A replica Colt 1873 revolver, showing case hardening colors on the frame

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 material's surface, forming a thin layer of a harder alloy. In Metallurgy, hardening describes techniques to increase the Hardness of a material Carbon steel, also called plain carbon steel, is Steel where the main alloying constituent is Carbon. An alloy is a Solid solution or Homogeneous mixture of two or more elements, at least one of which is a Metal, which itself has 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. Pattern welding is the practice in Sword and Knife making of forming a blade of several Metal pieces of differing composition that are forge-welded

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. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 A bloomery is a type of Furnace once widely used for Smelting Iron from its oxides. QtubIronPillarJPG|thumb|right| Iron pillar at Delhi India containing 98% wrought iron]] Wrought iron is commercially pure Iron. Cast iron usually refers to grey cast iron, but identifies a large group of Ferrous Alloys which solidify with a Eutectic. Since the high carbon iron is hot short, meaning it fractures and crumbles when forged, it was not useful without more smelting. A forge is the workplace of a smith or a Blacksmith. A forge is sometimes referred to as a smithy. The wrought iron, with nearly no carbon in it, was very malleable and ductile, but not very hard. For malleability in Cryptography, see Malleability (cryptography. Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires" without

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. Bones are rigid organs that form part of the Endoskeleton of Vertebrates They function to move support and protect the various organs of the body produce Charcoal' is the blackish residue consisting of impure Carbon obtained by removing water and other volatile constituents from Animal and Vegetation Leather is a material created through the Tanning of hides and Skins of Animals primarily Cattlehide The Tanning process HoofRearHoovesjpg|thumb|200px|right|Rear hooves of a horse]] A hoof is the tip of a Toe of an Ungulate Mammal, strengthened by a thick horny ( Salt is a Dietary mineral composed primarily of Sodium chloride that is essential for Animal life but toxic to most land plants Urine is a liquid waste product of the body secreted by the Kidneys by a process of filtration from Blood and Excreted through the Urethra. 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. Bluing is a Passivation process in which steel is partially protected against Rust, and is named after the blue-black appearance of the resulting protective 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. A replica is a copy that is relatively indistinguishable from the original A firearm is a Tool that projects either single or multiple Projectiles at high velocity through a controlled explosion The Colt Single Action Army Handgun (also known as the Colt Peacemaker, Single Action Army or SAA, Colt. rEVOLVEr is the fourth studio album by Swedish metal band The Haunted. Lever-action is a type of Firearm action which uses a lever located around the trigger guard area (often including the trigger guard itself to load fresh cartridges

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.

Chemistry

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. Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. This takes place with pure carbon, but unworkably slowly. Although oxygen is required for this process it's 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. Barium carbonate ( Ba[[Carbonate CO]]3 also known as witherite, is a Chemical compound used in Rat poison, Bricks

It's 'common knowledge' that case-hardening was done with bone, but this is misleading. Common knowledge is what "everybody knows" usually with reference to the Community in which the term is used Although bone was used, the main carbon donor was hoof and horn. Bone contains some carbonates, but is mainly calcium phosphate (as hydroxylapatite). Hydroxylapatite, also called hydroxyapatite, is a Mineral. It is a naturally occurring form of calcium Apatite with the formula Ca5(PO43(OH This doesn't have the beneficial effect on encouraging CO production and it can also supply phosphorus as an impurity into the steel alloy. Phosphorus, (ˈfɒsfərəs is the Chemical element that has the symbol P and Atomic number 15

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. Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 3% (see plain-carbon steel for more information). Carbon steel, also called plain carbon steel, is Steel where the main alloying constituent is Carbon. 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. Carburization (often referred to as carburizing) is a heat treatment process which iron or steel is heated to "below the melting point in the presence of a solid liquid Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Nitridization, also known as nitriding, is a process which introduces Nitrogen in the surface of a material Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. In chemistry a boride is a chemical compound between boron and a less electronegative element

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. Carburization (often referred to as carburizing) is a heat treatment process which iron or steel is heated to "below the melting point in the presence of a solid liquid 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. A cyanide is any Chemical compound that contains the cyano group (C≡N which consists of a Carbon Atom triple-bonded to a Ferrocyanide is the name of the Anion Fe ( C[[Nitrogen N]]64− ^[1][2]

Applications

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. A firing pin or striker is part of the firing mechanism used in a Firearm or explosive device e A bolt is a mechanical part of a Firearm that blocks the rear of the chamber while the powder burns The camshaft is an apparatus often used in Piston engines to operate Poppet valves It consists of a cylindrical rod running the length of the Cylinder bank 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. A trigger is a Mechanism that actuates the firing sequence of Firearms Triggers almost universally consist of Levers or buttons actuated by the Index

Another common application of case hardening is on screws, particularly self-drilling screws. Self-tapping is the ability of a Screw to advance when turned while creating its own thread 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

Case hardening in wood

The term case hardened is also used to describe lumber or timber that has been improperly kiln-dried. Lumber or timber is Wood in any of its stages from felling through readiness for use as structural Material for Construction, or Lumber or timber is Wood in any of its stages from felling through readiness for use as structural Material for Construction, or Kilns are thermally insulated chambers or Ovens in which controlled temperature regimes are produced If dried too quickly, wood shrinks heavily on the surface, compressing its still damp interior. Wood is hard fibrous lignified structural tissue produced as secondary Xylem in the stems of Woody plants notably trees but also shrubs This results in unrelieved stress. Stress is a measure of the average amount of Force exerted per unit Area. Case hardened wood may warp considerably and potentially dangerously when the stress is released by sawing. A saw is a Tool that uses a hard blade or wire with an abrasive edge to cut through softer materials

See also

• Carbonitriding
• Carburization
• Differential hardening
• Induction hardening
• Shot peening
• Surface engineering

References

External links

• Case Hardening
• Surface Hardening of Steels
• Case Hardening Steel and Metal