Diffusion hardening is a process used in manufacturing that increases the hardness of steels.  In diffusion hardening, diffusion occurs between a steel with a low carbon content and a carbon-rich environment to increase the carbon content of the steel and ultimately harden the workpiece. Diffusion only happens through a small thickness of a piece of steel (about 2.5 μm to 1.5 mm), so only the surface is hardened while the core maintains its original mechanical properties.  Heat treating may be performed on a diffusion hardened part to increase the hardness of the core as desired, but in most cases in which diffusion hardening is performed, it is desirable to have parts with a hard outer shell and a more ductile inside.  Heat treating and quenching is a more efficient process if hardness is desired throughout the whole part.  In the case of manufacturing parts subject to large amounts of wear, such as gears, the non-uniform properties acquired through diffusion hardening are desired.  Through this process, gears obtain a hard wear-resistant outer shell but maintain their softer and more impact-resistant core.