The short answer
ERW stands for electric resistance welded. ERW pipe is made from flat steel strip: the strip is roll-formed cold into a round tube, electric current heats its two edges, and squeeze rollers press them together into a weld. No filler metal is added, and the weld runs in a straight line along the pipe. Today most ERW pipe is welded with high-frequency current, so it is also called HFW pipe.
Seamless pipe is made a different way, from a solid round billet that is pierced and rolled into a hollow, so it has no weld at all. ERW pipe is used for structures, machinery, furniture and fluid lines, and as line pipe and casing made to the relevant standards. ERW mills make round tube from small sizes up to large pipe, and square and rectangular tube from the same lines.
What "electric resistance welded" means
A metal resists the flow of electric current, and that resistance turns into heat. In ERW, current is concentrated in the two edges of the formed strip where they come together. The edges heat to welding temperature in a fraction of a second, and squeeze rollers then forge them together. Oxides and molten metal are pressed out as a bead on the inside and outside, and the edges bond in the solid state.
Early ERW mills used low-frequency or direct current fed through rotating electrodes. Modern mills use high-frequency (HF) current, fed through an induction coil or through sliding contacts, which concentrates the heat in the edges far more effectively. Why HF current behaves this way, through the skin and proximity effects, is explained in our guide ERW Pipe Making Process: From Steel Coil to Finished Tube.
How ERW pipe is made, in one paragraph
A steel coil, slit to the right strip width, is unwound and joined end to end with the next coil. Roller stands bend the strip step by step into an open round tube. The edges are heated by HF current and forged together by squeeze rollers, and the outside weld bead is trimmed off in line. The tube is then cooled, sized to its final diameter, straightened, and cut to length by a flying saw while it keeps moving. Square and rectangular tubes are usually welded round first and then shaped in the sizing section. Each stage is described in ERW Pipe Making Process: From Steel Coil to Finished Tube.
ERW vs seamless pipe
- Starting material: ERW pipe starts from flat steel strip in coils. Seamless pipe starts from a solid round billet.
- How it is formed: ERW strip is roll-formed cold and its edges are welded, without filler metal. A seamless billet is heated, pierced and rolled into a hollow, or extruded, and then sized.
- Weld: ERW pipe has one straight weld line along its length; the outside bead is trimmed, and the inside bead is often left unless the product needs it removed. Seamless pipe has no weld.
- Wall thickness: ERW wall thickness comes from the rolled strip, so it is usually very even around the pipe. Seamless wall thickness can vary more around the circumference.
- Production: ERW runs continuously from coil at high speed. Seamless production has more hot-working steps.
- Typical choice: ERW covers most structural, mechanical and low- to medium-pressure fluid uses. Seamless is usually chosen for high-pressure, high-temperature or other critical service, very heavy walls, or where a code or customer specifies seamless.
The pipe standard, steel grade and tests decide whether a pipe is fit for a use, not the process name alone. InGen MillTek builds ERW tube mill lines; we do not supply seamless pipe equipment.
ERW and other welded pipe
- ERW / HFW: welded by electric current without filler metal; a straight seam; made from strip on a continuous tube mill.
- Submerged-arc welded (SAW) pipe: welded with filler wire under a layer of flux. Longitudinal SAW (LSAW) pipe is formed from plate and has a straight seam; spiral SAW (SSAW) pipe is formed from coil and has a helical seam. SAW processes are used mainly for larger diameters and heavier walls than ERW.
- Furnace butt-welded pipe: strip heated in a furnace and pressed together at the edges; used for small, low-pressure pipe.
Do not confuse a spiral welded pipe mill with the spiral accumulator on an ERW line: the accumulator only stores strip, so the mill keeps running during coil changes.
Pipe or tube?
The two words are often used loosely. In general, "pipe" carries fluids and is ordered by nominal size and wall schedule, such as NPS 2 Schedule 40. "Tube" is ordered by its actual outside diameter and wall thickness, and is used for structural and mechanical parts; square and rectangular hollow sections are tubes. ERW lines make both: the same mill can produce round pipe, round tube and, with sizing rollers for the purpose, square and rectangular tube.
Where ERW pipe and tube are used
- Structures: building frames, columns and trusses, scaffolding, greenhouse and solar mounting structures, fencing and guardrails, mostly as round, square and rectangular hollow sections.
- Mechanical and furniture: furniture frames, racks and shelving, machine parts, bicycle and fitness equipment, and automotive tubes.
- Fluids: water, gas, air and oil lines at low and medium pressure, and fire sprinkler pipe. Galvanized (GI) pipe can be made from pre-galvanized strip, with the weld seam re-coated in line.
