Carburization of Fe-Cr Alloys During Oxidation in Dry Carbon Dioxide

Iron‐chromium alloys (1–15 w/o [weight per cent] Cr) oxidized in dry carbon dioxide at 700°, 900°, and 1100°form duplex scales which are similar in structure to those generated in atmospheres, i.e. , a detached but continuous outer layer of iron oxides (predominantly ) and a porous inner layer of and Fe‐Cr spinel particles. Carburization accompanies this oxidation at all temperatures, and the resulting microstructural changes, the carbides formed, the hardness increases and the carbon content of homogeneous specimens are related to the equilibria of the Fe‐Cr‐C system. The carburization and the continued high oxidation rates, in spite of the detached duplex structure of the scale, require that a gaseous transfer of oxygen and carbon,occur within the voids separating the outer and inner oxide layers. This requirement suggests a carbon permeation of the dense outer scale and the generation of a atmosphere in these voids which is carburizing as well as oxidizing to the alloy.