Abstract
The behavior of carbon deposition on as polished or oxidized Fe–Al alloys containing 05.7 mass% Al in 10% CH2–H2 mixture at 1 203 K was studied by metallogrphy and thermogravimetry to elucidate the effect of alumina on carbon deposition on the alloys in carburizing gas. Aluminium works to accelerate the deposition of filamentous carbon by promoting Fe3C decomposition. However, carbon deposition on as polished alloys containing 2.0 mass% Al or more is markedly retarded because of the formation of Al2O3 on the surface despite of the carburizing atmosphere. Under this condition, Al2O3 may have formed by the reaction of aluminum in the alloy with small amounts of oxidizing gases such as O2 and H2O present in the atmosphere. On oxidation in air, Fe2O3, Fe3O4, and FeO form on iron and small additions of aluminum (≤2.7 mass% Al) retard the formation of Fe3O4 and FeO and a further addition of aluminum up to 5.7 mass% causes the formation of Al2O3 besides FE2O3. The exposure of the alloys containing 2.0 mass% Al or less to 1–%CH4–H2 mixture after the oxidation in air causes a rapid mass gain. It may be due not only to the promoting action of aluminum for filamentous carbon deposition but also to the formation of active iron by reducing iron oxides and to the increase in the reaction area by spalling the scale. Carbon deposition on the alloys containing 2.7 mass% Al or more, however, is greatly depressed because Al2O3 forms or remains even during carburization.
