Carburizing, also known as case hardening, is a heat treatment process in which a steel component of low carbon content is heated in a carbon rich controlled endothermic gas. The process of heating the component in a high carbon environment via means of controlled gas allows for diffusion of the carbon atoms directly into the surface of the part. This produces a surface which is resistant to wear, while maintaining toughness and strength of the core. This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components. Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel, creating a high case hardness while retaining a substantially lower hardness in the core.
Higher temperatures and longer times with higher carbon potentials will increase the amount of carbon diffused into the surface and the depth of case. The hardening of both case and core material does not actually occur until the metal is rapidly quenched in oil.
The carburizing process is typically specified by a case depth and a surface hardness. Typically, the surface is specified at Rockwell C 58 to 62 (or equivalent), with other hardnesses occasionally specified for special applications. Carburized case depth is typically specified to an “effective” hardness, or the depth where a specified hardness is maintained, generally Rockwell C 50. Case depths from as light as 0.003″ to as deep as 0.250″ may be specified, depending on the service requirements of the product.
In some cases, it may be desirable to have the benefits of a carburized surface in only certain areas of the part. For these applications, a carburizing stop-off paint can be applied to the areas to be protected. The stopped-off, or localized, areas will remain carbon free and relatively soft. The paint is removed through normal processing in the heat treatment operation.
- Increases surface hardness
- Improves wear resistance and fatigue strength
- Typically works best with low carbon and alloy steels with a carbon content 0.05% to 0.3%