The seamless pipe manufacturing process pierces a solid, hot steel billet and rolls the hollow shell to size – no weld is ever made. The billet is heated to about 1,200–1,280 °C, cross-roll pierced into a thick “mother hollow”, elongated on a mandrel mill (MPM/PQF), plug mill or pilger mill, reheated, sized or stretch-reduced, then cooled, straightened, heat treated where required, hydrotested or NDE-tested and marked. Very large or heavy-wall sizes are forged, hot-expanded or extruded. Welded pipe (ERW/HFW, EFW, LSAW, HSAW) is formed from plate or coil and joined along a seam, which is why codes such as ASME B31.3 give seamless pipe a weld joint quality factor of Ej = 1.00 versus 0.85 for ERW.
Standards referenced in this guide (reviewed October 2026)
- API Spec 5L, 47th edition (2026) – Line Pipe; adds CO2-pipeline requirements and updates HFW quality, hydrotest, NDE, sour-service and lamination controls. Its ISO counterpart is ISO 3183.
- ASTM A106/A106M-26 – seamless carbon steel pipe for high-temperature service.
- ASTM A53/A53M-24 – seamless (Type S), ERW (Type E) and furnace-butt-welded (Type F) carbon steel pipe.
- ASTM A312/A312M-25 – seamless, welded and heavily cold-worked austenitic stainless steel pipe.
- ASTM A530/A530M and ASTM A999/A999M – general requirements for carbon/alloy and stainless pipe.
- ASME B36.10M-2022 and ASME B36.19M – pipe dimensions and wall schedules.
- ASME B31.3-2024 (process piping, Table A-1B weld joint quality factors) and ASME B31.8 (gas transmission, longitudinal joint factor).
- EN 10216 (seamless) and EN 10217 (welded) series – European steel tubes for pressure purposes.
1. What Is a Seamless Pipe?
A seamless pipe is a hollow steel tubular produced from a solid round billet without any longitudinal or spiral weld. Because the wall is formed entirely by hot working, its grain structure and mechanical properties are uniform around the full circumference. There is no fusion line, no heat-affected zone (HAZ) and no weld-related flaw population to inspect, so design codes credit seamless pipe with the full allowable stress (Ej = 1.00 in ASME B31.3).
That integrity is why seamless pipe is the default for boiler and superheater lines, high-pressure steam, hydrocarbon process piping, hydraulic systems, sour service, cyclic and high-fatigue duties, and as the parent material for seamless butt-weld fittings (elbows, tees and reducers to ASME B16.9). The trade-offs are higher cost, longer lead times for unusual sizes, slightly looser wall-thickness tolerance and eccentricity than welded pipe made from rolled plate, and a practical size ceiling: hot-rolled seamless production generally tops out around NPS 24–26 (DN 600–650), with larger sizes made by hot expansion or forging. See our pipe sizes and schedules guide for dimensions.
2. Pipe Manufacturing Route Map
Every steel pipe starts as either a solid billet (seamless routes) or a flat plate or coil (welded routes). The figure below shows how each route branches and where it sits in the size range.
3. Seamless Pipe Manufacturing Process – Step by Step
Although mills differ in the elongation stage, almost every hot-rolled seamless line follows the same nine steps.
- Steelmaking and billet casting. Steel is melted in an electric arc furnace (EAF) or basic oxygen furnace (BOF), refined in the ladle (and vacuum-degassed for sour-service or low-hydrogen grades) and continuously cast into round billets. Clean steel with low sulphur and controlled inclusions is essential because inclusions at the billet centre become internal laps after piercing.
- Billet inspection and cutting. Billets are checked for surface cracks and central porosity, then hot- or cold-sawn to the length that gives the required finished pipe weight.
- Heating. The billet is soaked in a rotary hearth furnace to roughly 1,200–1,280 °C, depending on grade. Uniform through-temperature controls piercing quality and wall eccentricity.
- Rotary (cross-roll) piercing. Two skewed barrel- or cone-shaped rolls rotate the billet and drive it over a piercing plug, producing a thick-walled mother hollow (hollow shell).
