Case Hardening

Carburising involves the diffusion of carbon into the surface layers of a low carbon steel at high temperatures. Controlled cooling after carburising (water, oil or polymer quenching) produces hard martensitic layers at the surface (this is due to the increased hardenability of the carbon enriched surface region).

Nitriding involves the diffusion of nitrogen into the surface layers of a low carbon steel at elevated temperature. The formation of nitrides in the nitrided layer provides the increased hardness. Nitriding is typically carried out in the temperature range of 500 - 575°C, this is in the ferritic state rather than the austenitic used for carburising. This is possible since ferrite has a much higher solubility for nitrogen than it does for carbon.