Evolution Mechanisms of MgO·Al₂O₃ Inclusions by Cerium in Spring Steel Used in Fasteners of High-speed Railway

Abstract

The effect of rare earth metal addition on the non-metallic inclusions in spring steel used in fastener of high speed railway was investigated by metallographic examination; SEM-EDS and component analysis, aiming at deform those harmful inclusions to improve service life of spring steel. MgO·Al₂O₃ inclusions were found in present experimental steel, which is also confirmed by the stability diagram of MgO/MgO·Al₂O₃/Al₂O₃ from thermodynamic consideration. After Ce addition, the evolution process of Al₂O₃·MgO inclusions was determined through the surface and line scanning. The effects of time and Ce content on the evolution of Al₂O₃·MgO inclusions were examined. It was indicated that Al₂O₃·MgO inclusions were wrapped by rare earth inclusions to form a ring like shape Ce-riched band around the inclusion, which would be useful to improve fatigue and corrosion resistance of spring steel. It was found that diffusion of Ce³⁺, Al³⁺ and Mg²⁺ in inclusions core and intermediate layer would be the limited step during evolutions of inclusions.