What is welded steel pipe ?

Welded steel pipe refers to steel pipe made by bending or deforming steel strips or plates into round, square, or other shapes, with a seam on the surface. The raw material used for welded steel pipe is steel plate or strip.

Since the 1930s, with the rapid development of continuous rolling production of high-quality strip steel and advancements in welding and inspection technology, weld quality has continuously improved, and the variety and specifications of welded steel pipe have continued to expand. Seamless vs welded pipe : welded steel pipe has lower cost and higher production efficiency than seamless steel pipe.

Advantages of Welded Pipe

1. Strong Corrosion Resistance

Welded pipe is typically made of metal materials such as stainless steel and nickel, exhibiting excellent corrosion resistance and resisting rust or corrosion even in harsh environments.

2. High Strength

Compared to ordinary steel pipe, welded pipe offers higher strength and superior load-bearing capacity.

3. Efficient Construction

Welded pipe is quick and easy to install, significantly reducing time and labor costs compared to traditional iron pipe.

4. Safety and Reliability

Welded pipe joints are strong, reducing safety hazards such as electrical leakage and water seepage, ensuring safe production.

Disadvantages of Welded Pipes

1. Unstable Welding Quality

If welding techniques are substandard, welded pipe joints may exhibit defects such as incomplete penetration and unsealed welds, affecting the airtightness and watertightness of the pipe after installation.

2. Susceptibility to Corrosion

Because welded pipe surfaces lack special anti-corrosion treatment, high humidity or high-oxidation environments may cause rust and wear on the surface, shortening its service life.

Welded Pipe Manufacturing Types

There are three types of welding: ERW (electrical resistance welding), LSAW (longitudinal submerged arc welding), and SSAW (spiral submerged arc welding).

1. ERW

Application:

The ERW process is generally suitable for pipes under 24 inches (609 mm).

Manufacturing Features:

The ERW process uses resistance heating to heat the edges of the steel strip to welding temperature, then pressurizes and welds the steel strip to form the weld. Modern processes generally use high-frequency welding (HFW), which offers fast welding speeds and smooth welds.

ERW/HFW Longitudinal Welded Steel Line Pipe (Resistance Welding) Manufacturing Process:

Uncoiling - Flattening - Shearing and Butt Welding - Coil Storage - Plate Uncoiling - Trimming - Forming - High-Frequency Welding - Internal and External Burr Removal - Ultrasonic Testing - Medium-Frequency Annealing - Air Cooling - Water Cooling - Sizing - Straightness - Cutting - Dimensional and Initial Surface Inspection - Pipe End Beveling - Hydrostatic Testing - Full Pipe Ultrasonic Testing - Offline Ultrasonic Testing - Pipe End Ultrasonic Testing - Dimensional and Final Surface Inspection - Corrosion-Resistant Coating - Marking - Pipe End Protection - Inspection and Warehousing

Advantages:

Smooth welds with high dimensional accuracy;

Fast welding speed and high production efficiency;

Suitable for high-pressure transmission pipelines (e.g., API 5L Grade B, X42–X70).

Typical Use:

Oil and gas line pipes, water pipelines, and pipes for mechanical structures (EN 10219).

2. LSAW

Application:

LSAW pipe is suitable for pipes with diameters ranging from 20 inches (508 mm) to 40 inches (1016 mm).

Manufacturing Features:

Utilizing the submerged arc welding (SAW) process, a layer of flux is applied to the weld seam, allowing the arc to burn beneath the flux, concentrating the heat and achieving high weld quality. Named JCOE pipe based on the manufacturing procedure, the pipe is available in J-, C-, O-, and E-shaped cold-expanded shapes.

LSAW Steel Pipe Production Process:

Plate Unwrapping - Vacuum Lifting - Ultrasonic Testing - Edge Milling - Pre-bending - J-Forming - C-Forming - O-Forming - CO2 Gas Shielded Spot Welding - Tab Welding - Internal Welding - External Welding - Tab Plate Removal - Cleaning - Rounding - X-ray Inspection - Ultrasonic Testing - Cleaning Before Expansion - Mechanical Expansion - Cleaning After Expansion - Straightness Inspection - Hydrostatic Testing - Ultrasonic Testing - Pipe End Weld Grinding - Mechanical Surface Treatment - X-ray Inspection of the Pipe Body and Both Ends - Ultrasonic Testing of the Pipe Ends - Magnetic Particle Inspection of Both Ends - Weighing and Length Measurement - Product Inspection - External Coating - Internal Coating - Marking - Packaging and Warehousing

Advantages:

Dense and High-Strength Welds;

Can Produce Large-Diameter, Thick-Walled Pipes;

High Dimensional Accuracy, 100% Weld Inspection.

Typical Use:

Submarine Oil Pipelines, High-Pressure Natural Gas Pipelines, Structural Support Columns, Pressure Vessels, etc.

3. SSAW

Scope of Application:

SSAW pipe is suitable for pipes with a maximum diameter of 60 inches (1524 mm).

Manufacturing Features:

Starting from hot-rolled strip steel, it is rolled into a pipe using a forming machine at a specific spiral angle and then welded to the pipe using double-sided submerged arc welding. The spiral angle is adjustable, allowing for flexible product sizes.

SSAW Steel Pipe Manufacturing Process:

Uncoiling - Flattening - Shearing and Butt Welding - Edge Milling - Pre-rolling - Forming - Internal and External Welding - Cutting - Ultrasonic Testing - X-ray Testing - Pipe End Weld Grinding - Pipe End Flaring - Hydrostatic Testing - Ultrasonic Testing - Pipe End Beveling - X-ray Testing - Production Inspection - Marking - Warehousing

Advantages:

Can manufacture pipes of any length and large diameter;

Uniform stress distribution and strong resistance to hoop stress;

Low cost and high material utilization.

Typical Use:

Long-distance oil, gas, and water pipelines, pile foundations, and bridge steel pipe piles.

Key Differences Between SAW (LSAW, SSAW) and ERW Pipes

From the manufacturing processes described above, we can see that SAW (LSAW, SSAW) and ERW pipes are completely different in terms of forming and welding. Consequently, the performance of the two pipe types differs in the following aspects:

1. Surface Quality Comparison

ERW pipes have a smoother surface and a higher surface finish than SAW (LSAW, SSAW) pipes.

2. Defect Comparison

ERW pipes have fewer defects than SAW pipes.

Because of solvent deposits in SAW pipes, they can produce more defects in addition to volumetric defects.

Defects found in ERW pipes are limited to linear and straight welds, making them easier to detect.

3. Residual Stress Comparison

ERW pipes have lower residual stress than SAW pipes.

The displacement of ERW pipes along both the axial and transverse cuts is smaller than that of SAW pipes made of the same material, making them easier to detect.

4. Durability Comparison

ERW steel pipes can be scrapped and welded if defects develop, while SAW steel pipes can be scrapped and welded, which can lead to cracking or corrosion. ERW steel pipes are more durable.

Summary

Welded steel pipes, due to their high production efficiency, low cost, and strong versatility, have become one of the most widely used pipe types worldwide. In oil and gas transportation, urban water supply, building structures, and foundation engineering, welded pipes, along with seamless steel pipes, are the two most popular choices.