Short answer: how is ERW pipe made?
ERW pipe is made on one continuous line. Steel strip is unwound from a coil and passed through a series of rollers that bend it into an open round tube. High-frequency (HF) current heats the two strip edges, and squeeze rollers press them together into a weld. The outside weld bead is trimmed off, and the tube is cooled, sized, straightened and cut to length while it keeps moving. The seam is a straight line along the tube, and no filler metal is used.
A complete ERW tube mill line has five sections: (1) the entry section, for uncoiling, leveling, coil joining and strip accumulation; (2) forming and welding; (3) cooling, sizing and straightening; (4) cut-off and run-out; (5) optional finishing. Electrical controls and plant utilities support the whole line. The sections below follow the strip from coil to finished tube; Section 2 is explained in two parts, forming and welding.
- Uncoiling: the uncoiler holds the coil and pays off strip, with a brake keeping back tension.
- Leveling: flattens the strip head during threading and guides it into the next machine.
- Coil joining: the shear & butt welder cuts the tail of one coil and the head of the next and welds them into one strip.
- Accumulation: the spiral accumulator keeps feeding the mill during the coil change, so the mill does not stop.
- Forming: a series of roller stands bends the flat strip step by step into an open round tube.
- HF welding: the heated edges are forged together by squeeze rollers, and the outside bead is trimmed in line.
- Cooling, sizing and straightening: the weld is cooled, the tube is rounded to size and straightened; square and rectangular tubes are shaped from round in this section.
- Cut-off and run-out: a flying saw or cold saw travels with the tube and cuts it to length, and the run-out table delivers the finished tube.
Raw material: steel coil and strip width
ERW lines commonly run on hot-rolled, cold-rolled or galvanized (GI) steel strip. Stainless steel, zinc-aluminum-magnesium coated strip and high-strength steel are reviewed grade by grade. Higher strength or thickness usually means a stronger leveler, larger cylinders and clamps on the shear & butt welder, and, for high-strength strip, a larger accumulator.
The strip width you need is set by the outside diameter (OD) and wall thickness of the finished tube. For a rough idea, convert from the outside perimeter: about OD × 3.14 for round tube (Φ76 gives about 239 mm) and the sum of the four sides for square and rectangular tube. This is a theoretical value only; the actual strip width is set by the roller design and is usually slightly less. See our guide to tube mill calculations. Steel mills usually supply wide coils, which are cut into narrow strips on a slitting line and recoiled before they go to the tube mill. When you plan a new plant, decide early whether you will buy slit strip or slit your own.
Section 1: entry section (uncoiler, leveler, shear & butt welder, spiral accumulator)
The entry section feeds the mill with continuous, flat strip. The uncoiler holds the coil on an expanding mandrel and pays off strip, with a brake keeping back tension. A double-head uncoiler has two mandrels back to back: one pays off while the other is loaded, then the head turns 180°. It suits narrow-strip tube lines. A single-head uncoiler works with a hydraulic coil car and suits wider, heavier coils.
During threading, the leveler bends the strip head up and down between staggered rolls to flatten it. The tube mill itself has a set of non-driven flattening rolls at its entry, so small mills running ordinary strip often do without a separate leveler. Our engineers decide this from the material strength, thickness and strip shape.
The shear & butt welder cuts the tail of the old coil and the head of the new coil square, lines them up, clamps them and welds them into one continuous strip. The joint must be flat: a raised weld bead can damage the rollers downstream. InGen MillTek offers three grades: Integrated Shear & Manual Welder, Semi-automatic Shear & Welder and Fully Automatic Shear & Welder.
The spiral accumulator sits between the shear & butt welder and the mill. Before a coil change, the feeder speeds up and fills the accumulator. While the uncoiler is stopped and the strip ends are joined, the mill keeps drawing strip from the center of the accumulator, so it does not have to stop. After the joint is made, the feeder runs faster than the mill to refill the accumulator. Inside, the strip stands on edge in a spiral: a flat support stand and adjustable twist stands turn it from flat to vertical, and a high turnover stand turns it flat again before the mill. Horizontal and floor type designs are available, and storage is sized in meters of strip, not in tons.
Uncoilers, leveling machines, shear & butt welders and spiral accumulators are built in-house at InGen MillTek. They can be supplied as a complete entry section or as single machines to replace equipment on an existing line.
Section 2a: forming the strip into a round shape
The strip is formed cold, step by step: it passes through driven horizontal roller stands and vertical (side) roller stands that bend it a little more at each stand.
Forming has two parts. In the breakdown (open) passes, the upper rollers are convex and the lower rollers concave, and they start bending the strip. A cluster of non-driven side rollers then guides it. In the fin (closed) passes, the upper rollers carry a fin that controls the shape and position of the two edges and keeps the seam in place.
A standard layout is called “7+6”: seven horizontal stands for forming, six horizontal stands for sizing, plus two Turk's heads. Variants such as 8+6 are used for small thin-wall tube and square tube.
