Evolution of the Enriched Layer at the Oxide/Metal Interface during Oxidation of Recycled Steels with Copper and Copper+Nickel Residuals

Abstract

The presence of the residual element copper in recycled steels causes a surface cracking phenomenon during thermo-mechanical processing which is known as “hot shortness”. The cracks result from a copper-rich liquid that forms at the oxide/metal interface and subsequently embrittles austenite grain boundaries. Minimizing formation of the liquid phase would reduce or eliminate cracking. Thus, the evolution of the liquid layer is an important consideration when designing an optimal thermomechanical processing cycle in scrap-based steel plants. The time evolution of the liquid phase is dependent on the competing processes of enrichment rate due to iron oxidation and the rate of copper back-diffusion into the steel. This paper presents a fixed grid finite difference model that predicts the evolution of the enriched region as a result of a given oxidation kinetics and solution of Fick’s 2nd law. The model predictions are in agreement with measured data for the case of an iron alloy containing 0.3 wt% copper oxidized in air at 1150°C. Model predictions indicate that initial copper content, oxidation kinetics, and alloy microstructure (i.e. grain boundary diffusion) have the most significant influence on the copper-rich layer whereas the solubility increase due to nickel additions was not found to have an appreciable influence.