Why the Same Size Comes in Four Manufacturing Routes

The word “carbon steel pipe” covers more than one product. On the same procurement enquiry you can be quoted seamless (SMLS), electric resistance welded (ERW), longitudinal submerged arc welded (LSAW), or spiral submerged arc welded (SSAW), with the price difference between them reaching a factor of three on the same size and grade. The reason is that each route was developed to serve a specific diameter and pressure window, and no single process covers the full range economically.

A buyer who treats the four routes as interchangeable will either overpay for the wrong product or, worse, under-specify a critical line. The two mistakes are equally common, and both come from the same root: a data sheet that names a standard without naming the manufacturing route. Adding the route explicitly — SMLS vs ERW vs LSAW vs SSAW — is the single most effective clarification a procurement team can make on a carbon steel order.

The Four Manufacturing Routes in One View

The diameter ranges above are not absolute. Modern mills can stretch SMLS into 26-inch territory for special orders, and ERW lines have crept up to 24 inches. But on a project enquiry, the four-way split is the starting point. If the line is above 24 inches and not piling, the conversation is almost always about LSAW.

When Seamless (SMLS) Is the Right Answer

Seamless pipe is formed by heating a solid steel billet to around 1200–1280 °C and forcing it over a piercing mandrel to form a hollow shell. The shell is then elongated, sized, and heat-treated. The result is a tube with no weld seam, uniform wall thickness around the full circumference, and a continuous grain structure along the longitudinal axis.

That continuous grain structure is what makes SMLS the default for the most demanding services: high-pressure boiler tubes, superheater tubes, refinery steam lines, hydrocracker reactor piping, and any line that handles sour hydrocarbons (NACE MR0175). It is also the only practical choice for the heavier end of the wall schedule range at small diameters — a 4-inch SCH 160 in ASTM A106 Grade B is almost universally supplied as seamless because ERW mills rarely hold the dimensional tolerance at that wall.

The chemistry that makes seamless pipe work

Carbon and carbon-manganese steels dominate SMLS production for refinery and power plant service. ASTM A106 Grade B is the most common grade, with a typical chemistry of C ≤ 0.30%, Mn 0.29–1.06%, P ≤ 0.035%, S ≤ 0.035%, and a minimum tensile strength of 415 MPa with a minimum yield of 240 MPa. ASTM A106 Grade C lifts tensile to 485 MPa and yield to 275 MPa for thicker headers and superheater inlet lines. For low-temperature service down to −46 °C, ASTM A333 Grade 6 takes over; for elevated creep resistance, the alloy-steel family governed by ASTM A335 (P11, P22, P91, P92) enters the picture.

When ERW Is the Right Answer

ERW pipe is produced by uncoiling a steel strip, forming it through a series of rolls into a cylindrical shape, and welding the edges together using the resistance heating generated by passing a high-frequency electric current across the seam. The heat-affected zone is narrow (typically 2–3 mm on each side of the seam), and the weld is metallurgically sound enough to be accepted for most fluid, structural, and mechanical applications.

ERW is the most economical route for diameters from about 1/2 inch to 20 inches, and it dominates the ASTM A53, ASTM A252, and ASTM A500 markets. ASTM A53 Grade B ERW is the workhorse for low-pressure water, air, and steam service; ASTM A252 ERW is the default for steel pipe piling; ASTM A500 Grade C ERW covers cold-formed structural tubing. None of these are exotic applications, but together they account for a large share of global carbon steel pipe tonnage.

The two engineering limits on ERW are pressure and sour service. For sour (H₂S-containing) hydrocarbons, NACE MR0175 explicitly restricts the use of ERW pipe unless the weld has been subjected to full-body normalizing heat treatment. For very high-pressure hydrostatic service (above about 250 bar in small diameters), the seam is a fatigue concern and SMLS or LSAW becomes the safer choice.

When LSAW and SSAW Are the Right Answer

Above 24 inches, SMLS becomes uneconomical and ERW is no longer technically practical. Two welded routes cover the large-diameter market. LSAW pipe is formed from plate in a U-press and O-press, then welded along one or two straight longitudinal seams using submerged arc welding. SSAW pipe is formed from coil in a helix, then welded along a continuous spiral seam using the same submerged arc process.

LSAW is the standard for long-distance, high-pressure oil and gas transmission, with API 5L PSL2 grades up to X80 supplied in diameters from 18 to 56 inches. The straight longitudinal seam gives uniform mechanical properties around the circumference and is preferred for pressurised service. SSAW is the standard for piling, large-diameter water transmission, and structural applications where pressure is low and cost is the deciding factor. The spiral geometry means the seam crosses the hoop stress direction at an angle, which limits the pressure rating but allows the same coil to produce a wide range of finished diameters.