- Elongation. The hollow is rolled over a mandrel or plug to reduce the wall and lengthen the shell – in a mandrel mill (MPM or PQF), plug mill, pilger mill, Assel mill or push bench.
- Reheating. The shell is reheated in a walking-beam or reheating furnace to restore rolling temperature.
- Sizing or stretch-reducing. A multi-stand sizing mill sets the final OD; a stretch-reducing mill uses inter-stand tension to reduce both OD and wall, producing many small sizes from one shell size.
- Cooling, cutting, straightening and heat treatment. Pipe is cooled on a cooling bed, cut to length, rotary-straightened and, where the specification requires, normalized, quenched-and-tempered or solution-annealed.
- Finishing, testing and marking. Ends are faced or bevelled, then each length is hydrotested or tested by NDE (UT, eddy current or flux leakage), dimensionally checked, weighed, stencilled (for example “SMLS”) and coated or capped.
4. Rotary Piercing – The Mannesmann Process
Rotary piercing, invented by the Mannesmann brothers in the 1880s, is the step that turns a solid billet into a tube. The two work rolls are set with their axes skewed (feed angle typically about 6–14°), so they both rotate the billet and pull it forward. As the round billet is compressed between the rolls it is alternately squeezed and released many times per revolution. This cyclic loading creates secondary tensile stresses at the billet centre – the Mannesmann effect – which opens the centre just ahead of the plug. The plug, held on a thrust (mandrel) bar, then smooths and sizes the bore. Guide shoes or driven Diescher discs control the shell OD.
Modern cone-type piercers with high feed angles and controlled cross-roll (“toe”) angles reduce redundant shear, which improves inner-surface quality and allows harder grades and thinner, longer hollows. The outside diameter of the piercing plug is roughly equal to the bore of the mother hollow.
5. Seamless Pipe Mills Compared
The elongation mill defines a seamless plant’s size range, tolerances and productivity. Use the table to match a pipe size and wall to the process most likely to have produced it – useful when assessing supplier capability or reviewing mill test reports.
| Process | Typical OD range | Wall reduction method | Strengths | Typical products |
|---|---|---|---|---|
| Mandrel mill – MPM (2-roll stands) | ~2–16 in (60–406 mm) | Single continuous pass over a retained (controlled-speed) mandrel bar | High output, good wall tolerance, long lengths | A106, A53 Type S, API 5L / 5CT, boiler tube hollows |
| Mandrel mill – PQF (3-roll stands) | ~2–16 in (60–406 mm) | Continuous pass, three rolls per stand around a retained mandrel | Best concentricity and wall uniformity; handles thin walls and high alloys | OCTG, line pipe, high-alloy and premium process pipe |
| Plug mill | ~6–16 in (168–406 mm) | Two-high rolls over a short stationary plug; 2 or more passes with 90° rotation | Flexible, lower capital cost, heavier walls | Medium-size process and structural pipe; increasingly replaced by PQF |
| Pilger (pilgrim) mill | ~10–26 in (273–660 mm) | Oscillating eccentric rolls forge the shell in small increments over a mandrel | Large diameters and heavy walls from a single shell | Large-bore and heavy-wall seamless pipe |
| Assel mill | ~2–10 in (60–250 mm) | Three skewed rolls over a floating mandrel | Very good concentricity on thick walls | Bearing tubes, mechanical tubing, thick-wall hollows |
| Push bench | ~1–6 in (25–170 mm) | Cupped hollow pushed through a series of roller dies on a mandrel | Economical for small sizes | Small-bore mechanical and structural tube |
| Hot extrusion | ~½–12 in (12–300 mm) | Ram forces billet through a die around a mandrel in one stroke | Works hard-to-pierce alloys; complex sections | Stainless, duplex, nickel alloy and titanium tube |
| Hot expansion | Up to ~48 in (1,219 mm) and beyond | Heated seamless mother pipe is pushed or drawn over a conical mandrel | Seamless sizes above the rolling-mill limit | Large-diameter seamless headers and line pipe |
| Press forging (+ boring) | ~12–60 in, very heavy walls | Hydraulic press pierces an ingot in a die, then forged and machined | Thickest walls, special alloys, short lengths | Main steam headers, P91/P92 hot reheat headers, high-pressure manifolds |
| Cold drawing / cold pilgering | ~¼–10 in | Cold reduction of a hot-finished hollow over a plug or mandrel | Tight tolerances, fine surface, thin walls | Instrument, hydraulic, heat-exchanger and boiler tubes |
5.1 Mandrel Mill Process (MPM and PQF)
In a continuous mandrel mill the pierced hollow is threaded onto a long, lubricated mandrel bar and rolled through five to eight stands in one continuous pass. Each stand reduces the wall between grooved rolls and the mandrel. In the classic floating-mandrel mill the bar travelled with the pipe; modern MPM (Multi-stand Pipe Mill) lines hold the bar at a controlled speed (retained mandrel), and PQF (Premium Quality Finishing) lines use three rolls per stand, which spreads the deformation more evenly and gives tighter wall tolerance and lower eccentricity. After rolling, the mandrel is extracted, the shell passes an extracting/sizing mill, is reheated in a walking-beam furnace and then sized or stretch-reduced to finished dimensions.
