Seamless pipe, straight seam welded pipe, and spiral welded pipe are the three main types of steel pipes, and their characteristics and manufacturing processes determine their different application areas.
Let’s learn how these different types of steel pipes are manufactured.
Integrated steel manufacturing throughout the entire process
I. Seamless Steel Pipes
Key feature: It has no seams along the entire circumference. It is made by piercing a solid steel billet.
Seamless tube manufacturing (Mannesmann rolling process)
Main manufacturing processes:
1. Hot Rolling (Hot Extrusion) Process (Main Method):
Steps: Round solid tube billet → Heating in furnace to a plastic state → Piercing with a piercing mill (forming a hollow tube) → Rolling on a tube mill (elongation, thinning, sizing) → Finishing with a sizing/reducing mill → Cooling → Straightening → Cutting → Inspection.
Representative Processes: Mannesmann piercing method, skew rolling piercing, etc.
Characteristics: High production efficiency, capable of producing large-diameter, thick-walled tubes; it is the mainstream process.
2. Cold Drawing (Cold Rolling) Process:
Steps: Using hot-rolled tubes as raw material → Pickling to remove oxide scale → Phosphating/Saponification lubrication → Cold drawing through a die or cold rolling → Heat treatment (to relieve internal stress) → Straightening → Finishing.
Characteristics: High dimensional accuracy, good surface finish, and superior mechanical properties, but high production cost and lower output. Commonly used for small-diameter, precision, or thin-walled tubes.
Seamless tube manufacturing (hot extrusion – hot hollow forging)
Advantages:
Uniform Mechanical Properties: Seamless construction with uniform microstructure in both circumferential and length directions, resulting in strong pressure resistance.
High Pressure and Corrosion Resistance: Suitable for high-pressure, extreme temperature, and corrosive environments (e.g., boiler tubes, hydraulic cylinders).
Diverse Cross-sectional Shapes: Capable of producing complex cross-sections such as circles, squares, and ellipses.
Disadvantages:
High Production Costs: Complex process flow, high energy consumption, and significant metal loss (low yield).
Difficulty in Controlling Wall Thickness Uniformity: Especially for thick-walled tubes, internal wall eccentricity and surface defects may exist.
Limited Size Specifications: Limited by raw materials and processing equipment, the length and maximum diameter of a single tube (typically ≤Φ660mm) are limited.
Typical applications:
Petrochemical industry (high temperature and high pressure pipelines), power plant boilers, hydraulic systems, bearing sleeves, gun barrels/cannon barrels, high-precision mechanical structural components.
II. Straight Seam Welded Pipe
Key feature: The weld seam is a straight line parallel to the axis of the steel pipe. It is formed by welding rolled steel plates or strips.
Resistance welded pipe
Main manufacturing processes:
1. Straight Seam High-Frequency Welded Pipe:
Process: Continuously form steel strip (coil) → Utilize the skin effect and proximity effect of high-frequency current to instantly heat the weld edge to a molten state → Achieve solid-state welding under the pressure of extrusion rollers (no welding wire required).
Features: High speed, high efficiency, low cost. Small heat-affected zone of the weld.
Common Standards: ASTM A500 (for structural use), JIS G3444 (for mechanical structures).
2. Straight Seam Submerged Arc Welded Pipe:
Process:
JCOE Forming: First, pre-bend the steel plate at the edges, then gradually form it into a cylinder through multi-step pressing with J-type, C-type, and O-type molds, and finally expand the diameter.
UOE Forming: First, pre-bend the edges of the steel plate, then press it into a U-shape, then press it into an O-shape, and expand the diameter after welding. Requires large equipment investment, suitable for mass production.
Welding: Submerged arc welding is used on both the inside and outside of the formed straight seam (the electric arc burns under the flux layer, resulting in a high degree of automation and good weld quality).
Features: Capable of producing large-diameter (up to Φ1620mm and above) and thick-walled steel pipes with strong pressure resistance.
Resistance welded pipe
Advantages:
High production efficiency and low cost: Especially high-frequency welded pipes, which can achieve continuous high-speed production.
High dimensional accuracy and good surface quality: Using pre-processed plates, the wall thickness is uniform and the appearance is good.
High flexibility: Steel pipes of different diameters can be produced by changing the width of the raw material.
Disadvantages:
One longitudinal weld seam exists: The weld seam is a potential weak point, requiring extremely high welding quality.
Diameter limited by plate width: The diameter of the steel pipe generally cannot exceed π times the width of the steel plate (actually limited by equipment).
Typical Applications:
High-Frequency Welded Pipe: Building structures (scaffolding), furniture, low-pressure fluid transportation, automotive drive shaft pipes.
Submerged Arc Welded Pipe: Long-distance oil and gas pipelines, offshore platform structural pipes, urban pipe networks, wind turbine towers.
III. Spiral Welded Pipe
Key feature: The weld seam is spiral-shaped and surrounds the pipe body. It is also made of rolled and welded steel plate/strip.
Argon arc welded pipes and tubing (SAWH pipes produced by spiral process)
Main manufacturing processes:
Forming and Welding:
A steel strip (coil) of a certain width is continuously rolled at a specific helix angle (forming angle) to form a tube.
During the forming process, double-sided submerged arc welding technology is used to weld the inner and outer helical seams.
By changing the steel strip width and helix angle, steel pipes of different diameters can be produced from the same width of steel strip.
Post-processing: Cutting to length, weld inspection, hydrostatic testing, diameter expansion (optional), etc.
Advantages:
Various diameters of steel pipes can be produced using strip steel of the same width, offering extremely high flexibility.
The weld seam avoids the principal stress direction: The spiral weld seam forms an angle with the principal stress direction, resulting in a relatively balanced load-bearing capacity of the pipe body in that direction.
Weld defects are less likely to penetrate and propagate: The spiral shape makes crack propagation paths longer.
Relatively low equipment investment, suitable for producing medium to large diameter pipes.
Disadvantages:
Longer weld seam length: 30% to 100% longer than straight seam pipes, increasing welding workload and instability factors.
Dimensional accuracy and geometry (roundness, straightness) are generally inferior to straight seam welded pipes.
Higher internal stress: The internal stress generated during forming and welding is more complex.
Relatively slower production speed.
Typical applications:
Low-pressure fluid transportation (water, gas), pipe piles, pipe piles, structural support (especially large-diameter thin-walled pipes), and some onshore oil and gas transportation.