Inside an LSAW pipe order

A typical LSAW order for an API 5L X65 trunk line will specify plate from a normalised fine-grain steel, formed and welded in two passes (inside and outside the seam), then mechanically expanded for dimensional accuracy. The seams are 100% ultrasonically tested, with radiographic testing on the weld and magnetic particle inspection on the weld toes. Hydrostatic test is performed on every length. A mill that holds all of this in one production line — and ships the lengths with a complete EN 10204 3.2 certificate book — is the kind of supplier a major pipeline project can work with.

Reading a Data Sheet: Where the Route Lives

On a typical mill data sheet, the manufacturing route appears as a two- or three-letter code. SMLS is seamless; ERW is electric resistance welded; LSAW (also written SAWL) is longitudinal submerged arc welded; SSAW (also SAWH or HSAW) is spiral submerged arc welded. Most standards accept several routes for the same nominal pipe, but each route is paired with its own NDT and heat-treatment requirements. The code matters because it determines the test scope, the documentation scope, and the price.

A common procurement mistake is to write “ASTM A106 Grade B” on the order without specifying the route. The mill will then default to the lowest-cost option that meets the chemistry, which may be ERW even though the project actually requires seamless for a high-pressure line. The fix is to name the route in the line item: “ASTM A106 Grade B, SMLS, SCH 40” for example. Adding those four letters is the difference between a 30-year service life and a weld seam at the worst possible point in the piping.

How the Connectors Around the Pipe Change the Answer

The pipe is rarely the only item on a carbon steel procurement order. The matching pipe fittings — elbows, tees, reducers, caps — are usually produced by a different forming method (butt-welding of plate or hot-pushing of seamless shells), and the flanges that join the assembly are cut from plate or forged from billet. The route question on the pipe is therefore not isolated; it sits inside a larger question of how the whole assembly is going to be welded, inspected, and documented.

A practical procurement approach treats the pipe, the pipe fittings, the pipe flanges, and the gaskets and stud bolts as one material-traceable package. When all of these come from the same supplier, the heat number, the chemical analysis, the test plan, and the shipping schedule stay coherent. When they come from three or four different vendors, the documentation chain is the first thing to break, and the survey becomes a long series of phone calls.

Valve selection follows the same logic. A high-pressure carbon steel line that uses SMLS pipe with butt-weld fittings and welded flanges still needs an industrial valve with the same body material, the same pressure class, and the same end connection. A mismatch on any one of those three points is a leak path or a corrosion cell, and either one will surface during the first hydrostatic test.

A Practical Decision Table for Buyers

The Documentation That Proves the Route

Buying SMLS instead of ERW is meaningless unless the mill test certificate proves it. A clean EN 10204 3.1 certificate identifies the manufacturing process, the heat number, the chemical analysis, and the mechanical test results for the specific heat that the pipe was produced from. A 3.2 certificate adds independent third-party witnessing on top. For sour service, nuclear, and offshore applications, the 3.2 certificate is typically mandatory.

Beyond the certificate, the NDT report is what proves the route was executed correctly. SMLS pipe is typically eddy-current or ultrasonic tested on the body and hydrostatically tested on every length. ERW pipe is ultrasonically tested on the full seam length, with the seam heat-affected zone inspected for defects. LSAW and SSAW pipe carry radiographic or ultrasonic test reports on every weld seam, plus visual and dimensional inspection on the body.

A buyer who receives all of this in one binder, with one heat number per line, has a traceable chain from the steel heat to the finished pipe. A buyer who receives three binders from three different suppliers has a documentation project to run before the order can be accepted. The route decision is the first step; the documentation decision is what makes the first one stick.

Common Buyer Questions, Answered Briefly

Can ERW pipe be used for high-pressure service?
For non-sour, non-toxic, low-to-medium-pressure service up to about 250 bar in small diameters, yes — and ERW is widely accepted in that envelope. Above that pressure, or for any service in hydrogen sulphide, SMLS is the right answer.

Is LSAW always better than SSAW?
For pressurised service, yes. The straight longitudinal seam of LSAW handles hoop stress more cleanly than the spiral seam of SSAW. For piling and large-diameter low-pressure water, SSAW is the more economical choice and is universally accepted.

Does the route affect lead time?
Significantly. ERW is a stock-driven process; standard A53 and A252 sizes are often on the yard. SMLS in standard A106 sizes is typically 15–25 days. LSAW for large-diameter line pipe is usually 30–60 days depending on plate availability and inspection scope. SSAW is the shortest lead time for piling sizes because the forming process is fast and the input coil is widely available.

A Closing Note for the Procurement Team

The four manufacturing routes for carbon steel pipe are not competitors; they are four different tools for four different jobs. The engineering work is to match the tool to the service, then specify the route in the order so the mill is not forced to guess. A good supplier will help with that conversation. A great one will also hold stock in the routes that the local market buys most often, and will line up the matching pipe fittings, pipe flanges, gaskets, stud bolts, and industrial valves on the same delivery so the documentation stays in one chain.