5.2 Plug Mill and Pilger Mill Processes
The plug mill (automatic mill) shares the heating and piercing steps but reduces the wall over a short, stationary plug held between two grooved rolls. The shell is returned by stripper rolls, rotated 90° and rolled again, usually in two passes, so wall reduction is progressive rather than continuous. A reeling mill then burnishes the inside and outside surfaces and removes the plug marks, and a sizing mill sets the final OD. Plug mills suit medium diameters and heavier walls, but most new capacity uses PQF technology because of its better tolerances.
The pilger (pilgrim step) mill is used for large and heavy-wall seamless pipe. A thick hollow on a mandrel is fed backwards and forwards between two rolls with an eccentric (tapered) groove: on each revolution the rolls bite a small section of the shell, forge it down and push it back, then the shell is rotated and advanced slightly. The process is slow but produces seamless pipe up to about 26 in OD with heavy walls.
5.3 Forged and Hot-Expanded Seamless Pipe
Forged seamless pipe is made when the size or wall exceeds rolling-mill capability, for example main steam and hot reheat headers in ASTM A335 P91/P92 or forged-and-bored pipe to ASTM A369 and high-pressure manifolds. A heated ingot is placed in a die and a hydraulic press drives a punch (mandrel) into it, displacing metal upward around the punch to form a cup or open shell. The shell may be further drawn or forged over a mandrel, then heat treated, bored and turned to final dimensions. Forging gives excellent through-wall integrity on the heaviest sections but is slow and expensive.
Hot expansion is the economical route to large seamless sizes: a smaller seamless mother pipe is heated (often by induction) and pushed or pulled over a conical mandrel, increasing the diameter by up to roughly 30–50% per pass. Wall thickness falls slightly during expansion and must be allowed for when ordering.
5.4 Extrusion and Cold-Finishing Processes
In hot extrusion (the Ugine-Séjournet process with glass lubrication is the best-known), a pre-pierced or expanded billet is loaded into a heated container. A hydraulic ram forces the billet forward so the metal flows through the annular gap between the die and a mandrel, producing a seamless tube in a single stroke. Extrusion handles austenitic and duplex stainless steels, nickel alloys and titanium that are difficult to cross-roll pierce, and is often used to make mother hollows for cold finishing.
Cold drawing (over a plug or mandrel) and cold pilgering reduce a hot-finished hollow at room temperature to achieve tight dimensional tolerances, smooth surfaces and thin walls. Because cold work raises hardness and lowers ductility, the product must be heat treated afterwards: ASTM A106 requires cold-drawn pipe to be heat treated after the final cold-draw pass at 650 °C (1200 °F) or higher, while austenitic stainless pipe to ASTM A312 is solution annealed and rapidly cooled to restore corrosion resistance.
