GUIDE
Welded or seamless tube: which should you choose?
The question comes up on almost every enquiry, and it is often settled by inherited preference rather than by a criterion. Here is what actually separates them — and the two cases where the choice is not yours.
Two processes, two starting geometries
Seamless tube starts from a solid billet, pierced hot then drawn. It has no longitudinal weld. In exchange, its wall thickness varies more, which is inherent to piercing.
Welded tube starts from strip, roll-formed then welded along its length — most often by TIG or laser welding without filler, followed by a solution heat treatment. The weld bead is then rolled or dressed. Wall thickness, inherited from rolled strip, is markedly more consistent.
Corrosion: what the heat treatment changes
The idea that the weld is a corrosion weak point comes from older processes. On a welded tube meeting current standards — EN 10217-7, ASTM A312, ASTM A249 — the tube undergoes solution treatment after welding, which restores the structure and composition of the weld. The result is that the weld is no longer a preferential site of attack.
Two reservations are worth knowing, however.
On duplex grades, the austenite/ferrite balance of the weld is the issue; it is controlled at the mill by the heat treatment, but that is one more reason to insist the standard is met and the certificate supplied.
In very severe media — seawater, acids — some specifications require seamless as a matter of principle. That requirement is often conservative rather than demonstrated, but it is contractual: it is not open for discussion at delivery.
The real technical criterion: joint efficiency factor
This is the point that decides most cases where seamless is legitimately required.
Pressure equipment design codes apply a joint efficiency factor to welded tube — a factor of 1 or less that reduces the allowable stress. Seamless tube takes 1. Welded tube takes a lower value, depending on the process and the level of non-destructive examination applied.
The direct consequence: at the same pressure and grade, a welded tube may require a greater wall thickness. Depending on size and process, that extra thickness cancels some or all of the saving on welded tube.
This is therefore a calculation, not a preference. And that calculation belongs to the engineering department, working from the code applicable to the project — the exact value of the factor is not something to improvise.
Availability and cost
Welded tube is generally cheaper, by around 10 to 25% depending on grade and size, and it dominates large diameters — above a certain point, seamless becomes rare and expensive.
Seamless keeps the advantage on small diameters in heavy wall, and on grades where strip availability is limited.
One practical point matters more on site than people expect: lead time. For a given grade and size, what is in partner stock is often available in only one of the two processes. Specifying seamless without a calculation to justify it sometimes means choosing six extra weeks for no gain.
Heat exchanger tube
On bundles, welded tube is the majority and thoroughly established: ASTM A249 for welded, ASTM A213 for seamless, and both are routinely specified. Welded even has an advantage of its own — its consistent wall thickness gives a more uniform heat transfer coefficient across the bundle.
Seamless remains preferred where the tube will undergo severe bending — on tight-radius U-bends, where ovality and extrados thinning are critical.
Comparison
| Criterion | Welded | Seamless |
|---|---|---|
| Cost | 10 to 25% cheaper | reference |
| Large diameters | readily available | rare and expensive |
| Wall consistency | better | more variable |
| Corrosion, standard met | equivalent | equivalent |
| Joint efficiency factor | below 1 | 1 |
| Severe bending | possible | preferred |
| Usual standards | EN 10217-7, A312, A249 | EN 10216-5, A312, A213 |
Frequently asked questions
- Is welded tube weaker?
- No, where the standard is met: the guaranteed mechanical properties are those of the standard, identical in both processes. The difference appears in the calculation, through the joint efficiency factor, not in the material itself.
- How can I tell a welded tube?
- The longitudinal weld remains visible inside with some processes, or detectable by examination. On a dressed and heat-treated tube, visual identification becomes difficult — the material certificate is what counts.
- Can I replace seamless with welded on a repair?
- Only after checking the applicable design code and the original specification. If the calculation used a factor of 1, welded tube of the same wall thickness does not meet the same allowable stress.
- Which should I choose if my specification says nothing?
- The cheaper and more readily available in the size required, which most often means welded — unless the application is pressure-bearing and the calculation was not done with the appropriate factor. In that case, redo it before deciding.