An HF tube mill welds the seam with high-frequency current, either by contact or induction, and the quality of that weld depends on three things working together: the impeder inside the tube, the squeeze rolls holding the edges, and a stable strip feed. We supply and match all three.
Welding Side of the Line
- Solid-state HF welders with matching boards and cables.
- Impeders and ferrite cores sized to the tube and line speed.
- Fin and squeeze rolls designed with the weld V in mind.
- Scarfing tool systems for the inside and outside bead.
HF faults are usually a component problem, not a mill problem — a weak impeder, a burnt board or worn squeeze rolls. Send a photo and the machine model and we will identify the part before quoting a replacement.
Why Frequency, Not Temperature
An HF welder does not heat the tube and then weld it. It pushes current into the strip edges at 200 to 450 kHz, and at that frequency two effects do the work: the skin effect keeps the current in the outer fraction of a millimetre of the steel, and the proximity effect pulls the current to the two edges facing each other across the V. The result is a narrow band of steel at forging temperature, roughly 20 mm long, and a weld made without filler metal.
Change the frequency and you change how deep the current sits. Too low for a thin wall and the heat spreads through the wall and the weld becomes wide and weak; too high for a heavy wall and the current never penetrates far enough to fuse the full thickness.
Solid-State or Tube Oscillator
| Solid-state (IGBT / MOSFET) | Tube oscillator | |
|---|---|---|
| Efficiency | About 85 to 92% | About 55 to 70% |
| Frequency control | Set and held electronically | Tuned mechanically, drifts with wear |
| Footprint | Small, no high-voltage anode supply | Large, needs a high-voltage transformer and rectifier |
| Spares | Boards and modules, replaceable | Valves, increasingly hard to source |
| Best for | All new lines | Old machines kept running |
On a line running two shifts, the difference in efficiency is not a rounding error. A 200 kW tube oscillator that wastes 35% of its input is throwing away roughly 70 kW of electricity every hour it runs, plus the cooling to remove it.
Sizing the Welder: Start With Weld Area per Second
The power a welder needs is set by how much steel passes the weld point per second — thickness times speed — and by the wall it has to fuse through. As a first approximation, and before any supplier’s derating curves are applied, plan on the weld area per second being the dominant term and the wall thickness the second.
| Line | OD × wall | Speed | Typical HF rating |
|---|---|---|---|
| HG32 | 32 × 1.2 mm | 80 m/min | 40 to 60 kW |
| HG50 | 50 × 1.5 mm | 70 m/min | 60 to 90 kW |
| HG76 | 76 × 2.0 mm | 55 m/min | 80 to 120 kW |
| HG114 | 114 × 3.0 mm | 40 m/min | 150 to 220 kW |
| HG165 | 165 × 4.0 mm | 28 m/min | 250 to 350 kW |
| HG219 | 219 × 5.0 mm | 20 m/min | 350 to 500 kW |
| HG355 | 355 × 8.0 mm | 12 m/min | 600 to 900 kW |
A welder bought at the bottom of its range will run the small sizes beautifully and limit the mill the day a heavier order arrives. A welder bought at the top is money sitting idle. The number to ask for is not the nameplate rating but the running power at your worst case, at line speed, with margin for a cold coil.
Impedance Matching Is Where Cheap Installations Fail
A high-frequency welder, a welding transformer, a water-cooled secondary and the strip itself form a matching network. If the secondary is too long, laid in the wrong route, or made of the wrong section, the load no longer matches the oscillator and the power that leaves the cabinet does not arrive at the V. The symptoms look like a welder fault — power instability, board trips, poor penetration — and the cause is a cable run.
Cooling Water and the Ferrite Core
- Water conductivity and flow: a slow flow raises the temperature of the ferrite, and a hot ferrite loses its properties and stops impeding.
- Water quality: scale in the impeder housing insulates the core and quietly reduces weld efficiency.
- Impeder position: a few millimetres too far back and the current spreads; too far forward and it is damaged by the weld heat.
- Spare impeder: the cheapest insurance on the line, and the part most often ordered in a hurry.
Faults That Look Mechanical but Are Electrical
- A weld that wanders side to side — often an unstable impedance match, not a roll alignment problem.
- Power that spikes at coil changes — strip thickness step or a poor strip join, not a board failure.
- Intermittent penetration at constant settings — contact tip wear on a contact welder, or a drifting oscillator frequency.
- Welds that look correct and fail the flattening test — insufficient power density, usually because the line is running faster than the welder was sized for.
What to Ask Before Buying an HF Welder
- What is the running power at my heaviest size, at my line speed, with a cold coil?
- What is the frequency range, and can it be changed without changing hardware?
- What is the efficiency figure, and is it measured at the output terminals or at the wall?
- Which boards are on the shelf, and what do they cost?
- What is the impeder life on my wall thickness, and what does a replacement cost?
- What water flow and quality does it need, and what happens if they are not met?
From the Knowledge Centre
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