6. Welded Pipe Manufacturing Processes
Welded pipe starts as hot-rolled coil, strip or plate. The flat product is formed into a cylinder – by continuous roll forming for coil, by press forming (U–O or J–C–O) or roll bending for plate, or by helical forming for spiral pipe – and the edges are joined with or without filler metal. Welded pipe is cheaper per tonne, offers very tight wall tolerance (wall thickness is set by the rolled plate or coil) and has no practical upper diameter limit. Its weak point is the seam: the weld and HAZ must be qualified, heat treated where required and inspected along their full length. API 5L uses the designations SMLS, HFW, LFW, LW (laser), SAWL, SAWH, COWL and COWH (combination GMAW + SAW) to identify the product type on the certificate.
6.1 ERW / HFW Pipe Manufacturing Process
Coil is uncoiled, levelled, end-sheared and butt-welded to the previous coil so the line runs continuously through an accumulator. The strip edges are trimmed or milled, then a series of forming rolls (breakdown, fin pass) bends the strip into an open tube. At the weld point the edges converge in a “V”; a high-frequency current – induced by a coil (HFI) or applied through sliding contacts – flows along the V edges by the skin and proximity effects and heats them to forging temperature. Squeeze rolls forge the edges together without filler metal, expelling oxides in the upset. The external and usually internal flash is scarfed off, and the weld zone is heat treated by an in-line induction seam normalizer to refine the as-welded microstructure. The tube is then cooled, sized, flying-cut to length and inspected by automated UT of the seam, hydrotest and, for some grades, flattening tests.
Modern HFW (high-frequency welding, typically ≥ 70 kHz, often 200–450 kHz) replaced the old low-frequency ERW (LFW) used before about 1970, whose seams were prone to cold welds, hook cracks and selective seam corrosion. API 5L PSL2 does not permit LFW pipe and requires the HFW weld seam to be heat treated. ERW/HFW lines are fast and economical for roughly ½–24 in OD; ASTM A53 Type E, ASTM A135 and API 5L HFW pipe are typical products.
EFW (electric-fusion-welded) pipe is the stainless-steel counterpart: strip or plate is formed and fusion welded, usually by GTAW (TIG), plasma or laser, with or without filler, then solution annealed. ASTM A312 welded grades and ASTM A358 (plate, with filler) are EFW products.
6.2 LSAW Pipe Manufacturing Process (UOE and JCOE)
Submerged arc welding (SAW) feeds one or more consumable wire electrodes under a blanket of granular flux that shields the arc and the weld pool. Because filler metal is deposited, SAW suits thick walls and large diameters. LSAW (SAWL) pipe has one straight longitudinal seam (two seams for very large diameters).
- UOE: plate edges are milled and crimped, the plate is pressed into a U, then closed into an O, continuously tack-welded (usually GMAW), welded by multi-wire SAW from the inside (ID) and then the outside (OD), and finally mechanically Expanded by roughly 0.8–1.5% to round the pipe and relieve residual stress. UOE is the high-volume route for API 5L X52–X80 transmission pipe of roughly 16–56 in OD.
- JCOE: the plate is progressively bent into a J, then a C, then an O by step-pressing; welding and expansion follow as for UOE. JCOE is more flexible for smaller order quantities and heavier walls.
- RBE (roll bending): three- or four-roll bending is used for structural and lower-volume pipe.
Each seam is welded from both sides to guarantee full penetration. After expansion the seam is 100% ultrasonically tested, the pipe ends and any UT indications are radiographed (RT or digital radiography), and each pipe is hydrostatically tested.
6.3 HSAW (Spiral) Pipe Manufacturing Process
HSAW (SAWH) or spiral-welded pipe is made from hot-rolled coil fed into a forming head at an angle, so the strip wraps into a continuous helix. The helical seam is welded by SAW inside and outside. Because the forming angle sets the diameter, one coil width can produce many pipe sizes:
where B is the coil (strip) width, D the pipe outside diameter and α the forming angle between the strip feed direction and the pipe axis. Example: a 1,500 mm coil making 40 in (1,016 mm) pipe needs α = arccos(1,500 / (π × 1,016)) ≈ 62.0°; the same coil makes 32 in (813 mm) pipe at about 54.0° and 64 in (1,626 mm) pipe at about 72.9°.
