Due to the alternating electromagnetic field, eddy currents are induced in the surface of the material. The depth of the hardening can be controlled by controlling the current through the induction coil.
After the heating, the material is quenched in a cooling medium, such as oil. Carbon alloy steels are generally hardened in this manner.
Gears, pinion shafts, bearing races are some of the components which can be hardened by induction hardening.