Evolution and Distribution of the Copper-rich Phase during Oxidation of an Iron-0.3wt%Copper Alloy at 1 150°C

Residual copper contents in low carbon steels lead to a cracking phenomenon known as surface hot shortness. This phenomenon is caused by a copper-rich liquid layer that forms due to copper enrichment as iron oxidizes during casting, reheating and/or hot rolling. Evolution of the copper-rich liquid layer is dependent on the competing processes of enrichment due to iron oxidation and diffusion of copper back into the metal. This paper presents comparisons between experiments and calculations of a fixed grid finite difference model that predicts the evolution of the copper-rich region. Experiments involved oxidizing an iron-0.3wt% copper alloy in a gold-image furnace equipped with thermogravimetric balance. Samples were oxidized at 1 150 degrees C in three atmospheres, dry air, wet air (15 vol% H2O), and argon-15vol%H2O. Model predictions agree with measured data for dry air oxidation at 1 150 degrees C for 60, 300, 420, 600, 900, and 1 200 s. Agreement was also obtained for iron oxidized for 1 800 and 2 700 s in argon-15vol%water vapor. However, model predictions deviated for samples oxidized 3 600 s in dry air, 3 600 s in water vapor, and 600 s in wet air. The deviations arise due to grain boundary penetration and diffusion of copper. Results suggest a critical amount of separated copper is necessary for substantial grain boundary penetration to occur and the required amount decreases when the gas contains H2O. The model was also used to estimate the evolution behavior of the liquid copper phase under industrially relevant conditions.