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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 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 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 certain kinds of stress. 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. == History == Early iron melting made use of bloomeries, which produced two layers of metal: one with a very low carbon content that is worked into wrought iron, and the rest a high carbon cast iron. Since the high carbon iron is ''hot short'', meaning it fractures and crumbles when forged, it was not useful without more smelting. 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. The traditional method of applying the carbon to the surface of the iron involved packing the iron in a mixture of ground bone and charcoal or a combination of leather, hooves, salt and urine, all inside a well-sealed box. This carburizing package is then heated to a high temperature but still under the melting point of the iron and left at that temperature for a length of time. The longer the package is held at the high temperature, the deeper the carbon will diffuse into the surface. Different depths of hardening is desirable for different purposes: sharp tools need deep hardening to allow grinding and resharpening without exposing the soft core, while machine parts like gears might need only shallow hardening for increased wear resistance. 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 colouring'' 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Case-hardening」の詳細全文を読む スポンサード リンク
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