Decarburization

Decarburization(ordecarbonization) is the process of decreasingcarboncontent, which is the opposite ofcarburization.

The term is typically used in metallurgy, describing the decrease of the content ofcarboninmetals(usuallysteel). Decarburization occurs when the metal is heated to temperatures of 700 °C or above when carbon in the metal reacts with gases containingoxygenorhydrogen.^[1]The removal of carbon removes hardcarbidephases 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 asrolling) 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.^[2]

Chemical reactions

The most common reactions are:

{\ce {C + CO2 <=> 2CO}}

also called theBoudouard reaction

{\ce {C + H2O <=> CO + H2}}

{\ce {C + 2H2 <=> CH4}}

Other reactions are^[1]

{\ce {C + 1/2O2 -> CO}}

{\ce {C + O2 -> CO2}}

{\ce {C + FeO -> CO + Fe}}

Electrical steel

Electrical steelis one material that uses decarburization in its production. To prevent the atmospheric gases from reacting with the metal itself, electrical steel isannealedin an atmosphere ofnitrogen,hydrogen,andwater vapor,where oxidation of theironis specifically prevented by the proportions of hydrogen and water vapor so that the only reacting substance is carbon being oxidized intocarbon monoxide(CO).^[1]

Stainless steel

Stainless steelcontains additives which are highly oxidizable, such aschromiumandmolybdenum.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.^[1]

As a secondary effect

Incidental decarburization can be detrimental to surface properties in products (where carbon content is desirable) when done duringheat treatmentor 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 orreduced-pressureatmosphere, applyingresistive heatingfor a short duration, by limiting the time that the material is submitted to a high heat, as it is done in a walking-beam furnace, or through restorative carburization, which uses ahydrocarbonatmosphere to transfer carbon into the surface of the material during annealing.^[1]The decarburized surface of the material can also be removed bygrinding.^[1]

References

^^a ^b ^c ^d ^e ^f ^gShvartsman, L.A.(1973). "Decarburization".The Great Soviet Encyclopedia(Print) (3rd ed.). New York: Macmillan.Available in Englishhereand in the original Russianhere.
^Alvarenga H.D.; Van de Putte T.; Van Steenberge N.; Sietsma J.; Terryn H. (Apr 2009). "Influence of carbide morphology and microstructure on the kinetics of superficial decarburization of C-Mn Steels".Metall. Mater. Trans. A.46:123–133.doi:10.1007/s11661-014-2600-y.S2CID 136871961.

External links

Protecting Against Decarburization with Cress Furnaces Archived2019-07-02 at theWayback Machine

[TSB-1] ^^a ^b ^c ^d ^e ^f ^gShvartsman, L.A.(1973). "Decarburization".The Great Soviet Encyclopedia(Print) (3rd ed.). New York: Macmillan.Available in Englishhereand in the original Russianhere.

[Alvarenga-2] Alvarenga H.D.; Van de Putte T.; Van Steenberge N.; Sietsma J.; Terryn H. (Apr 2009). "Influence of carbide morphology and microstructure on the kinetics of superficial decarburization of C-Mn Steels".Metall. Mater. Trans. A.46:123–133.doi:10.1007/s11661-014-2600-y.S2CID 136871961.

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