Section 2b: high-frequency welding and bead removal
HF welding uses two electrical effects. The skin effect makes high-frequency current flow near the surface of a conductor, and the proximity effect pulls opposite currents in nearby conductors toward each other. As a result, the current runs along the two strip edges to the point where they meet (the vee apex), and the edges heat up quickly to welding temperature. The squeeze rollers then force the edges together. Molten metal and oxides are squeezed out as inside and outside beads, and the edges bond in the solid state.
In the direction of strip travel, the welding area has: seam guide, squeeze roller stand, outside bead scarfer and ironing rollers. The HF welder (solid-state power source, induction coil, impeder and water cooling) is sized for the tube OD, wall thickness and line speed, and can be configured as part of a complete line.
The outside bead is cut off in line by an outside scarfer, which usually has two tool holders that can be used in turn. The inside bead is normally left in place; an inside scarfer can be added if your product needs it.
Section 3: cooling, sizing, straightening and round-to-square forming
After welding, the tube is cooled by sprays and a water trough, and there are also coolant nozzles between the roller stands in forming and sizing. Mill cooling water and HF cooling water come from a pool or cooling tower and pumps in the customer's plant. These utilities should be planned early when you build a new factory.
The sizing section reduces the welded tube's OD by a small, fairly constant amount and makes it round, so the final size is stable. After sizing there are usually two Turk's heads (four-roller heads) that can be adjusted up and down, sideways and in rotation. They straighten the tube and shape square and rectangular sections. A measuring wheel with an encoder, pressed onto the tube by an air cylinder, sends the length signal to the flying saw.
Square and rectangular tubes are usually made by round-to-square forming. The tube is first welded round, then shaped square or rectangular stand by stand in the sizing section, keeping the centerline fixed and raising the lower rollers a little at each stand. The round tube used for this is called the “mother tube.”
A square or rectangular tube can share the forming rollers with its mother tube (the round tube with the same perimeter) and needs only one extra set of sizing rollers (a “half set”). So if you already make the mother tube, adding the matching square or rectangular size often needs only that set of sizing rollers.
Section 4: cut-off and run-out
The tube is still moving when it is cut, so the saw must travel with it. There are two common options. A hot friction flying saw tracks the tube and uses a blade spinning at high speed to melt through the wall by friction; it suits general cut-length requirements. A servo cold saw cuts with a slow-turning blade, giving a flat cut with little burr and higher accuracy, but the speed it can reach depends on cut length, tube size and wall thickness. The two options differ in accuracy, cut quality and investment, so compare them as separate items in a quotation.
After cutting, the run-out table carries the tube away. Types include two-side tipping, push-off and kick-off, and the rollers are usually nylon or polyurethane-covered to protect the tube surface. The run-out table length is set on the project layout drawing.
Section 5 (optional): finishing and testing
Depending on the product and the standard it must meet, finished tubes may need end facing and chamfering, hydrostatic testing, eddy current testing, stacking and bundling. For GI pipe made from galvanized strip, a zinc repair unit can be added to recoat the weld seam; it covers the seam only and does not hot-dip galvanize the whole pipe. Common quality tests include flattening, flaring, hydrostatic and eddy current tests.
These finishing machines are selected per project. When you ask for a quotation, tell us the product standard and test requirements, so the roller design and finishing equipment can be matched to them.
What InGen MillTek supplies on this line
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: forming & sizing units and flying saws for six standard models (YJ32, YJ50, YJ76, YJ89, YJ114 and YJ165), with HF welders, cold saws and automatic stacking configured to suit the project. Larger-diameter mills, YJ219, YJ254 and YJ273, are designed and built to order: our engineers design each one for the tube sizes in that 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. Prices are quoted for each configuration.
Frequently asked questions
- Q: Is ERW pipe the same as seamless pipe? A: No. ERW pipe is formed from strip and welded, so it has a straight weld seam. Seamless pipe is made without a weld by a different process, which InGen MillTek does not supply.
- Q: Is a spiral accumulator the same as a spiral welded pipe mill? A: No. A spiral accumulator stores strip on a straight-seam (ERW) tube line so the mill keeps running during coil changes. A spiral welded pipe mill is a different machine that makes pipe with a helical seam.
- Q: Does ERW welding use filler wire? A: No. The edges are heated by high-frequency current and pressed together by squeeze rollers; no filler metal is added.
- Q: How are square and rectangular tubes made on an ERW line? A: Usually by round-to-square forming: the tube is welded round (the mother tube), then shaped square or rectangular stand by stand in the sizing section. A square or rectangular tube can share the forming rollers with its mother tube, so only one extra set of sizing rollers is needed.
- Q: Does the line stop for a coil change? A: Not with a spiral accumulator. Stored strip keeps feeding the mill while the new coil is loaded and joined, and the feeder refills the accumulator afterwards.
- Q: Can one tube mill make every size I need? A: Each model covers a set OD and wall thickness range. Choose the smallest model that covers your maximum OD and maximum wall thickness; if your size range is wider than one model, plan two lines. See our guide “10 Steps to Specify an ERW Tube Mill.”
About this article
Prepared by the InGen Engineering Team. This is a general process guide, not a specification or performance guarantee for any project. Process values are common reference values; final parameters are set in the equipment proposal, roller design and technical agreement.