Article
Seamless pipe is often treated as the better product and welded pipe as the compromise. That is not how a piping engineer sees it. Each is made a different way, each has a size range it is good at, and a modern welded pipe tested to its standard is fully fit for a great deal of pressure service. The right question is which one the design code and the service condition call for.
How seamless pipe is made
A solid round billet is heated and pierced to form a hollow shell, which is then rolled or drawn down to the finished diameter and wall thickness. There is no weld anywhere in the pipe. Smaller sizes are often cold drawn after hot forming, which gives a closer tolerance and a better surface.
How welded pipe is made
Welded pipe starts as flat strip or plate, which is formed into a cylinder and joined along its length.
- ERW (electric resistance welded): the edges are heated by electric current and forged together without filler metal. Common for carbon steel in smaller and medium sizes.
- EFW (electric fusion welded): the edges are fused, usually by TIG or plasma, sometimes with filler. The normal route for stainless steel welded pipe.
- SAW (submerged arc welded): used for large diameters. LSAW has a straight longitudinal seam; SSAW (spiral) has a helical one.
Side by side
| Seamless | Welded | |
|---|---|---|
| Weld seam | None | One longitudinal or spiral seam |
| Size range | Small to medium diameters; large sizes become costly | Very wide, up to large diameters |
| Wall thickness | Suits heavy walls | Suits thin to medium walls; very heavy walls less common |
| Wall uniformity | Wider tolerance on eccentricity | Very uniform, rolled from plate |
| Cost | Higher, especially in larger sizes | Lower, and more readily available in big sizes |
| Typical standards | ASTM A106, A333 (seamless), A335, A312 (seamless) | ASTM A53 Type E, A358, A312 (welded), API 5L ERW / SAW |
Where seamless is usually specified
- High-pressure and high-temperature lines, such as steam and boiler piping, where the design code or the client's specification requires it.
- Heavy-wall pipe in small and medium sizes.
- Hydrogen, sour or cyclic service where the project standard excludes longitudinal welds.
- Hydraulic and instrumentation tubing where a weld bead would be a problem.
Where welded is the right choice
- Large diameters, where seamless is expensive or simply not made.
- Water, utilities, fire-fighting and general process lines at moderate pressure.
- Stainless steel process piping in thin walls, where EFW pipe to ASTM A312 or A358 is standard practice.
- Structural and mechanical use.
The weld joint efficiency factor
Pressure design codes such as ASME B31.3 apply a longitudinal weld joint quality factor to welded pipe. It reduces the allowable stress used in the wall thickness calculation, depending on the type of weld and how much of it has been examined by radiography. Fully radiographed welded pipe can reach a factor of 1.0, the same as seamless. This is why the specification of the examination matters as much as the choice between seamless and welded.
What to put on the enquiry
State the manufacturing route explicitly, not just the standard, because several standards cover both: ASTM A312, for instance, covers seamless and welded stainless pipe. Add the size, schedule or wall thickness, the grade, the end finish, and any examination requirement such as 100% radiography.
See ASTM A106 Grade B seamless pipe, SS 304/304L welded pipe, or the full pipes and tubes range.
Frequently asked questions
Is seamless pipe always stronger than welded?
Not necessarily. Modern welded pipe with proper testing performs very well; codes may apply a weld joint factor in design.
Why is welded pipe cheaper?
It is made from strip or plate, which is less costly to produce than pierced seamless pipe, especially in larger sizes.
When is seamless pipe mandatory?
When the project specification or design code requires it, typically for high pressure, high temperature or critical service.
Key takeaways
- Seamless suits high pressure, high temperature and critical service.
- Welded offers better wall uniformity and lower cost in larger sizes.
- Follow the piping specification and design code.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.