An ERW tube mill forms steel strip into a round section and welds the seam with high-frequency current — no filler metal, no consumable weld, just the strip welded to itself. It is the process behind most of the steel tube used in construction, furniture, automotive and general industry.
The Main Groups
- Entry end: uncoiler, shear and butt welder, strip accumulator.
- Forming and welding: breakdown and fin passes, HF welder, impeder.
- Sizing: shaping and sizing stands that set the final diameter.
- Cut-off and packing: flying cold saw, run-out table, stacker, strapping.
We build ERW lines from HG16 (ø8 mm) to HG355 (ø355 mm) and supply the roll tooling and cut-off as our own products. Send your tube specification and we will come back with a configuration and a price.
The Weld Is Made in About Twenty Millimetres
In an ERW mill, the weld happens in a zone roughly 20 mm long. The strip edges are brought together into a V, heated by high-frequency current to a narrow band of plastic steel, and squeezed closed by the weld rolls. Every weld defect that will ever appear on your tube is decided in that 20 mm — and everything upstream exists to keep that 20 mm stable.
Contact or Induction: Two Ways to Put Current Into the Strip
| Contact welding | Induction welding | |
|---|---|---|
| How current enters | Sliding contacts touching the strip near the V | Induction coil around the tube, no contact |
| Wear parts | Contact tips and the strip surface under them | Coil and impeder only |
| Surface marking | Possible contact marks and arcing | None |
| Efficiency | Higher, direct conduction | Slightly lower, but consistent |
| Best for | Small and medium tube, thin wall, high speed | Larger tube, galvanized and coated strip, sensitive surfaces |
The choice is usually made by the product, not by preference. Galvanized and pre-painted strip is almost always induction welded, because a sliding contact would strip the coating and start an arc.
What the Impeder Is Actually Doing
The impeder sits inside the tube, just upstream of the weld point. Its ferrite core concentrates the magnetic flux and raises the impedance of the current path so the current is forced to travel along the strip edges instead of taking a shortcut around the tube body. Without it, most of the power goes into heating the whole circumference and the weld never forms properly.
A cracked core, a wet impeder, or a housing that has lost its cooling shows up as a wandering weld, a sudden rise in power draw, or a weld that looks fine and fails the flattening test. It is a cheap part that causes expensive scrap, which is why we keep them on the shelf rather than making them to order.
Setting the V-Angle and the Squeeze
The V-angle is normally set between 3° and 7°. Too open and the current path is long, the heat spreads, and the weld becomes wide and weak. Too closed and the edges arc at the apex before they are plastic, which shows as spatter, a burned edge and a notch in the finished seam.
Weld roll pressure has one job: hold the two edges against each other while they are plastic, without thinning the wall. Too little pressure gives an under-filled weld with a groove inside; too much gives an internal and external overlap, a heavy bead and a thinning at the weld line that fails a wall measurement.
Reading a Weld Without Cutting It Up
- Bead shape — a smooth, even bead with a slight crown. A lumpy or intermittent bead means unstable power or edge alignment.
- Heat-affected zone width — a narrow, even band either side of the seam. A wide HAZ means too much power or too slow a line.
- Scarfing chip — a continuous chip with a bright finish. A blue or torn chip means the tool is dull or set too deep.
- Flattening test — press a sample flat; the weld should not open before the wall tears elsewhere.
- Macro etch — the only way to see whether the edges fused through the full wall or only at the surface.
Where the Speed Window Closes
| Wall thickness | Typical line speed | What limits it |
|---|---|---|
| 0.6 to 1.0 mm | 80 to 130 m/min | Edge alignment and weld power at the top end |
| 1.0 to 2.0 mm | 50 to 90 m/min | Weld power, cooling, cut-off cycle time |
| 2.0 to 3.5 mm | 30 to 60 m/min | Weld power and the saw’s travel and clamp time |
| 3.5 to 6.0 mm | 18 to 40 m/min | Heating time in the V, drive torque, blade life |
| over 6.0 mm | 12 to 25 m/min | Weld power density and cooling capacity |
These are starting points, not limits. Real speed is set by the seam quality the customer accepts, and the fastest way to find it is a weld window trial: run three speeds at fixed power, cut macros, and pick the middle of the band that passes.
Defects and What Actually Causes Them
| Defect | Usual cause |
|---|---|
| Cold weld (no fusion) | Power too low, V too open, speed too high, dirty strip edges |
| Stitch weld (intermittent fusion) | Unstable contact, worn impeder, fluctuating power supply |
| Burn-through | Power too high, V too closed, edge mismatch from a bad slitter |
| Hook crack | Excess squeeze, edges not parallel at the apex |
| Penetrator | A piece of ferrite or scale trapped in the seam |
| Grey band / oxide inclusion | Poor shielding at the apex, wet or dirty strip, coolant ingress |
Notice how few of those are fixed by turning a knob. Cold weld and penetrator are strip and tooling problems, hook crack is a setup problem, and grey band is usually a water or cleanliness problem. This is why weld troubleshooting starts at the uncoiler, not at the welder.
What the Operator Should Adjust, and What Should Be Locked
Power, speed and the V-angle are working settings and belong to the operator, within a recorded window. Weld roll pressure, impeder position and coil alignment belong to the setup sheet and should only change with a reason written down. Mills that drift are usually mills where somebody adjusted a locked setting at three in the morning to get through a shift.
From the Knowledge Centre
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