The process of heating and cooling of steel in a controlled manner that renders the base metal to have harder properties..
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 (called the "case") at the surface. For steel or iron with low carbon content, which has poor to no hardenability of its own, the case hardening process involves infusing additional carbon into the case. 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. The term face hardening is also used to describe this technique, when discussing modern armour.
Because hardened metal is usually brittler than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for applications where the metal part is subject to certain kinds of stress. In such applications, case hardening can provide a part that will not fracture (because of the soft core that can absorb stresses without cracking) but also provides adequate wear resistance on the surface.
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Case Hardening 1914 2014-01-09
Machinery's Reference Book