms of quality and safety, with LSAW being the primary choice.

Structural field:

Large-scale structures: Truss structures for stadiums and airports.

Bridge Construction: Steel tube concrete arch ribs used as bridge piers and for arch bridges.

Pile driving: foundation piles for docks and high-rise buildings.

Fluid transport:

Urban pipeline network: water supply and heating pipelines.

Industrial piping: Process piping within a factory.

V. In-depth Comparison with Other Steel Pipes

To more intuitively illustrate the position of straight-seam welded pipes within the steel pipe family, the following figure provides a comprehensive comparison—with respect to “relative cost” and “performance/reliability”—against spiral-welded pipes and seamless pipes. As clearly shown in the figure above:

Helical welded pipes are positioned as a cost-effective, large-diameter solution, but their performance and reliability are relatively lower.

Straight-seam welded pipe strikes an excellent balance in terms of cost, performance, and reliability, making it the preferred choice for many high-pressure, high-strength applications.

Seamless pipes represent the pinnacle of performance and reliability, but they also come with a high cost.

In addition to the core positioning shown in the chart, they also have the following key differences in technical details: