Seamless vs Welded Steel Pipe: Strength, Cost and Pressure Rating

For most general applications welded pipe is the better buy: it costs less, comes in tighter wall tolerances and covers the same size range. Choose seamless pipe when the service is high pressure, high temperature, cyclic, or safety-critical — or when the specification simply names a seamless grade such as ASTM A106 or API 5L seamless. The two are not interchangeable by default, and the deciding factor is usually the code and certificate, not the raw strength of the steel.

How the two pipes are made

Welded pipe starts as steel strip or plate that is formed into a cylinder and joined along one longitudinal seam. The common processes are ERW (electric resistance welding) for smaller sizes, LSAW (longitudinal submerged arc) for large-diameter line pipe, and SSAW (spiral submerged arc) for very large diameters. Because the strip is rolled to an exact thickness, the wall thickness around the circumference is very consistent.

Seamless pipe is made from a solid steel billet that is pierced and rolled into a hollow shell with no seam. There is no weld to inspect, but the piercing process leaves a small amount of wall eccentricity, so wall thickness is slightly less uniform than ERW.

Side-by-side comparison

Does the theoretical weight differ?

No. Theoretical weight is a function of geometry — outside diameter and wall thickness — so a seamless pipe and a welded pipe with the same OD and wall have the the same calculated weight per metre. The choice between them changes cost, tolerance and pressure rating, not the number on the weight chart. The reference below uses the standard formula W (kg/m) = (D − t) × t × 0.02466.

Note: values are theoretical, calculated from nominal dimensions at a steel density of 7.85 g/cm³. For commercial settlement the actual weighed mass applies, and the basis (theoretical or actual) is agreed on the order. Work out any other diameter and wall thickness with our steel pipe weight calculator.

Why the weld changes the pressure rating

Pressure design codes such as ASME B31.3 and ASME B31.1 calculate the allowable pressure from the pipe’s allowable stress, the wall thickness and a longitudinal weld joint factor. Seamless pipe carries a factor of 1.00 because it has no longitudinal weld. Welded pipe carries a factor below 1.00, the exact value depending on the specification and the extent of non-destructive testing of the seam. The practical effect is simple: for the same size, wall and grade, a welded pipe is rated for a lower allowable pressure than a seamless pipe, and the gap narrows as the weld is more thoroughly tested.

Which one should you specify?

When a drawing names a standard, follow the standard. A welded pipe cannot simply be substituted for a pipe specified as ASTM A106 seamless: the mill certificate would reference a different specification, and an inspector is entitled to reject it on that basis alone. If you want the cheaper option, ask for it at the enquiry stage so it can be quoted and documented correctly.

What to put in your enquiry

1. Process and standard — e.g. “ASTM A106 Grade B seamless” or “EN 10255 welded”, not just “steel pipe”.
2. Size — nominal size, or outside diameter × wall thickness in mm.
3. Length and ends — random or fixed length, plain, bevelled, threaded or grooved.
4. Surface — black, varnished, or hot-dip galvanized.
5. Documentation — mill test certificate to EN 10204 3.1, third-party inspection, or both.

Frequently asked questions

Is seamless pipe stronger than welded pipe?

The base steel is comparable; the difference is the longitudinal weld. Because a weld introduces a joint factor in pressure design, seamless pipe is rated higher for the same size, wall and grade. In structural service the section governs, so welded pipe is normally sufficient.

Is welded pipe cheaper than seamless pipe?

Yes, generally. Welded pipe is made from strip, which is cheaper to produce than the pierced billet used for seamless pipe, so welded pipe usually carries a lower price per tonne for the same size.

Can welded pipe be used for high pressure?

It can be used for medium and, with fully tested seams, fairly high pressure — but the allowable pressure is always lower than a seamless pipe of the same size and grade, because of the weld joint factor. For genuinely high-pressure service, seamless is the usual choice.

Do seamless and welded pipe weigh the same?

For the same outside diameter and wall thickness, yes — theoretical weight depends only on geometry. Process affects cost, tolerance and pressure rating, not the calculated weight per metre.

Which standards cover each type?

Seamless: ASTM A106, ASTM A53 (seamless), API 5L (seamless), GB/T 8163. Welded: EN 10255, GB/T 3091, ASTM A53 (welded), ASTM A135, API 5L (welded), with LSAW and SSAW for large diameters.

Which is better for galvanizing?

Both can be hot-dip galvanized to EN ISO 1461. Seamless pipe avoids a weld seam that must be coated, while ERW pipe is galvanized on both inside and outside surfaces. Tell us the coating standard and the intended service so the right surface finish is quoted.

Where can I buy both types from China?

We supply both seamless and welded steel pipe in the sizes and standards above. See our seamless steel pipe and welded steel pipe pages, check how we inspect and certify each order, or request a quotation with your specification. More guidance is in our steel knowledge base.