Choosing between carbon steel seamless pipe and welded pipe is one of the most common specification calls in pressure and process service. This guide explains where each is the right choice, the mechanical and cost differences, and how to write the RFQ clearly.
Carbon steel pipe is produced by two fundamentally different processes: seamless, where a solid billet is pierced and drawn into a hollow tube with no weld seam; and welded, where a strip or plate is formed into a tube and welded along the seam (ERW, LSAW, HSAW, or DSAW depending on size and method). The mechanical properties of the two are similar in modern production, but the microstructure, the response to high temperature and pressure cycling, and the cost differ significantly.
Where Seamless Carbon Steel Pipe Is Required
Seamless pipe is required for high-temperature service above about 400 °F (200 °C), for high-pressure boiler and superheater service, and for any application where the pipe will see severe pressure cycling or fatigue loading. The absence of a weld seam eliminates the heat-affected zone that can be a preferential path for creep damage and corrosion, so seamless is the conservative default for refinery hot lines, power plant boiler tubes, and process pipe in ASME B31.1 Power Piping. Seamless is also required for very thick walls (above about 25 mm) where welded pipe is not produced in that schedule.
Where Welded Pipe Is the Right Specification
Welded pipe is the right specification for low- and moderate-pressure service at ambient or moderate temperature: utility water, fire water, compressed air, low-pressure steam, structural pipe, piling, and low-pressure oil and gas gathering. Modern ERW and LSAW pipe has weld seams that meet or exceed the base metal strength in static loading, so for non-cyclic, non-corrosive service the seam is not a limitation. Welded pipe is also significantly cheaper than seamless, especially in larger diameters, so using seamless where welded would suffice is wasted money.
Diameter, Wall Thickness and Cost
Welded pipe is more economical in larger diameters (above NPS 16 or DN400) because producing a seamless pipe of that size requires a very large billet and heavy piercing equipment. LSAW (longitudinal submerged-arc welded) and HSAW (helical submerged-arc welded) are the dominant processes for line pipe diameters from NPS 20 to NPS 48. For small diameters (NPS 1/2 to NPS 6), ERW welded and seamless are close in price, and the choice is driven by the service condition rather than cost.
What to Specify on the RFQ
State the standard (ASTM A106 seamless, ASTM A53 Type S/E/F, API 5L PSL1/PSL2), the grade, the NPS and schedule, the manufacturing process (seamless, ERW, LSAW, DSAW), the delivery condition (hot finished, cold drawn, normalized), and any supplementary testing (Charpy, NACE, hydrostatic). For high-temperature and high-pressure service, default to seamless; for utility and structural service, welded is acceptable and saves cost. Always specify the hydrostatic test pressure and the mill test certificate per EN 10204 3.1.
Related Resources from Baobin Steel
- Carbon steel pipe catalog – seamless and welded options
- Mild steel overview – grades and standards
- Request a quote – send your service spec for pipe pricing