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Case hardening
Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the materials surface, forming a thin layer of a harder alloy. Case hardening is usually done after the part in question 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.
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 involved packing the iron in a substance high in carbon, and heating it in that environment to encourage carbon to migrate into the surface of the iron. This formed a thin layer of higher carbon steel on the surface, with the carbon content gradually decreasing further from the surface. The resulting product had much of the toughness of the softer iron core, with the hardness and wear resistance of the outer 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. The resulting 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 would be needed for different purposes; sharp tools would need deep hardening to allow them to be ground and resharpened without exposing the soft core, while machine parts like gears might need only a shallow hardening for increased wear resistance.
The resulting case hardened part may show a distinct coloration on the surface. The iron darkens significantly, and shows a mottled pattern of black, blue and purple caused by the various compounds formed by impurities in the bone and charcoal. This oxide surface works similar to blueing, providing some degree of corrosion resistance, in addition to being considered by many to be a very attractive finish. Case coloring refers to this pattern, and is commonly encountered as a decorative finish on replica firearms such as those patterned after single action Colt revolvers and lever-action rifles.
With modern steelworking techniques, it is possible to make homogenous steels of low to high carbon content, removing much of the original motivation for case hardening. However, the heterogeneous nature of case hardened steel may still be desirable, as it provides a combination of hardness and toughness that cannot readily be matched by homogenous alloys.
Carbon itself is solid at case-hardening temperatures and so is immobile. Transport to the surface of the steel was as gaseous carbon monoxide, generated by the breakdown of the carburising compound and the oxygen packed into the sealed box. This takes place with pure carbon, but unworkably slowly. Although oxygen is required for this process its re-circulated through the CO cycle and so can be carried out inside a sealed box. The sealing is necessary to stop the CO either leaking out, or being oxidised to CO2 by excess outside air.
Adding an easily decomposed carbonate "energiser" such as barium carbonate breaks down to BaO + CO2 and this encourages the reaction
C (from the donor) + CO2 <—> 2 CO
increasing the overall abundance of CO and the activity of the carburising compound.
Its common knowledge that case-hardening was done with bone, but this is misleading. Although bone was used, the main carbon donor was hoof and horn. Bone contains some carbonates, but is mainly calcium phosphate (as hydroxylapatite). This doesnt have the beneficial effect on encouraging CO production and it can also supply phosphorus as an impurity into the steel alloy.
Modern use
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 usually formed by diffusing carbon (carburization), nitrogen (nitridization) 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.
The term case hardening is derived from the practicalities of the carburization process itself, which is essentially the same as the ancient process. The steel work piece is placed inside a case packed tight with a carbon-based case hardening compound. This is collectively known as a carburizing pack. The pack is put inside a hot furnace for a variable length of time. Time and temperature determines how deep into the surface the hardening extends. However, the depth of hardening is ultimately limited by the inability of carbon to diffuse deeply into solid steel, and a typical depth of surface hardening with this method is up to 1.5 mm. Other techniques are also used in modern carburizing, such as heating in a carbon rich atmosphere. Small items may be case hardened by repeated heating with a torch and quenching in a carbon rich medium, such as the commercial products Kasenit / Casenite or "Cherry Red". Older formulations of these compounds contain potentially toxic cyanide compounds, such as ferrocyanide compounds, while the more recent types such as Cherry Red do not.
Parts that are subject to high pressures and sharp impacts are still commonly case hardened. Examples include firing pins and rifle bolt faces, or engine camshafts. In these cases, the surfaces requiring the hardness may be hardened selectively, leaving the bulk of the part in its original tough state.
Firearms were a common item case hardened in the past, as they required precision machining best done on low carbon alloys, yet needed the hardness and wear resistance of a higher carbon alloy. Many modern replicas of older firearms, particularly single action revolvers, are still made with case hardened frames, or with case coloring, which simulates the mottled pattern left by traditional charcoal and bone case hardening.
Another common application of case hardening is on screws, particularly self-drilling screws. In order for the screws to be able to drill, cut and tap into other materials like steel, the drill point and the forming threads must be harder than the material(s) that it is drilling into. However if the whole screw is uniformly hard, it will become very brittle and it will break easily. This is overcome by ensuring that only the case is hardened and the core remains relatively soft. For screws and fasteners, case hardening is less complicated as it is achieved by heating and quenching in the form of heat treatment
For theft prevention, lock shackles and chains are often case-hardened to resist cutting while remaining less brittle inside to resist impacts. As case hardened components can only be cut by grinding, they are generally cut to length before case hardening.
Case hardening in wood
The term case hardened is also used to describe lumber or timber that has been improperly kiln-dried. If dried too quickly, wood shrinks heavily on the surface, compressing its still damp interior. This results in unrelieved stress. Case hardened wood may warp considerably and potentially dangerously when the stress is released by sawing.