Decarburization

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Steel heating is sometimes utilized for altering carbon content. Upon heating of steel within oxidizing environment, oxygen makes a combination with iron for forming iron-oxide layer. This layer protects steel from the decarburization. Upon conversion of steel into austenite, oxygen & iron combine for forming slag that does not provide any protection against decarburization. Formation of scale and slag actually enhances decarburization, because iron oxide maintains contact between oxygen and decarburization-zone even when steel is transferred to an oxygen free environment, for example coals of forge. Carbon atoms thus start combining with surrounding slag and scale for forming both carbon dioxide and carbon monoxide, which is released in to the air.

There is a small percentage of the carbon within steel. This carbon migrates freely within gamma iron. Upon exposure of the austenized steel to the air for longer times, carbon content within steel can thus be lowered. This process is opposite to the results that are obtained upon heating of steel within reducing environment, within which here is slow diffusion of carbon further in to the metal. Within oxidizing environment, carbon diffuses readily outwards, so that austenized-steel is susceptible to the decarburization. Mostly this is used for the cast steel, in which high carbon content is required to cast, but lower carbon content is required within finished product. Often it is used on the cast irons for producing malleable cast-iron, through a process known as white tempering. Such tendency of decarburization creates problem for other operations for example blacksmithing, in which austenization of steel is desirable for shortest possible time in order to prevent extensive decarburization.

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