Surface hardening of steel

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 iron or steel with low carbon content, which has poor to no hardenability of its own, the case-hardening process involves infusing additional carbon 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 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 more brittle than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for uses where the metal part is subject to sliding contact with hard or abrasive materials. In such circumstances, 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.

Early iron smelting made use of bloomeries which produced two layers of metal: one with a very low carbon content which is worked into wrought iron, and one with a high carbon outer layer. Since the high carbon iron is hot short, meaning it fractures and crumbles when forged, it was not useful without more smelting. As a result, it went largely unused in the west until the popularization of the finery forge. The wrought iron, with nearly no carbon in it, was very malleable and ductile but not very hard.

Case-hardening involves packing the low-carbon iron within a substance high in carbon, then heating this pack to encourage carbon migration into the surface of the iron. This forms a thin surface layer of higher carbon steel, with the carbon content gradually decreasing deeper from the surface. The resulting product combines much of the toughness of a low-carbon steel core, with the hardness and wear resistance of the outer high-carbon steel.

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