Electromagnetic Induction High Frequency Heating Furnace

What is a high frequency electromagnetic induction heating furnace, and how does it heat metal without touching it?

It heats a conducting workpiece by inducing eddy currents inside the part itself, with no flame, no element in contact with the work and no heat carried in from outside. An alternating current in a copper coil generates an alternating magnetic field, the field induces circulating currents in any conductor inside it, and the workpiece’s own resistance turns those currents into heat. Because the heat is generated in the part, the surface can reach austenitising temperature in seconds while the core stays cool, and the cycle starts and stops within a fraction of a second. A crucible duty works the same way, with the induced currents heating a charge or a graphite susceptor instead of a workpiece.

What is skin depth, and why does it decide the frequency of the machine?

Skin depth is the reference depth of the induced current and the single figure that decides where the heat goes: the depth in millimetres equals 503 times the square root of the resistivity divided by the product of the relative permeability and the frequency in hertz. One skin depth is the distance at which current density has fallen to about 37 per cent of its surface value, and about 86 per cent of the heat the current generates is produced inside that layer. Because the depth varies inversely with the square root of frequency, frequency is the control that sets how thin or deep the heated layer is, which is why a depth requirement has to become a frequency before a machine can be selected.

Which frequency should be used for which case depth?

Start from the published depth bands. Medium frequency from about 1 to 10 kHz buys 3 to 10 mm and is applied to large gears, crankshafts and large shafts. High frequency from 10 to 100 kHz buys 1 to 4 mm for shafts, cams, medium gears and CV joints, and in heat treatment it is the band usually meant by the words high frequency furnace. Very high frequency from 100 to 400 kHz buys 0.5 to 2 mm for small gears, thin sections and pins, and above 1 MHz the layer is under half a millimetre. The band is only half the decision: the part’s section must be large relative to the skin depth, because when the two are comparable the fields induced from opposite sides cancel.

What is the difference between a high frequency induction heating furnace and a melting furnace?

They are different duties that share one name. A heating furnace holds a workpiece in place and brings a surface or a local zone to temperature, and its design unit is the case depth in millimetres at a stated hardness threshold. A melting furnace puts a charge into a crucible and brings the whole charge to liquid so it can be poured, and its design unit is the charge weight and the pour temperature. Frequency, coil, power supply rating, cooling and acceptance evidence all differ between the two, which is why they are quoted against different units, and why a crucible melt belongs with the melting and smelting equipment rather than with a surface heating schedule.

Why does the heating change when steel passes about 770 °C?

Because steel stops being magnetic there. Below roughly 770 degrees Celsius, or 1,420 degrees Fahrenheit, a magnetic steel part has a high relative permeability, the skin depth is thin and the part couples strongly to the coil. At the Curie point the material becomes non-magnetic, the permeability falls towards one and the skin depth increases, so the same coil presents a different load to the inverter in the middle of the heat. A machine whose power stage and programme are matched only to cold steel loses control at about the temperature where hardening actually begins, which is why the ramp, the tuning and the power rating are specified across the transition.

How is case depth controlled, and what is effective case depth?

Three controls, and only one of them is frequency. Frequency sets the ceiling on the layer that can be heated; power density in watts per square centimetre sets how fast the surface is brought up; and heat time lets conduction carry that heat deeper, with case depth in millimetres at about 0.5 times the square root of the heat time in seconds as a starting approximation for medium frequency on plain carbon steel. Effective case depth is the measurement convention: the depth at which hardness has fallen to 550 HV, or to 80 per cent of the minimum surface hardness, depending on the applicable standard, so the acceptance figure must name both the depth and its threshold.

How long does an induction hardening cycle take?

It is calculated from the depth and the section rather than quoted as a general figure. A shallow case on a small part in the very high frequency band can be made in a fraction of a second for the heating part of the cycle, a deeper case on a large part runs to several seconds of heating before the quench, and a scan duty on a shaft takes as long as the coil needs to traverse the length at the controlled speed. The relationship behind the cycle is the same one that gives a case depth of about 0.5 times the square root of the heat time, so the cycle is set by measurement on your own part.

What carbon content does a part need to be induction hardened?

Roughly 0.35 to 0.55 per cent carbon is the working range for direct hardening, and hardness rises with carbon: published values run from about 54 to 58 HRC at 0.40 per cent carbon, to 58 to 62 HRC at 0.55 per cent, and to 60 to 64 HRC on a 0.58 per cent chromium-molybdenum-vanadium grade. Below about 0.25 per cent carbon a quench alone cannot deliver useful hardness and the part must be carburised or carbonitrided first. The starting microstructure matters too: a normalised or well-tempered structure austenitises evenly, while a banded or segregated one produces an uneven case whatever the machine does.

What quench methods are used, and why does tempering matter?

Three methods are standard. Spray quenching from holes in the coil face or a following spray ring is usual for scan hardening; immersion quenching moves the part rapidly from the coil into a tank for single-shot hardening; and mass quenching relies on the cool core of a heavy part to remove heat from a thin case. The quenchant is usually a water-based polymer at about 5 to 20 per cent concentration, and its concentration decides how severe the quench is. Tempering matters because a freshly quenched martensitic case is brittle, and a part not tempered within about an hour of the quench can crack on its own, so the tempering temperature and time belong inside the hardening schedule.

