Seamless pipes, longitudinal welded pipes, and spiral welded pipes are the three primary types of steel pipes, and their distinct characteristics and manufacturing processes determine their respective applications.
Let's take a look at how these different types of steel pipes are manufactured.
Steel Pipe Manufacturing Methods
Seamless Steel Pipes
Key Characteristics : Seamless steel pipes have no weld seam around the entire circumference. They are produced by piercing a solid steel billet.
Seamless Pipe Manufacturing (Mannesmann Rolling Process)
Seamless Pipe Manufacturing (Hot Extrusion – Hot Hollow Forging)
Main Manufacturing Processes:
1. Hot Rolling (Hot Piercing/Compression) Process (Primary Method):
Steps:
Solid round billet → Heated in furnace to plastic state → Piercing mill creates hollow shell (forming a hollow rough tube) → Pilger mill rolling (elongation, wall reduction, diameter control) → Sizing/reducing mill for precision finishing → Cooling → Straightening → Cutting → Inspection.
Representative Processes:
Mannesmann piercing, oblique rolling piercing, etc.
Features:
High production efficiency and capable of producing large-diameter, thick-walled tubes; this is the dominant manufacturing method.
2. Cold Drawing (Cold Rolling) Process:
Steps:
Hot-rolled tube as blank → Acid pickling to remove oxide scale → Phosphating/saponification for lubrication → Cold drawing through dies (or cold rolling) → Heat treatment (to relieve internal stresses) → Straightening → Finishing.
Features:
High dimensional accuracy, excellent surface finish, and superior mechanical properties-but with higher production cost and lower output volume. Commonly used for small-diameter, precision, or thin-walled tubes.
Advantages:
- Uniform mechanical properties: No weld seam; homogeneous microstructure in both circumferential and longitudinal directions, resulting in high pressure resistance.
- High pressure & corrosion resistance: Suitable for demanding applications under high pressure, extreme temperatures, or corrosive environments (e.g., boiler tubes, hydraulic cylinders).
- Versatile cross-sections: Capable of producing complex shapes-including round, square, rectangular, and oval profiles.
Disadvantages:
- High production cost: Complex process flow, high energy consumption, and significant metal loss (low material yield).
- Difficulty in wall thickness uniformity control: Especially for thick-walled tubes, internal surfaces may exhibit eccentricity and surface defects.
- Size and specification limitations: Constrained by billet size and processing equipment; maximum single-length and outer diameter are limited (typically ≤ Φ660 mm).
Typical Applications:
Petroleum & chemical industry (high-temperature, high-pressure pipelines), power plant boilers, hydraulic systems, bearing sleeves, gun/barrel tubes, and high-precision mechanical structural components.
Longitudinal Submerged Arc Welded (LSAW) Pipes
Key Characteristics：The weld seam is a straight line parallel to the pipe's longitudinal axis. These pipes are manufactured by forming steel plate or coil into a cylindrical shape and then welding the seam.
Electric Resistance Welded (ERW) Pipe
Hot Electric Resistance Welded (Hot ERW) Pipe
Main Manufacturing Processes:
1. High-Frequency Electric Resistance Welded (HF-ERW) Pipe:
Process:
Steel strip (coil) is continuously formed → High-frequency current is applied, utilizing the skin effect and proximity effect to rapidly heat the weld edges to a molten state → Solid-state welding is achieved under pressure from squeeze rolls (no filler wire required).
Features:
High speed, high efficiency, low cost; minimal heat-affected zone (HAZ), ensuring good weld integrity.
Common Standards:
ASTM A500 (structural applications), JIS G3444 (mechanical applications).
2. Longitudinal Submerged Arc Welded (LSAW) Pipe:
Forming Processes:
JCOE Forming: Steel plate is first edge-bent, then progressively shaped via J-, C-, and O-forming steps into a cylindrical shell, followed by expansion (expanding to final diameter).
UOE Forming: Steel plate edges are pre-bent, then pressed into a U-shape, followed by an O-shape, and finally welded before expansion. This method requires high-capacity equipment and is ideal for large-scale production.