- Energy: ERW line pipe and casing are made to standards such as API 5L and API 5CT on mills configured and qualified for them.
Common standards for ERW pipe and tube
ERW products are made to the standard the buyer specifies. Commonly used ones include:
- ASTM A53: welded and seamless steel pipe; Type E is electric resistance welded.
- ASTM A500: cold-formed welded and seamless carbon steel structural tubing, round and shaped.
- API 5L: line pipe, seamless and welded, including HFW.
- EN 10219: cold-formed welded structural hollow sections.
- EN 10217: welded steel tubes for pressure purposes.
- GB/T 3091: welded steel pipe for low-pressure fluid service.
- GB/T 13793: longitudinally electric resistance welded steel pipe.
The standard sets dimensions, tolerances and tests such as flattening, flaring, hydrostatic and eddy current testing. Tell us which standard your product must meet, so that the roller design and finishing equipment can be matched to it.
ERW pipe sizes
Industry-wide, ERW mills make everything from small tube for furniture and automotive parts to large-diameter line pipe; the largest diameters and heaviest walls are made by the SAW processes instead.
Each tube mill covers a set range of outside diameter (OD) and wall thickness. InGen MillTek's 12 mill models cover round tube from Φ10 to Φ273 mm OD with walls from 0.4 to 10 mm, which by outside diameter spans pipe sizes from NPS 1/2 (21.3 mm) up to about NPS 10 (273.1 mm). Larger sizes, such as 250 × 250 mm and 300 × 300 mm square tube, and other custom models are available on request. Line speed falls as size rises, from up to 120 m/min on the YJ-32 to up to 30 m/min on the YJ-273. The YJ-219, YJ-254 and YJ-273 are large-diameter models designed and built to order.
OD alone does not decide the mill. For pipe ordered by schedule, the wall thickness is often the limit: NPS 2 Schedule 40, for example, is 60.3 × 3.91 mm, so the mill must cover both that OD and that wall in your material. Choose the smallest model that covers your largest OD and heaviest wall together; see our guide 10 Steps to Specify an ERW Tube Mill: Tube Size to Full Line.
The strip width for each size is close to the tube's outside perimeter, about OD × 3.14 as a theoretical value, for example about 189 mm for 60.3 mm pipe; the roller design sets the actual width. See ERW Tube Mill Calculations: Strip Width and Weight per Meter, and for preparing strip from wide coils, What Is a Slitting Line? How a Coil Slitter Works.
What InGen MillTek supplies
YinJiang (InGen MillTek) has focused on strip processing equipment since 1997. We specialize in the entry section of ERW tube mills: uncoilers, leveling machines, shear & butt welders and spiral accumulators are built in-house, and the spiral accumulator is our flagship product. We also supply complete HF ERW tube mill lines for round, square and rectangular tube, with forming & sizing units and flying saws, and HF welders, cold saws and automatic stacking configured to suit the project. The large-diameter YJ-219, YJ-254 and YJ-273 mills are designed and built to order. Slitting lines to prepare strip for the tube mill are also available.
Every line is configured for your tube sizes, material, coil weight, line speed and local power supply, and prices are quoted for each configuration.
Frequently asked questions
Prepared by the InGen Engineering Team. Values are reference values and are confirmed in the technical proposal and technical agreement.
- What does ERW stand for? Electric resistance welded (or welding): the strip edges are heated by electric current and pressed together, without filler metal.
- Is HFW the same as ERW? HFW is ERW welded with high-frequency current. Most ERW pipe today is HFW.
- Is ERW pipe weaker than seamless pipe? Not as a rule. ERW pipe made and tested to the applicable standard meets that standard's requirements; for high-pressure, high-temperature or other critical service, codes or buyers often specify seamless.
- Does ERW pipe have a seam inside? The outside bead is trimmed in line. The inside bead is normally left in place; an inside scarfer can be added if your product needs it.
- Can ERW pipe be galvanized? Yes. It can be made from pre-galvanized strip with the weld seam re-coated in line, or hot-dip galvanized after it is made, which is a separate process outside the tube mill.
- Can one ERW mill make round and square tube? Yes. Square and rectangular tubes are usually welded round and then shaped in the sizing section; they share forming rollers with the round tube of the same perimeter and need one extra set of sizing rollers.
- What sizes can your tube mills make? Our 12 models cover round tube from Φ10 to Φ273 mm OD with walls from 0.4 to 10 mm; larger sizes and custom models are available on request. The range of each model is on our Tube Mill Lines page.
About this article
Prepared by the InGen Engineering Team. This is a general technical guide, not a specification or performance guarantee for any project. Final parameters are set in the project's technical proposal and technical agreement.