Effect of residual Cu and Ni on high-temperature oxidation behavior of 40Cr steel

To elucidate the influence of residual Cu and Ni on the high-temperature oxidation behavior of 40Cr medium-carbon alloy steel, a comparative analysis was conducted between steels produced via the electric arc furnace (EAF) with scrap route and the blast furnace-basic oxygen furnace (BF-BOF) route. Thermogravimetric analysis (TGA) demonstrated that the residual Cu and Ni increased the parabolic oxidation rate from 3.18 × 10 −4 mg cm −2 s −1 to 4.59 × 10 −4 mg cm −2 s −1 at 1100 °C. Microstructural characterization confirmed that the synergistic enrichment of copper and nickel at the oxide scale/metal interface acts as a selective barrier, effectively suppressing internal oxidation. The integration of TKD analysis and electrochemical measurements corroborates that the occlusion of voids within the oxide scale by Cu and Ni significantly enhances the scale density. Furthermore, nanoindentation experiments were employed to quantify the elastic modulus mismatch across the interface, innovatively demonstrating that these residual elements enhanced the adhesion between the oxide scale and the substrate by 41.4%. Molecular dynamics (MD) simulations further provided theoretical validation at the atomic scale. Under the synergistic influence of residual Cu and Ni, the thermodynamic activity for inward oxygen diffusion is reduced. The outward migration of Fe cations through the Cu-rich liquid phase emerged as the predominant diffusion pathway. This transition in the diffusion regime restricts sub-surface internal oxidation in favor of external scale development, ultimately yielding a more adherent and compact scale/metal structure.