Induction Melting
Coreless and channel induction melting for foundry, laboratory, and precious metal applications — with electromagnetic stirring for homogeneous alloys.
How It Works
A water-cooled copper coil surrounds a refractory crucible containing the charge material. The alternating field induces eddy currents directly in the metal charge, heating it to the melting point and beyond.
Electromagnetic Lorentz forces cause natural stirring — the melt forms a meniscus and circulates, producing homogeneous composition and temperature distribution throughout the charge. This is a key advantage over resistance or fuel-fired furnaces.
Coreless furnaces use frequencies from 50 Hz (large foundry, > 1 tonne) to 10 kHz+ (small laboratory crucibles, < 5 kg). Higher frequency gives stronger stirring force in smaller charges. The rule of thumb: stirring intensity scales with power and inversely with frequency and charge diameter.
Vacuum induction melting (VIM) operates under 10⁻³ to 10⁻¹ mbar for reactive metals (titanium, zirconium) and nickel-base superalloys, preventing oxidation and gas pickup that would degrade mechanical properties.
Typical Parameters
| Application | Frequency | Specific Power | Temp Range | Charge Size |
|---|---|---|---|---|
| Foundry iron/steel (coreless) | 50 – 500 Hz | 200 – 600 kW/tonne | 1500 – 1650°C | 0.5 – 30 tonne |
| Foundry aluminium (coreless) | 200 Hz – 3 kHz | 150 – 400 kW/tonne | 700 – 780°C | 0.1 – 5 tonne |
| Precious metals (Au, Ag, Pt) | 3 – 50 kHz | 30 – 100 kW/kg | 1000 – 1800°C | 0.01 – 10 kg |
| Laboratory / R&D melting | 10 – 100 kHz | 50 – 200 kW/kg | Up to 2000°C | 1 g – 1 kg |
| Vacuum induction melting (VIM) | 500 Hz – 3 kHz | 300 – 800 kW/tonne | 1400 – 1700°C | 10 kg – 10 tonne |
| Copper / brass (coreless) | 200 Hz – 1 kHz | 200 – 500 kW/tonne | 1100 – 1250°C | 0.1 – 10 tonne |
Key Considerations
- Crucible material must withstand the melt temperature and be electromagnetically transparent. Common choices: alumina, magnesia, zirconia for metals; graphite for certain non-ferrous applications.
- Frequency affects stirring intensity — too vigorous stirring in reactive melts causes turbulence and gas entrainment. For superalloys under vacuum, moderate stirring preserves melt cleanliness.
- Sintering of scrap and turnings requires careful startup protocol. Loose charge has poor coupling until it consolidates; a heel of molten metal from the previous melt improves startup efficiency.
- Slag and dross removal is simplified with induction — the meniscus pushes impurities to the crucible wall where they can be skimmed. Inert gas cover further reduces re-oxidation.
- Energy efficiency varies by material: 55–75% for steel, 40–60% for aluminium (lower due to low resistivity and high thermal conductivity), 70–85% for precious metals in small crucibles.
- Thermal shock to the crucible on startup is a primary failure mode. Pre-heating the crucible to 300–500°C before charging extends crucible life by 2–3×.
Common Coil Geometries
Multi-Turn Solenoid (Coreless)
The standard melting coil. Heavy copper tubing with water cooling wraps around the crucible. Turn spacing and taper can be adjusted for melt stirring control.
Channel Coil (Channel Furnace)
For continuous holding and superheating. Molten metal flows through a refractory-lined channel that acts as the secondary winding of a transformer.
Cold Crucible (Segmented Copper)
For extremely reactive metals (Ti, Zr). The crucible itself is water-cooled copper with longitudinal slits — the melt levitates within the field, contacting only a thin skull layer.