Decarburization

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For the corrosion caused by exposure to hydrogen, see Hydrogen embrittlement.

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 carbon in the metal reacts during heating with oxygen- or hydrogen-containing gases present in the atmosphere, starting at 700°C.^[1]

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).

Chemical reactions

The most common reactions are:

${\mathrm {C+CO_{2}\ \rightleftharpoons \ 2\ CO\quad }}$

also called the Boudouard reaction

${\mathrm {C+H_{2}O\ \rightleftharpoons \ CO+H_{2}}}$

${\mathrm {C+2\ H_{2}\ \rightleftharpoons \ CH_{4}}}$

Other reactions are^[1]

${\mathrm {C+{\tfrac {1}{2}}O_{2}\ \Rightarrow \ CO}}$

${\mathrm {C+O_{2}\ \Rightarrow \ CO_{2}}}$

${\mathrm {C+FeO\ \Rightarrow \ CO+Fe}}$

Electrical steel

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.^[1]

Stainless Steel

Because the components (like chromium and molybdenum) of some stainless steels are very oxidizable, these 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.^[1]

As a secondary effect

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.^[1] This can be prevented by using an inert or negative 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.^[1] The carburized surface material can also be removed using a grinding machine.^[1]

References

↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Schwartzman, LA (1973). The Great Soviet Encyclopedia (Print|format= requires |url= (help)) (3rd ed.). New York: Macmillan. Available in English here and in the original Russian here.

External links

Protecting Against Decarburization with Cress Furnaces

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