Modern “two-step” HSAW mills form and tack-weld the pipe on line, then complete the SAW welds off line at controlled speed, which significantly improves seam quality. HSAW is the most economical route for very large diameters and is widely used for water transmission, penstocks, piling and structural pipe, and – where the owner’s specification allows – onshore oil and gas transmission up to high strength grades. Offshore, high-strain (strain-based design), sour and some critical gas services often still specify seamless or LSAW because the longer seam increases the weld length to be inspected and coil-origin material can have different through-thickness properties.
7. Seamless vs Welded Pipe – Engineering Comparison
Seamless and welded pipe are made to the same dimensional standards (ASME B36.10M / B36.19M), but they differ in integrity, tolerances, size range, cost and code treatment.
| Property | Seamless (SMLS) | ERW / HFW | LSAW (SAWL) | HSAW (SAWH) |
|---|---|---|---|---|
| Starting material | Solid round billet, bloom or ingot | Hot-rolled coil | Heavy plate | Hot-rolled coil |
| Weld seam | None | Straight, no filler | Straight, SAW ID + OD | Helical, SAW ID + OD |
| Typical OD range | ~⅛–26 in (hot-rolled); larger by hot expansion / forging | ~½–24 in | ~16–60 in | ~20–120 in |
| Wall-thickness tolerance | Wider; eccentricity possible (A106: −12.5%) | Tight (from coil) | Tight (from plate) | Tight (from coil) |
| ASME B31.3 Ej | 1.00 | 0.85 | 0.95 (API 5L SAW) | 0.95 (API 5L SAW) |
| Relative cost | Highest | Lowest | Medium–high | Low (large sizes) |
| Typical service | High pressure / temperature, sour, cyclic, boiler, hydraulics, fittings stock | Utilities, low–medium pressure process, gathering and transmission lines | High-pressure oil and gas transmission, offshore, risers | Water mains, piling, penstocks, onshore transmission where permitted |
| Main inspection focus | Full-body UT / EMI for laps, seams, laminations, wall | Seam UT, flattening, seam heat treatment | Seam UT + RT of ends, expansion check | Seam UT + RT, longer seam per metre |
8. Weld Joint Quality Factors and a Worked Example
The design codes account for the seam by multiplying the allowable stress by a longitudinal weld joint quality factor. The values below are taken from the code tables most often used in plant and pipeline design; always confirm against the edition named in your project specification.
| Pipe type (example spec) | ASME B31.3 Ej (Table A-1B) | ASME B31.8 E (gas pipelines) | Remarks |
|---|---|---|---|
| Seamless (A106, A53 Type S, A312, API 5L SMLS) | 1.00 | 1.00 | No seam; full allowable stress |
| ERW / HFW (A53 Type E, A135, API 5L HFW) | 0.85 | 1.00 (API 5L) | B31.3 does not raise Ej for ERW by extra radiography |
| EFW, double-butt seam (e.g. A358, A312 welded) | 0.85 | – | 1.00 with 100% radiography of the seam |
| EFW, single-butt seam | 0.80 | – | 1.00 with 100% radiography of the seam |
| SAW straight or spiral (API 5L SAWL / SAWH) | 0.95 | 1.00 | Full-length seam UT required by API 5L |
| Furnace butt-welded (A53 Type F) | 0.60 | 0.60 | Continuous-weld pipe; utility service only |
Worked example – why the seam changes the schedule. Process line, NPS 8 (D = 219.1 mm), design pressure P = 9.0 MPa, design temperature ≤ 200 °C, carbon steel with S = 138 MPa, Y = 0.4, W = 1.0, no corrosion allowance, mill under-tolerance 12.5%. ASME B31.3 Eq. (3a):
| Case | Ej | Pressure design t (mm) | Min. nominal tm (mm) | Selected schedule (B36.10M) |
|---|---|---|---|---|
| Seamless (A106 Gr. B) | 1.00 | 6.96 | 7.96 | Sch 40 (8.18 mm) |
