What it is
Induction hardening is a surface heat treatment. A coil carrying alternating current generates a magnetic field around the component; that field induces eddy currents in the steel; the steel’s own resistance turns those currents into heat. Within seconds the surface passes its austenitising temperature. Quenching immediately afterwards traps the carbon in a hard martensitic structure.
The reason only the surface transforms is the skin effect. Alternating current concentrates near the surface of a conductor, and the higher the frequency, the thinner that layer. The core never reaches transformation temperature, so it keeps the toughness the part was forged or machined with. You get a component that resists wear on the outside and shock on the inside, which is not a combination any single bulk heat treatment gives you.
The process is normally followed by tempering. As-quenched martensite is very hard and very brittle; a tempering cycle gives back a large amount of toughness for a small loss of hardness.
Why induction
Against furnace or flame hardening, induction wins on four counts that matter on a production floor.
Speed: seconds per part rather than minutes or hours, with no furnace to bring up to temperature first and no waiting for a batch.
Placement: the coil decides where the heat goes. A journal can be hardened while the adjacent thread, spline or shoulder is left soft, which is impossible in a furnace and unreliable with a flame.
Repeatability: the energy is set electrically and the cycle is held by a PLC, so part 1 and part 5000 receive the same treatment. Flame hardening depends on an operator’s judgement.
Working environment: no open flame, no combustion products, no radiant heat filling the shop, and the machine is only hot while it is running.
