What Is Seamless Pipe? How It's Made
A seamless pipe (SMLS) is a tubular product manufactured without a welded seam. The absence of a weld joint gives seamless pipes superior strength under pressure, making them the default choice for critical process piping in oil and gas, petrochemical, and power generation plants.
| Property | Detail |
|---|---|
| Manufacturing | Hot rolling or cold drawing from a solid steel billet |
| Weld seam | None |
| Size range | NPS 1/8 to NPS 26 (DN 6 to DN 650) |
| Wall thickness | SCH 5S to SCH XXS (ASME B36.10/B36.19) |
| Common specs | ASTM A106, ASTM A53 Type S, API 5L (PSL1/PSL2) |
| Pressure rating | Higher than equivalent welded pipes |
| Temperature range | Cryogenic (ASTM A333) to high-temp (ASTM A335) |
| Cost | 20-40% more than ERW for equivalent sizes |
How Seamless Pipes Are Made
The process starts with a solid round billet heated to approximately 1,200-1,300°C. A piercing mill forces a mandrel through the center to create a hollow shell. The shell then passes through an elongation mill (plug mill or mandrel mill) to achieve the target diameter and wall thickness. Final sizing and straightening follow.
Two main routes exist:
- Hot-finished seamless: Produced entirely at elevated temperatures. Covered by ASTM A106 Grade B/C for carbon steel.
- Cold-drawn seamless: Additional cold working after hot forming refines tolerances and surface finish. Common in instrumentation tubing and heat exchanger tubes.
For carbon steel pipes, the most widely used specification is ASTM A106 Grade B, with a minimum yield strength of 240 MPa (35 ksi) and tensile strength of 415 MPa (60 ksi).
Where Seamless Pipes Are Used
Seamless pipes dominate applications where failure is not an option:
- Process piping in refineries and chemical plants (high pressure, high temperature)
- Upstream oil and gas: wellhead connections, flow lines, production tubing
- Boiler and superheater tubes per ASTM A335 grades (P5, P9, P11, P22)
- Subsea pipelines where weld integrity concerns are amplified
- Low-temperature service per ASTM A333 Grade 6
Size and Schedule
Seamless pipes follow ASME B36.10 (carbon/alloy) and ASME B36.19 (stainless) for dimensional standards. Wall thickness is designated by schedule numbers (SCH 10, 20, 40, 80, 160, XXS). A 6-inch SCH 40 seamless pipe has an OD of 168.3 mm and a wall thickness of 7.11 mm.
Seamless pipes are tested and inspected per the relevant ASTM or API standard, including hydrostatic testing, ultrasonic examination, and mill test certificates (EN 10204 3.1 or 3.2).
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