How seamless pipe is made

A solid round billet is heated and pierced to form a hollow shell, which is then rolled or extruded over a mandrel to the required diameter and wall thickness. Larger sizes are hot finished; smaller sizes and thinner walls are cold drawn or cold pilgered for closer tolerances and a better surface. The pipe is then heat treated – normalised for carbon steel, solution annealed for stainless steel – straightened, tested and inspected. Because there is no seam, the pipe has uniform properties around its circumference.

How welded pipe is made

Welded pipe starts as coil, strip or plate. ERW (electric resistance welded) pipe is formed from coil and welded by high-frequency current without filler metal – used for carbon steel in small and medium sizes. Stainless welded pipe is formed from strip and welded by TIG or laser, usually without filler, then solution annealed. EFW (electric fusion welded) and LSAW pipe is formed from plate and welded with filler metal, for large diameters and heavy walls. Spiral welded pipe is used for large-diameter water and piling. Welds are inspected by ultrasonic, eddy-current or radiographic testing.

Key differences

Pressure ratings and codes

In piping codes such as ASME B31.3, the allowable stress of welded pipe is multiplied by a weld joint quality factor. For ERW pipe this is commonly 0.85, while seamless pipe and fully radiographed EFW pipe have a factor of 1.0. This means welded pipe may need a slightly thicker wall for the same pressure, unless the weld is fully examined. For stainless steel pipe to ASTM A312, welded pipe with additional non-destructive examination can be used at full allowable stress. Boiler and high-temperature codes often require seamless pipe for critical lines.

Standards for each type

When to choose seamless

• High-pressure process, hydraulic and gas lines
• High-temperature steam and hydrogen service, and boiler tubes
• Cyclic, vibrating or fatigue-loaded lines
• Sour service where the specification excludes welded pipe
• Heavy walls and small bores, such as Sch 160 and XXS
• Heat-exchanger tubes that will be expanded into tube sheets, where specified

When to choose welded

• Water, utility, air and low-pressure process lines
• Large diameters, where seamless is costly or unavailable
• Thin-wall stainless piping (Sch 5S and 10S) for chemical, food and pharmaceutical plants
• Pipelines, where ERW and LSAW line pipe is standard
• Hygienic and instrument tubing, where uniform wall and smooth bore matter
• Structural and mechanical uses

Quality and inspection of welded pipe

Modern welded pipe is very different from the welded pipe of decades ago. For ERW pipe, the weld zone is heat treated to normalise the structure, the weld bead is trimmed inside and out, and the seam is inspected along its full length by ultrasonic or eddy-current testing. Stainless welded pipe to ASTM A312 is solution annealed after welding, so the weld and base metal have the same corrosion resistance, and the weld can be cold worked to refine its structure. EFW and LSAW pipe is radiographed. With these controls, welded pipe is accepted for many pressure and process services, as the code and project specification allow.

Wall thickness, ovality and fit-up

Seamless pipe has a wider tolerance on wall thickness – often ±12.5% – and the wall can vary around the circumference, because it is formed from a pierced billet. Welded pipe, made from strip or plate of controlled thickness, has a very uniform wall and closer tolerances. This makes welded pipe easier to fit up and weld, especially by automatic orbital welding in thin-wall stainless systems, and gives more predictable heat transfer in tubes. Seamless pipe remains preferred where heavy walls are needed, which welded processes cannot easily produce in small diameters.

Cost and availability

For the same size and material, welded pipe is generally less costly than seamless, and the difference grows with diameter and alloy cost. Welded pipe is also available in a wider range of sizes and from more mills. In expensive alloys such as duplex, super duplex and nickel alloys, choosing welded pipe where the specification allows can reduce project cost significantly. Seamless pipe in large diameters may require special mill runs and long lead times.

Making the decision

The project piping specification normally sets whether seamless, welded or either is allowed for each line class, based on the fluid, pressure, temperature and code. Where the choice is open, welded pipe is usually more economical, especially in larger sizes and thinner walls, while seamless gives extra assurance for severe service. We supply both – see seamless pipes and tubes, welded pipes and tubes, stainless steel seamless pipes, stainless steel welded pipes and carbon steel ERW pipes. More guides are on the technical resources page.