Four pipes can meet the same nominal size, carry the same grade stamp, and still differ by 30% in price and by a wide margin in what they will survive in service. The difference is how the pipe was made — and that is decided on the purchase order, not in the mill.

This guide compares the four manufacturing routes honestly: what each one is good at, where each one gets specified by mistake, and the few inspection points that actually tell you whether the pipe you bought is the pipe you ordered.

1. Four routes, one pipe

Every steel pipe starts as either a solid billet or a coil of strip. How it becomes a tube determines everything downstream:

The single most useful way to read this table: seamless is limited by how big a billet you can pierce; welded is limited by how wide a coil or plate you can handle. That is why large-diameter heavy-wall pipe is welded almost without exception, while small-diameter high-pressure pipe is almost always seamless.

2. Seamless: when the no-seam premium is worth paying

A seamless pipe has no weld and therefore no heat-affected zone — no localized region of altered microstructure, no seam that can carry a defect invisible to a dimensional check.

Pay for it when any of these is true:

• The service is cyclic or high pressure, and a seam defect would be a leak rather than a wet patch on a floor.
• The pipe will be machined, threaded, or bent after delivery — threads cut across a weld seam are a known failure point.
• The specification says so: ASTM A106 for high-temperature service, ASTM A335 for alloy elevated-temperature work, and most oil country tubular goods under API 5CT are seamless by default.
• The diameter is modest and the wall is heavy — the geometry where seamless rolling is most economical.

Where seamless is the wrong choice: large diameters above roughly 24 inches, thin walls, and long structural runs where cost per metre dominates. A 1,200 mm seamless pipe is not a cost problem, it is a manufacturing impossibility at most mills.

3. ERW: the workhorse, and the one seam you should inspect

Electric resistance welding forms a coil into a tube and welds the edges with high-frequency current, without filler metal. It is fast, dimensionally accurate, and cheap per metre — which is why ERW covers most of the world's line pipe, water main, structural tube, scaffold, and fire protection demand.

Its weakness is concentrated in one place: the weld seam and its heat-affected zone. The weld cools quickly, the HAZ can end up harder and less tough than the parent metal, and a poorly controlled weld can leave a cold lap or a stitch defect that passes a hydrostatic test but fails in service.

Three things to require, in writing:

1. Seam normalizing or full-body normalizing for line pipe grades — this restores HAZ toughness. For API 5L grades above X42, ask specifically whether the weld is normalized and at what temperature.
2. In-line non-destructive testing of the seam — ultrasonic or eddy current, with the test standard named on the certificate. Not "100% inspected": named, and with an acceptance criterion.
3. A flattening or bend test result on the lot certificate. It is the cheapest proxy for whether the seam is metallurgically sound rather than merely closed.

Where ERW is the wrong choice: very heavy walls at large diameter, sour service where hardness control across the HAZ is difficult to guarantee, and any application where the end user's specification explicitly excludes a welded seam.

4. LSAW: large diameter without giving up pressure

LSAW (longitudinal submerged arc welded) takes plate, forms it into a cylinder, and welds one straight seam with submerged arc — a process that deposits far more metal per pass than ERW and tolerates much thicker walls.

It is the default for large-diameter, high-pressure transmission: 24-inch and above, wall thickness from about 6 mm to 32 mm, API 5L grades to X80, onshore and offshore. Because the seam is straight and accessible, it can be inspected by radiographic or ultrasonic methods along its full length, and repaired locally if needed.

Two practical notes:

• Dimensional accuracy is better than SSAW, because the plate is formed against a fixed geometry rather than a moving helix. For pipe that has to be field-welded end to end, that matters — ovality and end squareness drive fit-up time.
• Double-seam (DSAW) versions exist for very large diameters where plate width is insufficient for a single seam. Two seams mean twice the inspection; confirm which construction you are being quoted, because the designations are often used interchangeably in offers.

Where LSAW is the wrong choice: small diameters (below roughly 14 inches, ERW or seamless wins on cost) and thin walls, where forming and welding plate is simply inefficient.

5. SSAW: the diameter champion, with a caveat

Spiral submerged arc welded pipe feeds coil at an angle, so the seam runs helically around the tube. This lets a relatively narrow coil produce a very large pipe — up to 4 m diameter — which no other route can do economically.

Its structural honesty is worth understanding: the seam is longer. A helical weld on a given length of pipe is roughly 1.2 to 1.5 times the length of a straight seam depending on the helix angle, and every metre of that seam is a potential defect site. That is not a reason to avoid SSAW, but it is the reason inspection intensity on large spiral pipe is higher, and the reason SSAW is usually specified for water, drainage, piling, and low-to-medium pressure transmission rather than for high-pressure gas where LSAW is available.

Where SSAW wins: water transmission mains, large outfall and drainage lines, structural piling, and any project where the diameter is the constraint and the pressure is modest.

6. The decision, in the order it should be made

Work down this list; the answer usually falls out by item three.