| ERW (A53 Type E Gr. B) | 0.85 | 8.16 | 9.32 | Sch 60 (10.31 mm) |
9. Mechanical Properties of Common Pipe Grades
| Specification & grade | Product form | Min. yield MPa (ksi) | Min. tensile MPa (ksi) | Typical use |
|---|---|---|---|---|
| Carbon steel – process and utility | ||||
| ASTM A106 Gr. B | Seamless | 240 (35) | 415 (60) | High-temperature process, steam, hydrocarbons |
| ASTM A106 Gr. C | Seamless | 275 (40) | 485 (70) | Higher-stress high-temperature lines |
| ASTM A53 Gr. B | Seamless (S) or ERW (E) | 240 (35) | 415 (60) | Utilities, water, air, structural |
| ASTM A333 Gr. 6 | Seamless or welded | 240 (35) | 415 (60) | Low-temperature service, impact tested to −45 °C |
| Line pipe – API 5L (47th ed.) / ISO 3183 | ||||
| API 5L Gr. B (L245) | SMLS / HFW / SAW | 245 (35.5) | 415 (60.2) | Gathering and low-pressure pipelines |
| API 5L X52 (L360) | SMLS / HFW / SAW | 360 (52.2) | 460 (66.7) | Oil and gas transmission |
| API 5L X65 (L450) | SMLS / HFW / SAW | 450 (65.3) | 535 (77.6) | High-pressure and offshore pipelines |
| API 5L X70 (L485) | HFW / SAW (SMLS less common) | 485 (70.3) | 570 (82.7) | Large-diameter gas transmission |
| Stainless steel – ASTM A312 | ||||
| A312 TP304 / 304L | Seamless, welded (EFW) | 205 / 170 (30 / 25) | 515 / 485 (75 / 70) | General corrosion service |
| A312 TP316 / 316L | Seamless, welded (EFW) | 205 / 170 (30 / 25) | 515 / 485 (75 / 70) | Chloride-bearing and chemical service |
10. Testing and Inspection by Pipe Type
| Test / inspection | Purpose | Seamless | ERW / HFW | SAW (LSAW / HSAW) |
|---|---|---|---|---|
| Chemical (heat + product) analysis | Composition, carbon equivalent, weldability | Required | Required | Required |
| Tensile test | Yield, tensile, elongation | Body | Body + cross-weld (as spec) | Body + cross-weld |
| Flattening / bend test | Ductility, seam soundness | Flattening (A106, A53) | Flattening at weld 0°/90° | Guided bend of weld |
| Charpy V-notch impact | Toughness (PSL2, low-temperature, sour) | Body | Body + weld + HAZ | Body + weld + HAZ |
| Hydrostatic test | Proof of pressure integrity | Each length (or NDE alternative where the spec allows) | Each length | Each length |
| Ultrasonic testing (UT) | Laminations, laps, seam flaws, wall | Full body (when specified / PSL2) | Full-length weld seam | Full-length weld seam + plate/coil edges |
| Eddy current / flux leakage | Surface and near-surface flaws | Common alternative to hydro | Common on small sizes | – |
| Radiography (RT / DR) | Weld volumetric flaws | – | – | Pipe ends and UT indications |
| Hardness, HIC / SSC tests | Sour-service qualification (NACE MR0175 / ISO 15156) | When specified | When specified | When specified |
| Dimensional & visual | OD, wall, ovality, straightness, bevel, weight | Each length | Each length | Each length |
11. Typical Pipe Manufacturing Defects
| Defect | Pipe type | Typical cause | Detection |
|---|---|---|---|
| Wall eccentricity | Seamless | Uneven billet heating, plug or mandrel misalignment | UT wall mapping, end measurement |
| Internal laps / seams | Seamless | Centre porosity or inclusions in billet opened during piercing | Full-body UT, visual of bore |
| Rolled-in scale, scabs | Seamless | Poor descaling, surface defects in billet | Visual, MT, eddy current |
| Laminations | All | Non-metallic inclusions or segregation in billet, plate or coil | UT (body and pipe ends) |
| Cold weld / lack of fusion | ERW / HFW | Low heat input, wrong V-angle or squeeze pressure | Seam UT, flattening test |
| Hook cracks, penetrators | ERW / HFW | Upturned inclusions at edges, oxides trapped in bond line | Seam UT, flattening, metallography |
