Welded Steel Pipe

Welded Steel pipe refers to steel pipe made by bending hot-rolled or cold-rolled steel plates or strips into shape and then welding them with a straight seam on welding equipment. Its weld seam is parallel to the longitudinal direction of the steel pipe, which is the core characteristic distinguishing it from spiral welded pipe.

The core advantages of welded steel pipe are reflected in the following aspects:
Simple production process: The forming and welding processes are relatively simple, requiring less equipment investment.
High production efficiency: Continuous and automated production is possible, resulting in high output.
Low cost: High raw material utilization rate, with overall costs lower than seamless steel pipes.
Many varieties and specifications: A wide range of specifications from extremely small to large diameters can be produced.
High dimensional accuracy: Especially for high-frequency resistance welded (ERW) steel pipes, the outer diameter is highly accurate, with good ovality, uniform wall thickness, and smooth inner and outer surfaces.

Production Process
1. High-Frequency Resistance Welding (ERW/HFW) Process
High-frequency resistance welding uses steel strip (coil) as raw material. The core process flow is:
Steel strip (coil) uncoiling → Leveling → Edge trimming → Forming (roll bending) → High-frequency welding → Weld treatment (scraping, heat treatment) → Sizing → Cutting → Inspection → Packaging

High-frequency welding is based on the principle of electromagnetic induction and utilizes the skin effect to achieve fillerless welding. In recent years, the forming process of ERW welded pipes has been continuously developing, with roll bending technology gradually moving towards greater flexibility.

2. Straight Seam Submerged Arc Welding (SAWL) Process
Straight seam submerged arc welding uses a single steel plate as raw material. The core process flow is as follows:

Steel plate → Ultrasonic testing (plate inspection) → Milling → Pre-bending → JCO/UO forming → Pre-welding → Inner welding (multi-wire submerged arc welding) → Outer welding (multi-wire submerged arc welding) → Diameter expansion → Testing (ultrasonic/X-ray flaw detection/magnetic particle) → Hydrostatic test → Corrosion protection treatment

Taking the JCO forming method as an example: After ultrasonic testing, the steel plate is milled and pre-bent to form a specific curvature, and then progressively stamped to complete the pipe body forming. The inner and outer welds are welded using longitudinal multi-wire submerged arc welding (up to four wires).

Key differences between welded steel pipe and seamless steel pipe

Core Performance Characteristics
1. Good Dimensional Accuracy
ERW welded steel pipes have high outer diameter accuracy, good ovality, uniform wall thickness, and smooth inner and outer surfaces. 1. Flexible length options: Straight seam welded pipes can be produced in any length from 4 to 20 meters, with a yield rate exceeding 95%.
2. Excellent mechanical properties: Straight seam welded pipes possess good bending and compressive strength. The short weld length minimizes the chance of defects. Dimensional accuracy and mechanical properties can be further improved through processes such as full-length diameter expansion.
3. Good processability: Straight seam welded pipes have low stiffness and are easy to bend, facilitating on-site construction and installation.
4. High-quality weld: Straight seam submerged arc welded pipes utilize a double-sided submerged arc welding process, ensuring reliable weld quality. High-frequency welded pipes do not require filler metal during welding, resulting in rapid heating and a small heat-affected zone.

Welded steel pipes, with their core advantages of simple production processes, low cost, and high dimensional accuracy, are widely used in fluid transportation, building structures, machinery manufacturing, and energy and power industries, and are continuously developing towards higher strength, process innovation, and product diversification. Compared to seamless steel pipes, straight seam welded pipes have welds and lower pressure resistance, but offer significant cost advantages, making them suitable for low-pressure applications.

Welded steel pipe as an important steel pipe, longitudinal welded pipe carries the transportation tasks of many important projects. The following are the current standards of some countries that are commonly used:

GB standards: 

The national standards implemented in China are GB/T 13793-2016 "Straight Seam Electric Welded Steel Pipes" and GB/T 3091-2015 "Welded Steel Pipes for Low Pressure Fluid Transmission".

ASTM Standards: 

The American Society for Testing and Materials has developed many standards for straight welded pipe. These include ASTM A53 "Ordinary and Machining Carbon Steel Longitudinal Welded Pipe", ASTM A106 "Seamless Carbon Steel Straight Welded Pipe", ASTM A252 "Spiral Seam Longitudinal Welded Pipe" and so on.

British Standard: 

BS EN 10217-1:2019 "Technical delivery conditions for welded steel pipes - Welded steel pipes for non-alloy steel pipes".

Japanese Standard: 

JIS G 3452:2019 "Carbon Steel Pipe", which specifies carbon steel straight seam welded steel pipes for general purposes.

These standards stipulate the size, material, manufacturing process, chemical composition, mechanical properties, inspection methods and other requirements of welded steel pipe to ensure that the quality and performance of longitudinal welded steel pipes meet the regulations.

The specifications of welded steel pipes are expressed as "nominal diameter" or "outer diameter × wall thickness".

Large‑diameter Specifications

General Size Range

The diameter range of longitudinal‑seam welded pipes is generally 14‑610 mm, with wall thickness from 1‑22 mm.
Large‑diameter longitudinal submerged‑arc welded (LSAW) pipes can reach an outer diameter of 406‑1829 mm and wall thickness of 6‑60 mm.

The national standards followed by welded steel pipes are GB/T 13793-2016 "Longitudinal Electric Welded Steel Pipes", GB/T 3091-2015 "Welded Steel Pipes for Low Pressure Fluid Transmission", which stipulates the manufacturing, quality requirements and testing of welded steel pipes method.

National standard for conventional wall thickness welded pipes:

The conventional wall thickness of the national standard for welded pipes of DN15~DN200 is 28mm~5.5mm. 

The national standard wall thickness of DN225~DN300 welded pipes is 4.0mm~7.5mm
The national standard wall thickness of DN350~DN400 welded pipes is 60mm~9.0mm.
The conventional wall thickness of the national standard for welded pipes of DN450~DN600 is 6.0mm~10.0mm.
The conventional wall thickness of the national standard for welded pipes above DN650 is 7.0mm~12.0mm.

The specific allowable error value will be stipulated according to the product requirements and relevant national standards.