Key takeaways
- Coreless induction melting of cast iron typically uses about 500 to 600 kWh per tonne, against a theoretical minimum near 350 to 400 kWh per tonne to melt and superheat from a cold, dense charge.
- The gap between typical practice and the theoretical minimum is where a foundry saves: it is set by charge quality, furnace utilisation, the power system, the coil and pouring practice, not by the metal itself.
- A modern IGBT converter recovers energy over an older SCR drive, a UHP coil couples power efficiently, and a heated press-pouring buffer avoids remelting metal that a cold line would send back.
- Energy is the largest variable cost in melting and the biggest lever on a foundry's carbon footprint, so kWh per tonne is a sustainability metric as much as a cost one. CIME builds coreless furnaces up to 10 MW.
The number and the benchmark
The single most useful energy figure in a melt shop is kWh per tonne: the electricity it takes to turn a charge into a tonne of good metal at pouring temperature. It is comparable across furnaces and shifts, it tracks straight to the energy bill, and it is the number worth driving down. For coreless induction melting of cast iron, typical practice sits around 500 to 600 kWh per tonne. That range is wide on purpose: the same nominal furnace can sit near the bottom of it in a well-run shop and near the top in one that charges wet scrap and idles between casts. The metal does not change; the practice does.
That figure only means something next to the theoretical minimum, the energy the metal itself has to absorb to melt and superheat. For cast iron that minimum is near 350 to 400 kWh per tonne. No furnace reaches it, because electrical, coil, thermal and holding losses all sit on top, but the distance between the two is exactly the efficiency a foundry can work on.
| Metal | Theoretical minimum | Typical practice |
|---|---|---|
| Grey and ductile iron | 350 to 400 | 500 to 600 |
| Carbon steel | 350 to 400 | 550 to 700 |
| Copper alloys | 150 to 200 | 250 to 350 |
| Aluminium | 300 to 330 | 550 to 650 |
Note
Read the table as ranges, not guarantees. The metal sets the theoretical minimum; everything above it is the furnace, the power system and the shop practice. Two foundries melting the same iron can sit 100 kWh per tonne apart on the same nominal furnace.
What drives your kWh per tonne
Five things decide where a foundry lands between the minimum and the typical figure. None of them is the metal.
Charge quality. A clean, dense, dry charge melts with the least wasted energy. Light or loosely packed scrap couples poorly and takes longer; rust and moisture cost energy to drive off and can slow the melt. Dense returns and pre-dried scrap couple better and shorten the melt, and a charge sized to the crucible avoids the slow bridging that wastes energy at the top of the bath. The single cheapest efficiency gain in many shops is a better charge.
Furnace utilisation. A furnace has fixed losses whether it is melting hard or idling on a holding load. Running full melts back to back spreads those losses over more tonnes, while long idle holds between casts burn energy for no metal. Scheduling melts back to back, matching furnace size to the melt rate and keeping holds short all pull the number down without buying anything. High utilisation is often worth more than any single piece of hardware.
Power-system efficiency. The converter that feeds the coil decides how much of the drawn power reaches the metal. A modern IGBT medium-frequency converter runs more efficiently than an older SCR drive and holds a power factor above 0.95, so less energy is lost as heat and reactive power on the way in. The full comparison is in IGBT vs SCR converters.
Coil and insulation. How the coil is built and how well the furnace is insulated set the electrical and thermal losses around the bath. A UHP coil couples power into the charge efficiently, and good insulation keeps heat in the metal rather than the surroundings. The coil side of this is covered in UHE vs UHP coil design.
Pouring practice. Energy already spent is wasted if the metal is remelted. A cold, ladle-fed line that stops has to cast waiting metal back into ingots and remelt it later, paying for the same tonne twice. A heated press-pouring buffer holds the metal ready instead, so the energy stays in the metal. The pouring side is in press pouring vs ladle pouring.
The metal fixes the floor; the furnace, the power system and the shop decide how far above it you run. kWh per tonne is a practice number, not a physics one.
How to cut it
The levers follow from the drivers, in rough order of what they return. Start with the charge: keep it clean, dense and dry, and size it to the furnace. A dry charge in particular pays back fast, because moisture that boils off is energy that never reaches the metal, and drying it is a safety gain as much as an efficiency one. Then run the furnace hard, with full melts and the shortest holds the schedule allows, so fixed losses are spread over metal rather than time.