| Selective seam corrosion | Legacy LFW / unnormalized ERW | Untempered bond-line microstructure | Prevented by HFW + seam normalizing |
| Porosity, slag inclusions | SAW | Moist flux, contamination, wrong parameters | UT, RT |
| Lack of penetration, offset | SAW | ID/OD weld misalignment (tracking error) | UT, RT, macro-section |
| Peaking / ovality | LSAW / HSAW | Forming error at seam, insufficient expansion | Profile gauge, dimensional check |
12. Applicable Standards and Specifications
| Standard (current edition) | Scope | Product forms |
|---|---|---|
| Material / product specifications | ||
| ASTM A106/A106M-26 | Carbon steel pipe for high-temperature service, Gr. A/B/C | Seamless only |
| ASTM A53/A53M-24 | Black and galvanized carbon steel pipe | Type S, E, F |
| ASTM A333/A333M | Pipe for low-temperature service | Seamless and welded |
| ASTM A335/A335M | Ferritic alloy pipe for high temperature (P11, P22, P91, P92) | Seamless only |
| ASTM A312/A312M-25 | Austenitic stainless steel pipe | Seamless, welded, heavily cold-worked |
| ASTM A358/A358M | EFW austenitic stainless pipe from plate | Welded with filler |
| ASTM A790/A790M | Duplex (ferritic/austenitic) stainless pipe | Seamless and welded |
| API Spec 5L, 47th ed. (2026) / ISO 3183 | Line pipe, PSL1 and PSL2, incl. sour, offshore and CO2 annexes | SMLS, HFW, LFW, LW, SAWL, SAWH, COWL, COWH |
| API Spec 5CT / ISO 11960 | Casing and tubing (OCTG) | Seamless and ERW |
| EN 10216 series | Seamless steel tubes for pressure purposes | Seamless |
| EN 10217 series | Welded steel tubes for pressure purposes | ERW/HFW, SAW |
| General requirements, dimensions and design codes | ||
| ASTM A530/A530M & A999/A999M | General requirements for carbon/alloy and stainless pipe | – |
| ASME B36.10M-2022 / B36.19M | Welded and seamless wrought steel / stainless pipe dimensions | – |
| ASME B31.3-2024 / B31.1 / B31.4 / B31.8 | Process, power, liquid and gas pipeline design codes | – |
| NACE MR0175 / ISO 15156 | Materials for H2S (sour) service | – |
| EN 10204 | Types of inspection documents (2.2, 3.1, 3.2) | – |
13. How to Specify Seamless or Welded Pipe
- Define the duty: fluid, design pressure and temperature, corrosion, sour (H2S) or cyclic service, and the governing code (B31.1, B31.3, B31.4, B31.8).
- Choose the product form: seamless for severe duty and small-to-medium bore; HFW, LSAW or HSAW where the code factor, owner specification and economics allow.
- Name the specification, grade and PSL – e.g. “ASTM A106 Gr. B, seamless” or “API 5L X65 PSL2 SAWL, Annex H (sour)”.
- State dimensions to ASME B36.10M/B36.19M: NPS, schedule or wall, length (SRL/DRL), and end finish (plain, bevelled to ASME B16.25).
- Add supplementary requirements: heat treatment condition, CVN impact temperature, hardness limits, extra NDE, hydrotest pressure and hold time.
- Specify documentation: EN 10204 3.1 or 3.2 certificates, heat traceability, marking and PMI for alloys.
- Plan receipt inspection: verify stencil (e.g. SMLS / HFW / SAWL), heat number, dimensions and visual condition before release to fabrication.
Common mistakes to avoid
- Ordering “A53 Gr. B” without the Type – the mill may supply ERW (Type E) when seamless (Type S) was intended.
- Using Ej = 1.00 in wall-thickness calculations for ERW or SAW pipe under ASME B31.3.
- Assuming every seamless pipe is hydrotested – some specifications permit NDE in lieu of hydrotest unless the purchaser requires both.
- Forgetting the 12.5% mill under-tolerance and hot-expansion wall loss when selecting a schedule.
