Induction Wire & Strip Heating
Continuous inline annealing, stress relief, galvannealing, and coating cure for wire, rod, strip, and tube at production line speeds.
How It Works
Wire or strip passes continuously through a solenoid coil. The dwell time — the time the material spends inside the heated zone — equals coil length divided by line speed. This is the fundamental relationship: change the line speed and the exit temperature changes proportionally.
Power requirement is proportional to mass flow rate: P = (mass/metre) × line_speed × Cp × ΔT / η. Double the line speed and you need double the power to maintain the same exit temperature.
For thin wire and strip, high frequency (100 kHz–2 MHz) is needed because the skin depth must be comparable to the wire diameter or strip thickness for efficient electromagnetic coupling. Copper and aluminium wire requires particularly high frequencies due to their low resistivity.
Multiple coils in series enable multi-stage heating — preheat, soak, and equalize zones — or can distribute the total power requirement across several smaller power supplies for flexibility.
Typical Parameters
| Application | Frequency | Power | Temp Range | Line Speed |
|---|---|---|---|---|
| Steel wire annealing (1–5 mm dia) | 50 – 400 kHz | 10 – 50 kW/m coil | 650 – 850°C | 10 – 100 m/min |
| Copper wire annealing (0.5–3 mm dia) | 200 kHz – 2 MHz | 5 – 30 kW/m coil | 400 – 600°C | 20 – 200 m/min |
| Strip annealing (0.2–3 mm thick) | 10 – 200 kHz | 20 – 100 kW/m coil | 600 – 900°C | 5 – 50 m/min |
| Galvannealing (zinc coating alloy) | 50 – 200 kHz | 10 – 40 kW/m coil | 480 – 560°C | 50 – 200 m/min |
| Coating / paint cure | 50 – 400 kHz | 5 – 20 kW/m coil | 150 – 350°C | 20 – 100 m/min |
| Tube stress relief | 3 – 30 kHz | 10 – 50 kW/m coil | 550 – 700°C | 2 – 20 m/min |
Key Considerations
- Line speed variation directly affects final temperature. Closed-loop pyrometer control with fast power supply response (< 100 ms) is essential for consistent product quality.
- Copper and aluminium wire require high frequency because their low resistivity makes electromagnetic coupling extremely poor at low frequencies. Typical: 200 kHz–2 MHz for Cu wire < 3 mm.
- Wire tension and guide rollers must not create electrical shorts or arcing near the coil. Use ceramic or insulating guides within 100 mm of the coil zone.
- Atmosphere control (N₂, H₂/N₂ mix) prevents oxidation for bright annealing applications. The induction coil housing doubles as the atmosphere chamber.
- Multiple strands through a single coil are possible but reduce coupling efficiency and temperature uniformity. Each strand should be equidistant from the coil wall.
- Power factor correction is critical at high frequencies. Capacitor banks must be rated for continuous duty and matched to the operating frequency range.
Common Coil Geometries
Multi-Turn Solenoid
Standard for wire and rod. Bore diameter matched to the wire bundle or strip width with minimal clearance for maximum coupling. Lengths from 0.3 m to 3 m.
Transverse Flux Coil
For wide, thin strip where conventional solenoid coupling is inefficient (strip width >> thickness). The field passes across the strip for more uniform heating.
Split Solenoid
Opens along the axis for easy wire threading without cutting. Two halves close for operation. Essential for production lines where downtime for threading is costly.