Decarburization (or decarbonization) is the process opposite to carburization, namely the reduction of carbon content.
The term is typically used in metallurgy, describing the reduction of the content of carbon in metals (usually steel). Decarburization occurs when the metal is heated to temperatures of 700 °C or above when carbon in the metal reacts with gases containing oxygen or hydrogen. The removal of carbon removes hard carbide phases resulting in a softening of the metal, primarily at the surfaces which are in contact with the decarburizing gas.
Decarburization can be either advantageous or detrimental, depending on the application for which the metal will be used. It is thus both something that can be done intentionally as a step in a manufacturing process, or something that happens as a side effect of a process (such as rolling) and must be either prevented or later reversed (such as via a carburization step).
The decarburization mechanism can be described as three distinct events: the reaction at the steel surface, the interstitial diffusion of carbon atoms and the dissolution of carbides within the steel.
The most common reactions are:
also called the Boudouard reaction
Other reactions are
Electrical steel is one material that uses decarburization in its production. To prevent the atmospheric gases from reacting with the metal itself, electrical steel is annealed in an atmosphere of nitrogen, hydrogen, and water vapor, where oxidation of the iron is specifically prevented by the proportions of hydrogen and water vapor so that the only reacting substance is carbon being made into carbon monoxide.
Stainless steel contains additives which are highly oxidizable, such as chromium and molybdenum. Such steels can only be decarburized by reacting with dry hydrogen, which has no water content, unlike wet hydrogen, which is produced in a way that includes some water and can otherwise be used for decarburization.
Incidental decarburization can be detrimental to surface properties in products (where carbon content is desirable) when done during heat treatment or after rolling or forging, because the material is only affected to a certain depth according to the temperature and duration of heating.This can be prevented by using an inert or reduced-pressure atmosphere, applying resistive heating for a short duration, by limiting the time that material is under high heat, as is done in a walking-beam furnace, or through restorative carburization, which uses a hydrocarbon atmosphere to transfer carbon into the surface of the material during annealing. The decarburized surface material can also be removed by grinding.
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The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today. It is named after its inventors, the German chemists Fritz Haber and Carl Bosch, who developed it in the first decade of the 20th century. The process converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using a metal catalyst under high temperatures and pressures:
Sulfuric acid (American spelling) or sulphuric acid (British spelling), also known as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with molecular formula H2SO4. It is a colourless, odourless, and viscous liquid that is soluble in water and is synthesized in reactions that are highly exothermic.
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Tungsten carbide is a chemical compound containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through a process called sintering for use in industrial machinery, cutting tools, abrasives, armor-piercing shells and jewellery.
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Lithium hydride is an inorganic compound with the formula LiH. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not soluble but reactive with all organic and protic solvents. It is soluble and nonreactive with certain molten salts such as lithium fluoride, lithium borohydride, and sodium hydride. With a molecular mass of slightly less than 8.0, it is the lightest ionic compound.
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A vacuum furnace is a type of furnace in which the product in the furnace is surrounded by a vacuum during processing. The absence of air or other gases prevents oxidation, heat loss from the product through convection, and removes a source of contamination. This enables the furnace to heat materials to temperatures as high as 3,000 °C (5,432 °F) with select materials. Maximum furnace temperatures and vacuum levels depend on melting points and vapor pressures of heated materials. Vacuum furnaces are used to carry out processes such as annealing, brazing, sintering and heat treatment with high consistency and low contamination.
Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on high-carbon, low-alloy steels. They are also used on medium and high-carbon steels, titanium, aluminium and molybdenum. In 2015, nitriding was used to generate unique duplex microstructure, known to be associated with strongly enhanced mechanical properties.
Lithium carbide, Li
2, often known as dilithium acetylide, is a chemical compound of lithium and carbon, an acetylide. It is an intermediate compound produced during radiocarbon dating procedures. Li
2 is one of an extensive range of lithium-carbon compounds which include the lithium-rich Li
5, and the graphite intercalation compounds LiC
12, and LiC
2 is the most thermodynamically-stable lithium-rich compound and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896 who reacted coal with lithium carbonate.
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Endothermic gas is a gas that inhibits or reverses oxidation on the surfaces it is in contact with. This gas is the product of incomplete combustion in a controlled environment. An example is hydrogen gas (H2), nitrogen gas (N2), and carbon monoxide (CO). The hydrogen and carbon monoxide are reducing agents, so they work together to shield surfaces from oxidation.