Decarburization of 60Si2MnA in Atmospheres Containing Different Levels of Oxygen, Water Vapour and Carbon Dioxide at 700-1000 °C

The decarburization behaviour of 60Si2MnA in atmospheres containing 0-21% O-2, < 20 ppm-17%H2O, and with or without 8%CO2, at 700-1000 degrees C, was investigated. The new findings of the current study were: (a) severe decarburization was associated with the formation of wustite (FeO) scale on the steel surface, (b) the carbon activity at the steel-FeO interface was most likely determined by the reaction equilibrium between FeO and dissolved carbon in steel, (c) when a ferrite layer was able to form, the decarburization tendency was determined by the relative carbon permeability (defined as the product of carbon concentration difference at the two interfaces of the ferrite layer and carbon diffusivity) through the ferrite layer, and therefore, (d) the decarburization tendency at 800 degrees C was greater than those at 700 and 900 degrees C as the relative carbon permeability at 800 degrees C was the greatest. If FeO was absent when heating in dry O-2-containing gases, however, possibly as a result of the formation of a SiO2 layer at the steel surface, decarburization was very much alleviated or avoided. At 1000 degrees C, the decarburization tendency was alleviated even when FeO was able to form because formation of a ferrite layer was not possible and carbon diffusivity in austenite was much lower than that in ferrite. A preformed oxide scale was effective in providing decarburization protection only when the steel was exposed to dry O-2-containing atmospheres.