1. Does the governing code name a type? ASTM A106, A335 and API 5CT tubulars are seamless by default. ASTM A795 fire protection pipe permits ERW or seamless, hot-dip galvanized. EN 10210 covers hot-finished sections (including seamless), while EN 10219 covers cold-formed welded hollow sections — the two are not interchangeable on a European structural job, and mixing them up is a common and expensive mistake.
2. What is the design pressure and consequence of failure? High pressure plus large diameter points to LSAW. High pressure plus small diameter points to seamless. Low pressure at any diameter leaves the welded options open on cost.
3. What is the diameter and wall? Above 24 inches, seamless is out. Above 1,600 mm, LSAW becomes difficult and SSAW takes over. Heavy wall at large diameter means LSAW; thin wall at large diameter means SSAW.
4. Will it be welded, bent, threaded, or machined on site? Threading across a seam is bad practice; tight-radius bending of welded pipe needs a forming trial, not an assumption.
5. Then compare price per metre, not per tonne. Two pipes with the same nominal size can differ in actual wall thickness by several percent within tolerance, and identical tonnage then buys different lengths. Convert quotes to cost per metre of usable pipe.

7. PSL1 or PSL2 — the choice inside each type

API 5L is often quoted without saying which specification level applies, and that single omission changes the price and the paperwork substantially.

PSL1 is the basic level: chemistry and mechanical properties to the standard, with a hydrostatic test that can be replaced by non-destructive testing by agreement. PSL2 adds specified chemistry limits, mandatory CVN impact testing, non-destructive inspection of the weld, and full traceability to heat number.

If the pipe is going into a gas transmission line, a sour service, or a low-temperature application, PSL2 is normally the correct requirement. If a supplier's quote looks unusually cheap for a line pipe enquiry, check whether PSL1 was assumed.

8. What to verify before the container is sealed

• Mill test certificate to EN 10204 3.1, issued by the mill's independent inspection department and tied to the specific heat and pipe number — not a 2.2 statement of compliance typed by the trader. Match the numbers on the certificate against the stencil on the pipe ends.
• Hydrostatic test record: pressure, duration, and the standard applied. "Tested" without figures is not a record.
• Seam NDT report for welded pipe, naming the method, coverage and acceptance criterion.
• Dimensional check: OD, wall thickness at several points, ovality, end squareness, and length. For field-welded lines, ovality and end squareness cost more in fit-up labour than any wall tolerance.
• Coating check where applicable: for hot-dip galvanized pipe, coating mass in g/m² (typically 120–500 g/m², or roughly 30–70 µm) and a thickness reading at multiple points around the circumference — not one reading on the smoothest face.
• Marking and traceability: grade, size, heat number, standard, and manufacturer's mark, legible on every length.

Sourcing all four types from one supplier

Jinhongtong Steel operates independent welded and seamless pipe lines, with a 70,000 m² plant and over 5,000 tons of pipe held in stock across common grades and sizes. The range covers seamless pipe to 720 mm OD, ERW to 26 inches, LSAW to 1,626 mm and SSAW beyond 4 m, plus hot-dip galvanized, stainless, alloy pipe and matching fittings — which means a mixed order can ship as one consolidated shipment instead of three.

Every shipment carries an MTC to EN 10204 3.1, and in-house processing covers cutting, beveling, threading, grooving and anti-corrosion coating, so pipe arrives ready to install rather than ready to rework.

If you are still deciding between two manufacturing routes for a project, send the design pressure, diameter, wall thickness and governing standard — we will come back with the type, the grade and the price per metre.

Related reading: Seamless steel pipe · ERW steel pipe · LSAW welded pipe · SSAW steel pipe · Galvanized steel pipe · Stainless steel pipe · Pipe fittings

FAQ

Is ERW pipe as strong as seamless pipe? In the body of the pipe, yes — both meet the same grade requirements. The difference sits at the weld seam and its heat-affected zone, which is why welded line pipe specifies seam normalizing and non-destructive testing. For high-cycle pressure service or post-delivery machining, seamless removes that variable entirely.

What is the maximum diameter for seamless steel pipe? Around 720 mm at most Chinese mills, and often less for heavy walls. Beyond that, the billet and mandrel geometry become impractical and welded pipe — LSAW or SSAW — takes over.

Which is cheaper, ERW or seamless? ERW is generally cheaper per metre in the sizes where both are available, because it starts from coil rather than a pierced billet. The gap narrows at small diameters and heavy walls, so compare quotes on cost per metre rather than assuming one route is always cheaper.

Can SSAW pipe be used for high-pressure gas lines? It can be, and it is used in some markets, but LSAW is generally preferred where the diameter allows it, because a straight seam is shorter per metre of pipe and easier to inspect fully. The governing pipeline code and the project specification decide, not the supplier.

Does hot-dip galvanizing change the pressure rating of a pipe? No. Galvanizing is a corrosion protection coating and does not alter the pressure-carrying capability of the pipe body. It does affect threading and welding practice — galvanized pipe is normally threaded rather than field-welded at the joint, and cut ends need re-protection.