What materials can be heated by induction, and which cannot?

Any electrical conductor can be heated by induction, which covers steel and most engineering alloys, stainless steels, copper and its alloys, aluminium, the precious metals and graphite. Magnetic materials heat more readily at low temperature because permeability concentrates the current, while austenitic stainless steel, copper and aluminium have different resistivities and need their frequency, power density and cycle calculated for the alloy rather than copied from steel. What cannot be heated is a non-conductor: glass, ceramics and most polymers do not couple to the field at all. Those materials can still be processed indirectly through a graphite or molybdenum susceptor, but that is a different design.

Can induction heating braze or solder dissimilar metals?

Yes, and it is one of the duties the machine is best at, because the heat is local and the cycle is short. Carbide tips on steel shanks, copper pipe joints, aluminium assemblies, electrical contacts and terminals are brazed or soldered by bringing only the joint zone to the filler metal’s flow temperature, so each parent material stays inside its own limits instead of both being taken to a compromise temperature. The coil is designed to follow the joint geometry and the filler is placed before the heat. Where the joint has to be oxide-free the same coil runs inside a controlled atmosphere or a vacuum chamber with no flux, which removes the cleaning operation flux would leave.

Can this furnace melt gold, silver and copper, and what about platinum and palladium?

The same power supply can drive a crucible coil, and gold, silver or copper is the natural small-batch melting duty: silver melts at about 961 degrees Celsius, gold at about 1,064 and copper at about 1,084, all inside the range of a crucible induction melt. Steel and iron are higher, at about 1,425 to 1,540 and about 1,538. The platinum group metals are a different matter, because palladium melts at about 1,554 degrees Celsius, platinum at about 1,772 and rhodium at about 1,960, so a crucible duty on those metals is matched to the metal and its crucible rather than to a general temperature claim, and is quoted against the charge weight on the melting and smelting pages.

How much power does the machine need, and how is the power rating stated?

The rating is stated as the power delivered to the work at the working frequency, with input kVA, duty cycle and site voltage given separately, because those are different numbers and a machine quoted on input kVA can be compared with nothing. The rating follows from the mass to be heated, the depth, the power density the cycle needs and the time available, and it is checked on both sides of the Curie transition, since the load changes when the part goes non-magnetic. A cycle repeated every few seconds on a large case needs a much larger rating than the same case produced more slowly, and the cooling system, the coil and the site supply all follow from that decision.

Why do soft spots, cracks and inconsistent case depth happen?

In published practice the common failures trace to three places, and the power supply is usually not one of them. Soft spots come from inconsistent coupling between coil and part, from quenchant fouling or from coil damage, all of which change the heat going into that spot without changing any setting on the panel. Cracking follows an excessive case depth, a sharp geometry transition or a delayed temper, which is why tempering inside about an hour of the quench is written into the schedule. Inconsistent case depth usually comes from the starting microstructure, because a banded or segregated structure austenitises unevenly, and distortion has a fourth cause in asymmetric heating or uneven quench agitation.

How is the machine commissioned and accepted?

In four stages, each signed off separately. The electrical set-up is verified first, with converter, resonant tank, workhead, coil and cooling circuit proved at the working frequency and flow, pressure and temperature recorded. The programme is then established on the customer’s own parts, with power density, heat time, the ramp across the Curie transition and the quench trigger brought to their scheduled values. The metallurgical result is measured next, and acceptance is written as measurements: case depth traverses at the specified threshold, surface and core hardness, distortion against the drawing, and joint sections where the duty is brazing. Training on the schedule and on the symptoms of a drifting process runs alongside the last two stages.

What cooling water and site services does the installation need?

The cooling circuit is a closed loop with pump, heat exchanger or chiller, reservoir, filtration and instrumentation, delivering the flow, pressure and temperature the coil, capacitor bank and power modules need at the design power. Water quality matters as much as quantity: scale in a coil’s water path raises its temperature and changes the circuit’s tuning, so conductivity or hardness is controlled and the loop is run on treated or deionised water where the local supply requires it. Beyond cooling, the site provides power at the stated voltage and frequency with its protection, the quench medium and its handling, compressed air where required, gas where an atmosphere is used, and the floor loading and lifting arrangement taken from the machine’s real mass.

What is the smallest part and the smallest job that makes an induction heating furnace worthwhile?

The threshold is not a size but three conditions, and all three have to be true. The part has to couple to a coil at a frequency that gives the required depth, which rules out parts whose section is comparable to the skin depth at any available frequency. The material has to meet the specification after the treatment, which for hardening means enough carbon or a carburised case. And the volume and repeatability requirement have to justify a coil, a fixture and a programme. Where a part fails either of the first two, the honest answer is a different process rather than a smaller machine: a resistance or convection furnace for a non-conductor, carburising first for a low-carbon part, and a batch furnace where the volume is low.