Diffusion

Diffusion refers to the movement of atoms in solid metals at elevated temperatures.

Without diffusion, there would be no heat treatment. During the heat treatment of steel, it is the smaller atoms, especially carbon and nitrogen, which move readily through the iron crystal structure. When the carbon content at the surface increases, it changes the composition of the steel and therefore its properties after hardening.

Atoms move very slowly in solid metals and it therefore requires treatment for a long time to give a very deep case. For example, a case depth of 6mm would typically require carburising for five days.