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the reduction of the carbon concentration in steels and alloys that takes place upon heating in oxidizing media, as well as in hydrogen (dry or wet).
Decarburization of steel and alloys may produce both harmful and beneficial effects. Decarburization during heat treatment or during heating prior to rolling or forging extends into the interior of the metal to a depth that depends on the temperature and duration of heating, and it leads to degradation of surface properties in the finished articles and to rejects.
Decarburization is the result of various complex reactions: C + ½O2 = CO; C + O2 = CO2; C + CO2 = 2CO; C + H2O = CO + H2; C + 2H2 = CH4; C + FeO = CO + Fe. The reactions are initiated and proceed at appreciable rates at temperatures above 700°C. Decarburization may be eliminated by heating in furnaces with a protective atmosphere or under vacuum. Decarburization in open furnaces is minimized by reducing the residence time of metal at high temperatures (for example, in walking-beam furnaces) or by using rapid electrical contact heating of the metal. Decarburized layers formed during production may be removed mechanically (on grinding machines). Annealing of metal in reducing gas mixtures containing natural gas or other hydrocarbons is also used; this results in enrichment of the surface of articles with carbon (restorative carburizing).
As a type of chemical heat treatment, decarburization improves the properties of metals and alloys in which carbon is an undesirable impurity (transformer steel and stainless steels). Refining decarburization is achieved by heating in gaseous media of fixed composition, which is selected to prevent the base metal from participating in chemical reactions. Transformer steel is annealed in N2-H2-H2O mixtures. In this case, the ratio of the H2 concentration to the H2O concentration is such that the iron is not oxidized, whereas the carbon forms CO and is removed. Stainless steels and similar alloys, which contain readily oxidizable alloying elements, are subjected to refining annealing in dry hydrogen.
L. A. SHVARTSMAN