In ancient times, it was thought a sword could be hardened by plunging it into the newly-decapitated body of a slave and stirring vigorously.
Modern metallurgy tells us that the secret of hardening certain kinds of steel is to heat them, then cool them rapidly. Swift cooling changes the way the elements in the alloy are arranged, especially the iron and carbon. The effect is to produce a harder material. In the case of this steel, the harder material consists of a mass of uniform needle-shaped crystals.
Induction hardening is done by inducing a high current flow into the component that needs hardening. This is done with an induction coil. After the component has been heated enough, it is quenched, usually in oil. A process called flame hardening can also be used, but in contrast the material is water-quenched.
The shape of the component may require different methods of heating and cooling but the principle is the same.
The process of quenching can make a material harder but it can also make it brittle. This brittleness can be reduced to make the steel more ductile and tougher by tempering.
Tempering means heating to a certain temperature, depending on the material, holding it at that temperature for a time, then letting it cool at a controlled rate. The temperature needed, how long it's held, and how the material is cooled all depend on the material, and the properties that are wanted.
