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6.5 Case hardening

Case-hardening or surface hardening is the process of hardening the surface of a metal object while
allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal at the
surface. For iron or steel with low carbon content, which has poor to no hardenability of its own, the
case-hardening process involves infusing additional carbon or nitrogen into the surface layer. Case-
hardening is usually done after the part has been formed into its final shape, but can also be done to
increase the hardening element content of bars to be used in a pattern welding or similar process (Fig.
6. 11).

Process description
Hardening is desirable for metal components that are subject to sliding contact with hard or abrasive
materials, as the hardened metal is more resistant to surface wear. However, because hardened metal
is usually more brittle than softer metal, through-hardening (that is, hardening the metal uniformly
throughout the piece) is not always a suitable choice. In such circumstances, case-hardening can
produce a component that will not fracture (because of the soft core that can absorb stresses without
cracking), but also provides adequate wear resistance on the hardened surface.

Benefits
The resulting case-hardened part may show distinct surface discoloration, if the carbon material is
mixed organic matter as described above. The steel darkens significantly, and shows a mottled pattern
of black, blue, and purple caused by the various compounds formed from impurities in the bone and
charcoal. This oxide surface works similarly to bluing, providing a degree of corrosion resistance, as
well as an attractive finish. Case coloring refers to this pattern and is commonly encountered as a
decorative finish on firearms.
Case-hardened steel combines extreme hardness and extreme toughness, something which is not
readily matched by homogeneous alloys since hard steel alone tends to be brittle.

Applications
Both carbon and alloy steels are suitable for case-hardening; typically mild steels are used, with low
carbon content, usually less than 0.3% (see plain-carbon steel for more information). These mild steels
are not normally hardenable due to the low quantity of carbon, so the surface of the steel is chemically
altered to increase the hardenability. Case-hardened steel is formed by diffusing carbon
(carburization), nitrogen (nitriding) and/or boron (boriding) into the outer layer of the steel at high
temperature, and then heat treating the surface layer to the desired hardness.

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