How is welded steel pipe made?

The basic manufacturing process of welded steel pipe involves bending steel plates or coils into a cylindrical shape and then welding the joints. However, based on the welding techniques and forming methods used, welded steel pipes are mainly divided into three types: resistance welded (ERW), longitudinal submerged arc welded (LSAW), and spiral submerged arc welded (SSAW).

In this guide, we will detail the manufacturing steps for each type of welded steel pipe, compare their characteristics, and analyze their advantages and disadvantages compared to seamless steel pipes.

1. Resistance Welded Steel Pipe Manufacturing Process

The manufacturing method for resistance welded (ERW) steel pipe is to cold-bend steel coils into a cylindrical shape.

Detailed Steps:

Uncoiling and Leveling: The steel coil is uncoiled and thoroughly leveled using a leveling machine. This eliminates residual coiling stress and ensures consistent flatness throughout the entire pipe manufacturing process.

Edge Treatment: The edges of the steel strip are trimmed and cleaned to avoid burrs and barbs, ensuring weld quality.

Roll forming: The steel strip is gradually bent into a circular tube blank through a series of rollers.

High-frequency welding (ERW): A high-frequency current (typically 100 to 500 kHz) is applied to the edges of the steel strip. The resistance generated by the current melts the edges, which are then pressed together by mechanical rollers to form a strong weld without the need for filler metal.

Weld heat treatment (annealing): The weld is heat-treated to eliminate residual stress and normalize the microstructure, ensuring that the weld strength is comparable to that of the base material.

Sizing and straightening: The pipe is sizing the tube to achieve a precise outer diameter and straightening it to meet tolerance standards.

2. LSAW Steel Pipe Manufacturing Process

LSAW steel pipe (longitudinal submerged arc welding) uses a single thick steel plate as raw material. It is commonly used for large-diameter, thick-walled, high-pressure oil and gas pipelines.

Process steps:

Steel plate milling: The edges of the thick steel plate are milled to the required shape and angle to ensure good weld penetration.

Forming (JCOE or UOE): Steel plates are sequentially pressed into "J," "C," and "O" shapes, ultimately expanding into a circular steel pipe. JCOE forming is the most common and widely used forming method.

Pre-welding: The formed steel pipes are temporarily fixed together with continuous welds.

Internal and External Submerged Arc Welding: The pipes are moved to the welding station for submerged arc welding (SAW). The welding wire melts under a protective layer of granular flux. The weld is performed first from the inside and then from the outside to ensure 100% penetration.

Mechanical Expansion: LSAW pipes typically require full-length internal expansion to improve dimensional accuracy and eliminate internal stress.

3. SSAW Steel Pipe Manufacturing Process

SSAW steel pipes (spiral submerged arc welded) are also manufactured using steel coils, but the coils are wound at a specific helix angle (forming angle) rather than longitudinally.

Process Steps:

Uncoiling and Joint Welding: The steel coils are uncoiled, and the ends of adjacent coils are welded together to form a continuous steel strip.

Helical Angle Forming: The steel strip is fed into a forming machine and bent into a cylinder at a precise angle. This allows slender spiral tubing to be processed into large-diameter pipes.

Continuous Submerged Arc Welding: Similar to LSAW, SSAW also uses submerged arc welding. Welding is performed simultaneously on both the inner and outer sides of the spiral weld as the pipe rotates forward.

Fitting to Length: A flying cutter cuts the continuously formed spiral pipe to the required length.

4. Key Post-Welding Treatment and Quality Control Steps

The manufacturing process of any welded steel pipe must undergo rigorous inspection and testing to ensure compliance with standards such as API 5L, ASTM A53, or EN 10217.

Non-Destructive Testing (NDT): All welds undergo 100% continuous online ultrasonic testing (UT) and X-ray inspection to detect any internal defects, cracks, or porosity.

Hydrostatic Testing: Each pipe is filled with water and pressurized to the specified pressure to ensure it will not leak or burst under on-site operating pressure.

Surface Treatment and Corrosion Protection: Depending on the application, the pipes will be straightened, the ends chamfered, and coated with a corrosion-resistant coating such as galvanizing, 3PE (triple-layer polyethylene), FBE (fusion-bonded epoxy), or black paint.

5. Comparison: ERW vs. LSAW vs. SSAW vs. Seamless Steel Pipes

To help you choose the best pipe for your project, here is a comprehensive comparison of the three welded technologies alongside welded vs seamless steel pipe characteristics.

Features	ERW	LSAW	SSAW	Seamless
Process	Cold formed, high frequency induction	Heavy single plate, JCOE press, SAW	Strip strip formed spirally, SAW	Billet piercing, hot rolled / cold drawn
Seam Style	Longitudinal, no filler metal	Longitudinal, double-sided filler	Spiral seam along the body	Seamless (None)
OD Range	21.3mm - 660mm	406mm - 1422mm	219mm - 3020mm	10.3mm - 914mm
Wall Thickness	Thinner (up to 30mm)	Extremely thick (up to 65mm)	Medium to thick (up to 25.4mm)	Extremely thick with a wide range of thicknesses (up to 100mm+).
Precision	Excellent, highly uniform WT	High, using mechanical expander	Moderate dimensional precision	Moderate, slight eccentricities
Pressure Class	Medium Pressure	Very High Pressure	Medium-Low Pressure, Structural	Ultra-High Pressure & Temp
Cost Impact	Highly Cost-Effective	High (due to plate & JCOE setup)	Moderate (High production speed)	Highest Premium Cost

6. FAQ: Frequently Asked Questions

Q1: What is the main difference between welded and seamless steel pipes?
The primary difference lies in the welded steel pipe manufacturing process which uses a weld seam to join a flat plate or coil into a tube. Seamless pipes are made by piercing a solid cylindrical billet, meaning they have no weld seams. This gives seamless pipes higher pressure ratings but makes them significantly more expensive than welded options.

Q2: Why is the weld seam annealed in ERW steel pipes?
During high-frequency welding, extreme heat is localized at the edges, changing the grain structure of the steel. Annealing (heat treating) the weld area refines the grain structure, removes internal stresses, and restores the mechanical properties so that the seam matches the strength of the rest of the pipe body.

Q3: Can welded steel pipes withstand high pressure?
Absolutely. Modern LSAW and heavy-duty ERW steel pipes undergo stringent NDT inspections and hydrostatic testing. When manufactured under strict API 5L standards, they can safely withstand the high pressures required for international oil and gas transmission.

Conclusion

Understanding how is welded steel pipe made gives you clarity on performance limitations and pricing structures. While ERW is ideal for cost-efficient, small-to-medium diameter projects, LSAW thrives in heavy-duty, large-diameter high-pressure environments, and SSAW offers a highly flexible solution for large-diameter liquid transport and piling.