About Equilibrium Mode Ruling Ferritic Transformation in Low-Alloy SGI

Abstract

Ferrite precipitating around the graphite nodules shaping the typical bull’s-eye microstructure could grow under negligible partitioning local equilibrium or under paraequilibrium conditions, as both imply that ferrite inherits the composition of the parent austenite. The first mechanism has been rejected by other researchers by means of simple calculations of the silicon spike width necessary for local equilibrium conditions to take place. Nevertheless, experimental analyses are necessary to verify this conclusion. In this study, transmission electron microscopy has been used to assess the presence of a silicon spike in front of the growing ferrite interface. The outcome allowed the authors to confirm that a paraequilibrium mode governs the transformation, supporting the conclusions of previous calculations. In addition, some issues about ferrite growth modeling are discussed.