Carburizing

Carburizing is a surface hardening process in which carbon atoms diffuse into the surface layer of low carbon steel at elevated temperatures, followed by quenching to harden the surface layer. As a result, the component acquires a hard, wear resistant outer case along with a tough, impact resistant core.

It is a preferred method for components requiring both surface hardness and core toughness, such as gears, shafts, and bearing rings.

Carburizing is typically applied to low carbon (0.10%–0.25% C) or low alloy steels. The part is heated in a carbon rich atmosphere to a temperature within the austenitic range. Because the solubility of carbon in austenite is high at these temperatures, carbon atoms on the surface diffuse inward into the material. At the end of the process, the part is quenched to transform the carbon enriched surface layer into a martensitic structure, while the low carbon core remains relatively soft and tough.

Carburizing alone does not harden the component; to achieve high hardness, the carbon enriched case must be quenched and subsequently tempered at low temperatures to relieve internal stresses. Therefore, carburizing should be planned as a complete processing sequence that incorporates the quenching and tempering steps.