Welding:
After forming, submerged arc welding (SAW) is applied both internally and externally-the electric arc burns beneath a granular flux layer, ensuring high automation and excellent weld quality.
Features:
Capable of producing large-diameter pipes (up to Φ1620 mm or larger) and thick-walled tubes with superior pressure resistance and structural strength.
Typical Applications:
HF-ERW Pipes: Structural frameworks (e.g., scaffolding), furniture, low-pressure fluid conveyance, automotive drive shafts.
LSAW Pipes: Long-distance oil & gas transmission pipelines, offshore platform structures, municipal water/gas networks, and wind turbine towers.
Advantages:
High production efficiency & low cost: Especially HF-ERW pipes enable continuous high-speed manufacturing.
High dimensional accuracy & excellent surface quality: Pre-processed raw materials ensure uniform wall thickness and an aesthetically pleasing finish.
Strong flexibility: Tube diameter can be adjusted by varying the width of the steel strip-enabling production of multiple diameters from a single coil.
Disadvantages:
Presence of a longitudinal weld seam: The weld joint is a potential weak point; thus, stringent welding quality control is essential.
Diameter limited by plate width: The maximum pipe diameter is generally constrained to ≤ π × steel plate width (in practice, further limited by forming equipment capacity).
Spiral Welded Pipes
Key Characteristics:
The weld seam spirals around the pipe body. Like longitudinal welded pipes, spiral welded pipes are also manufactured by forming steel plate or coil into a cylindrical shape and then welding the seam.
TIG (Tungsten Inert Gas) Welded Pipes
Submerged Arc Welded Helical (SAWH) Pipes produced via spiral welding process
Main Manufacturing Process:
Forming and Welding:
A steel strip (coil) of specified width is continuously formed at a predetermined helix angle (forming angle) into a cylindrical pipe shell.
During forming, double-sided submerged arc welding (SAW) is applied simultaneously to both the internal and external spiral seams, ensuring high weld integrity and productivity.
By adjusting the strip width and helix angle, pipes of varying diameters can be produced from the same-width steel coil-offering excellent flexibility in product configuration.
Post-Processing Steps:
Cutting to specified lengths, weld inspection (e.g., X-ray/UT), hydrostatic pressure testing, and optional pipe expansion (to improve dimensional accuracy and residual stress relief).
Advantages:
High flexibility: A steel strip of a given width can be used to produce pipes of multiple diameters, enabling highly adaptable production.
Weld seam avoids principal stress direction: The spiral weld forms an angle with the primary stress axis, resulting in more balanced load distribution and improved structural integrity under internal pressure.
Reduced risk of crack propagation: The helical geometry elongates the crack path, making it less likely for defects to propagate circumferentially-enhancing reliability.
Lower equipment investment: Compared to LSAW (UOE/JCOE) lines, spiral welding mills require relatively lower capital expenditure, making them ideal for medium- to large-diameter pipe production.
Disadvantages:
Longer weld seam length: The spiral weld is 30%–100% longer than that of a straight-seam pipe of equivalent diameter, increasing welding workload and introducing more potential sources of instability (e.g., arc fluctuations, flux coverage issues).
Lower dimensional accuracy and geometric tolerances: Roundness and straightness are generally inferior to those of ERW or LSAW pipes, especially for smaller diameters.
Higher residual internal stresses: Complex deformation during helical forming and welding leads to more intricate stress distributions, requiring careful post-weld treatment (e.g., stress relief annealing).
Relatively slower production speed: Due to the continuous helical forming and dual-side welding synchronization, output rates are typically lower than high-speed HF-ERW lines.
Typical Applications:
- Low-pressure fluid conveyance (water, gas)
- Piling tubes (sheet piling, structural piles)
- Casing and tubing (especially large-diameter, thin-walled variants)
- Structural support members (e.g., for bridges, buildings, offshore platforms)
- Some onshore oil & gas transmission lines (where cost-effectiveness outweighs stringent dimensional requirements)
