Effect of oxide scale on carbon deposition on Fe-Ni alloys in carburizing gas

The behavior of carbon deposition on preoxidized Fe-Ni alloys containing 0 to 57.0 mass pct Ni in 10 pct CH₄-H₂ mixture at 1203 K was studied by metallography and thermogravimetry. Nickel retarded carburization and carbon deposition by lowering the solubility limit of graphite in austenite and by reducing catalytic activity for the pyrolytic reaction of CH₄. On oxidation in air, the addition of nickel to iron depressed the development of FeO and, thereby, caused a significant decrease in the thickness of the scale. The exposure of the alloys to 10 pct CH₄-H₂ mixture after the oxidation in air led to a sudden mass loss in the early stage and then a rapid mass gain. This mass change is primarily ascribed to mass loss by reduction of iron oxides and to mass gain by carbon deposition. The rapid mass gain by carbon deposition is probably due to the formation of active iron by reduction of iron oxides and to the increase in the reaction area by spallation of the scale; the active iron formed may promote filamentous carbon deposition through Fe₃C formation and decomposition. Carbon deposition on the alloys containing 27.2 mass pct Ni or more was considerably retarded because of the formation of a thin oxide scale which consists of α-Fe₂O₃ and spinel (Ni_xFe_3−xO₄) and the reduction of catalysis by enrichment of nickel in the subscale. However, the amounts of carbon deposition increased compared with those on the as-polished alloys, owing to the presence of reducible iron oxides.