Surface Engineering by Expanded Austenite
Carburising Process
Hardening
Carburising is the hardening method being investigated and is the process of heating a low carbon content steel whilst in the presence of a ‘carbon donor’ such as carbon monoxide, or in the past charcoal. This enriches the surface of the material with carbon by means of diffusion (“the thermally-activated movement of atoms in a solid”) either interstitially or by substitutional means.
Interstitial being where the impurity atom is small enough to fit within the crystal lattice, thus introducing some local strain, acting as barrier to prevent dislocation movement, thus strengthening the material.
Substitutional being an atom of the solute material taking the place of the solvent atom within the lattice. This causes localised stress which opposes dislocation movements.
The carbon content depends on the time and the temperature of the carburising process, with an increase in time and temperature resulting in an increased depth of the diffusion zone. After the carburising process, the material is often subject to a subsequent quench (rapid liquid cooling) which further increases the hardness by preventing unfavourable phase transformation. This is followed by tempering which involves once again heating the material, but this time to a temperature below the critical temperature and then allowing it to cool via natural convection at room temperature. This gives a small reduction in the hardness of the material, but greatly enhances the ductility of the material.
Interstitial Impurity
Substitutional Impurity