- Treating legacy low-frequency ERW pipe as equivalent to modern HFW when assessing older pipelines.
Check wall thickness, schedules and test pressures with Aienginear’s free engineering calculators.
14. Frequently Asked Questions
Which is stronger, seamless or welded pipe?
For the same grade and wall, seamless pipe has uniform properties around the circumference and no weld or HAZ, so codes credit it with the full allowable stress (ASME B31.3 Ej = 1.00). Modern HFW and SAW pipe can match the base-metal strength, but the seam is still derated (0.85 for ERW, 0.95 for API 5L SAW under B31.3) because it is the most likely location for a flaw.
What is the maximum size of seamless pipe?
Hot-rolled seamless pipe from mandrel, plug or pilger mills is commonly available up to about NPS 24–26 (DN 600–650). Larger seamless sizes, up to roughly NPS 48 and beyond, are produced by hot expansion or press forging, at higher cost and longer lead time.
What is a mother hollow in seamless pipe production?
The mother hollow (hollow shell) is the thick-walled tube produced by the rotary piercer. It is then elongated over a mandrel or plug and sized to become the finished pipe.
What is the difference between a mandrel mill and a plug mill?
A mandrel mill (MPM or PQF) reduces the wall in one continuous pass through several stands over a long mandrel bar, giving high output and tight tolerances. A plug mill reduces the wall over a short stationary plug in two or more passes, with the shell rotated 90° between passes, and needs a reeling mill to smooth the surfaces.
What is the Mannesmann effect?
It is the opening of a cavity at the centre of a round billet when it is rotated and compressed between skewed rolls. The cyclic compression creates secondary tensile stresses at the axis, which the piercing plug then enlarges and smooths into a bore.
Can spiral-welded (HSAW) pipe be used for oil and gas?
Yes. API 5L includes SAWH pipe up to high strength grades, and modern two-step HSAW pipe is used for onshore gas and oil transmission where the owner specification allows. Many offshore, sour and strain-based-design projects still specify seamless or LSAW.
How can I tell if a pipe is seamless or welded?
Check the stencil and mill test certificate for the product type (SMLS, HFW, SAWL, SAWH or A53 Type S/E/F). Physically, welded pipe usually shows a straight or helical weld line or a scarfed seam on the bore; seamless pipe has no seam but may show slight wall eccentricity.
Why is seamless pipe more expensive than welded pipe?
Seamless production needs heavy piercing and rolling equipment, more energy (several reheats), more yield loss and longer cycle times. Welded pipe is formed from coil or plate at high line speeds, which lowers the cost per tonne, especially in large diameters.
15. Test Your Knowledge – Pipe Manufacturing Quiz
1. What creates the central cavity during rotary piercing?
2. Which mandrel mill uses three rolls per stand around a retained mandrel?
3. What is the ASME B31.3 weld joint quality factor Ej for seamless pipe?
4. What is the B31.3 Ej for ERW pipe such as A53 Type E?
5. Which LSAW route uses a U-press, an O-press and mechanical expansion?
6. Which pipe type lets one coil width produce many different diameters?
7. ASTM A106 cold-drawn pipe must be heat treated after the final cold draw at a minimum of:
8. Which welded pipe type is not permitted for API 5L PSL2?
9. What is the “mother hollow”?
10. Which mill under-tolerance is normally applied to A106 wall thickness in design?
Conclusion
Seamless pipe is pierced and rolled from a solid billet, so it carries no weld and earns the full allowable stress in design. Mandrel (MPM/PQF) mills dominate modern production, plug and pilger mills cover larger and heavier sizes, and forging, hot expansion and extrusion handle the extremes and special alloys. Welded routes – HFW, EFW, LSAW and HSAW – deliver economical large-diameter pipe with tight wall tolerance, provided the seam is properly made, heat treated and inspected. Match the process to the duty, name the specification, grade and product form, and apply the correct weld joint factor, and you will specify the right pipe the first time.
Written by Muhammad Raza
Founder of Aienginear. Engineering and digital solutions professional with 26+ years managing projects, plant shutdowns and turnarounds in oil & gas, petrochemical and power generation.
