Glossary

The temperature above which steel transforms to austenite, the face-centred cubic phase in which carbon dissolves readily. Hardening requires reaching it, because the rapid quench that follows traps that dissolved carbon as martensite. For most engineering steels it falls between roughly 800 and 900 degrees Celsius, depending on composition.

Induction hardening

Heating only one end of a long bar, so that the end can be forged while the rest of the bar stays cool enough to handle. The heated length is set by the coil, which makes it repeatable from bar to bar.

Bar and end heating

A short length of bar stock cut to the volume required for one forged part. Billet heating brings it to forging temperature before it reaches the press or hammer.

Billet heating

Joining two components with a filler metal that melts above 450 degrees Celsius but below the melting point of either parent metal, drawn into the joint by capillary action. Distinguished from soldering, which uses a filler melting below 450 degrees Celsius, and from welding, in which the parent metals themselves melt.

Induction brazing

How deep the hardened layer extends below the surface, measured on a sectioned part by microhardness traverse and reported to the depth at which hardness falls to a defined limit. It is the result of frequency, power density and heating time acting together, not a single machine setting.

Frequency, power and case depth

A cast iron ring used in wheel and drum assemblies, typically pre-heated before assembly so that it expands to the required fit. In alloy wheel production the ring and its bush are often heated together in one operation.

CI ring and bush heating

How efficiently the coil transfers energy into the workpiece. Coupling falls off rapidly with the gap between coil and part, which is why inductors are built to close tolerances around the component and why coil design is component-specific.

Inductor design

The temperature above which a ferromagnetic material loses its magnetism. For plain carbon steel it is approximately 770 degrees Celsius. Crossing it during a heating cycle increases the reference depth roughly tenfold, because magnetic permeability collapses, and any well-designed machine accounts for the change.

What is induction heating

Water with its dissolved mineral content removed, used to cool induction coils and power electronics. Ordinary water would deposit scale in the coil and conduct current where it should not. Induction machines carry a DM water circuit separate from any quench circuit.

A circulating electrical current induced in a conductor by a changing magnetic field. In induction heating these currents are the heat source: the workpiece resists them, and that resistance dissipates energy as heat inside the metal.

What is induction heating

How readily a steel forms martensite at depth, which is a function of alloy content rather than carbon content. Carbon determines how hard the martensite is; hardenability determines how far below the surface it forms for a given quench severity. Do not confuse the two.

Steels for induction hardening

The semiconductor switch used in modern induction power supplies and switch-mode rectifiers. Compared with the thyristor and valve designs it replaced, it switches faster and more efficiently, giving a smaller, lighter, cooler-running machine.

The copper conductor, usually water-cooled tube, that carries the alternating current and creates the magnetic field. It is tooling: shaped to the component, and the single largest determinant of whether the result is uniform.

Inductor design

The hard, highly strained structure formed when austenitised steel is quenched fast enough that carbon has no time to diffuse out of solution. It is what makes a hardened case hard, and it is brittle until tempered.

Steels for induction hardening

The lower induction frequency band, giving deeper penetration. Used for billet heating, bar and end heating, shrink fitting and pre-heating, where the section needs heating through rather than at the surface.

Power delivered per unit of heated surface area, usually stated in kilowatts per square centimetre. More informative than total power, because the same power supply feeding a large coil and a small one produces entirely different heating rates.

Frequency, power and case depth

A non-contact instrument that measures surface temperature from emitted infrared. On an induction machine it is what makes closed-loop temperature control possible, so that parts arriving at different starting temperatures all finish at the specified one.

Pyrometers

Rapid cooling immediately after austenitising, to trap carbon in solution and form martensite. Media in order of decreasing severity: water, polymer solution, oil. More severity means more hardness at depth but also more distortion and cracking risk.

Steels for induction hardening

The depth at which induced current density has fallen to about 37 percent of its surface value; roughly 86 percent of the heating occurs within it. It varies as the square root of resistivity divided by frequency and permeability, which is why higher frequency gives a thinner heated layer.

Reference depth calculator

Residual AC variation on a rectifier’s DC output. In electroplating it matters because deposition rate follows current, so ripple shows up as variation in the deposit and, on a bright finish, as a visible defect.

Electroplating rectifiers

Hardening in which the part traverses through a fixed coil, or the head traverses along the part, with quench following a set distance behind. Only a short band is hot at any moment, so a long shaft can be hardened by a far smaller machine than single-shot heating would require.

Vertical hardening scanners

Assembling an interference fit by heating the outer component so it expands, placing it over the inner component with clearance, and letting it contract into grip. Gives higher interference than a press fit, with no galling and no press force on the fit surfaces.

Shrink fitting

Hardening in which the entire treated area is heated simultaneously and quenched together, with no relative movement between coil and part. Faster per part than scanning, but it needs enough power to heat the whole area at once, so it suits short features.

The tendency of alternating current to concentrate near the surface of a conductor, more strongly at higher frequencies. In induction heating it is the primary control over where energy is deposited.

What is induction heating

Reheating a hardened part well below the austenitising range and holding it there, to relieve internal stress and restore toughness at a modest cost in hardness. The temperature chosen fixes where on that trade the part ends up.

Tempering

Hardening the full cross-section rather than a surface case. Normally a furnace operation, and metallurgically the opposite intent to induction hardening, which deliberately leaves the core untransformed and tough.

Induction versus furnace heating

The highest induction frequency band, giving a very shallow, very fast heat. Used for fine brazing, small joints and thin sections where a lower frequency would heat too much of the surrounding metal.