On the hardware side, a modern IGBT converter and an efficient UHP coil recover the electrical and coupling losses, and good lid and refractory practice keeps the thermal losses down. On the pouring side, a heated buffer that avoids remelting protects the energy already spent. None of these is exotic; together they are the difference between the top and the bottom of the typical range. See the melting furnaces and the IGBT power supply, or the core technology for how the platform is built for efficiency.
Measuring and tracking it
A number a shop does not measure cannot be cut. The practical way to track kWh per tonne is to meter the melting circuit directly and divide the energy it draws over a period by the tonnes of good metal poured in the same period, not the metal charged. Good metal is what counts, because scrap and returns carry the energy of everything that did not become a casting.
Then read the trend, not the single figure. A shop that logs kWh per tonne per shift sees the effect of a wet charge, a long holding weekend or a converter drifting out of tune long before it shows on the monthly bill. Benchmark the number against the typical range for the metal to know roughly where you stand, then stop comparing outward: the useful target is not another foundry's figure but a steady fall in your own, melt after melt. That is also the data a modern control system already has, since it meters the power it delivers.
Beyond the bill: the carbon side
kWh per tonne is a cost number and a carbon number at the same time. Melting is the largest energy user in most foundries, so it is also the largest lever on the emissions tied to the electricity a foundry buys. Every kWh per tonne cut is both money saved and carbon avoided, which is why energy efficiency sits at the centre of a foundry's sustainability case, not beside it. It also improves figures a foundry increasingly has to report, since energy per tonne and the emissions behind it now appear in customer audits and procurement questionnaires, not only the internal accounts.
Induction melting starts from a good position here: the heat is generated directly in the metal by the magnetic field, with no combustion and no flue losses, and the whole charge can be recovered and corrected in the bath. Push the five drivers above and the same furnace melts the same metal for fewer kWh, lower cost and lower carbon. That is the thread that runs into sustainability.
Key point
There is no single efficient furnace and no single number. There is a metal-set floor, a typical range above it, and five levers that decide where a foundry runs. Move the levers and the kWh per tonne, the cost and the carbon all fall together.
Frequently asked questions
How much energy does an induction furnace use per tonne?
Coreless induction melting of cast iron typically uses about 500 to 600 kWh per tonne to melt and superheat from a cold charge, against a theoretical minimum near 350 to 400 kWh per tonne. The exact figure depends on the metal, the charge quality, how fully the furnace is used and the efficiency of the power system, so it is best read as a range rather than a single number.
What is the theoretical minimum energy to melt iron?
The thermodynamic minimum to melt and superheat cast iron to pouring temperature is roughly 350 to 400 kWh per tonne, the energy the metal itself absorbs. Real furnaces sit above that because of electrical, coil, thermal and holding losses. The distance between the two is the efficiency headroom a foundry can work on.
Does an IGBT converter reduce energy consumption?
Yes. A modern IGBT medium-frequency converter runs more efficiently overall than the SCR drive it replaces and holds that efficiency across the power range, so fewer kWh reach the coil as loss. Because it also holds a power factor above 0.95, it avoids the reactive-power waste an SCR line corrects with capacitor banks.
What is the biggest way to cut kWh per tonne?
Charge quality and furnace utilisation usually move the number most: a clean, dense, dry charge melts with less wasted energy than light, rusty or damp scrap, and running full melts spreads the fixed losses over more metal than idling the furnace on a holding load. A modern power system, an efficient coil and a heated pouring buffer that avoids remelts then close the remaining gap.
How do I measure kWh per tonne in my foundry?
Meter the melting circuit and divide the energy drawn over a period by the tonnes of good metal poured in the same period, using good castings rather than metal charged, because scrap and returns carry energy that did not leave as product. Track it per shift and read the trend: a rising number flags a wet charge, long idle holds or a converter drifting out of tune, usually well before the monthly bill does.
Does furnace size change energy consumption per tonne?
It can. A larger furnace run at a high duty spreads its fixed electrical and thermal losses over more metal, so kWh per tonne tends to fall with scale when the furnace is kept busy. A furnace that is too large for the demand, held part-full or idling between casts, loses that advantage, so the right size is the one matched to the melt rate the shop actually runs.
References and sources
- Foundry Trade Journal, technical article on induction-melting energy efficiency, 2006.
- Cast Metal Times, coreless induction practice and specific energy, 2004.
- CIME S.r.l., internal process and engineering documentation, Turin.