Selective Oxidation and Sub-surface Phase Transformations in Chromium-bearing Austenitic Steels

A series of Fe–15Cr–(2–3)Mo–(0.7–2.5)C (compositions in weight percent) steels was oxidised at 850°C and P_{O_2} = 5.8 × 10^–20 atm, where iron oxide is unstable. All grew external Cr₂O₃ scales according to parabolic kinetics. Depletion of chromium from alloy subsurface regions led to dissolution of chromium-rich carbides if the original alloy carbon level was less than 1.2%. Simultaneous decarburisation caused a transformation of the original austenitic or austenoferritic structure into single-phase ferrite, stabilised by the molybdenum. Diffusion analysis of the concentration profiles within this transformed zone led to satisfactory agreement with the known diffusion coefficient for chromium in ferrite. At high carbon levels, decarburisation was slow, resulting in low chromium concentrations at the internal alloy–carbide interfaces. In these cases, the carbide dissolution did not proceed and chromia scaling rates were slowed.