Basic processes controlling the nucleation of graphite nodules in chill cast iron

Abstract

This paper describes the results of an extensive microstructural study designed to explore the basic processes controlling the nucleation of graphite nodules in chill cast iron. The experimental melts were nodularized with magnesium ferrosilicon and then inoculated, either with foundry grade ferrosilicon or Superseed (a ferrosilicon which contains a small quantity of strontium). The work elucidates the role played by the additives to the melt, both in terms of their abilities to increase the density of nodules in castings and the more fundamental question of how they influence the mechanism of graphite nucleation. In addition, the phenomenon of inoculant fade is investigated and an explanation is presented for this behaviour. The results clearly demonstrate that graphite nodules are nucleated heterogeneously on particles formed in the melt from elements introduced via the additives, together with trace elements present in the molten iron. Electron microscopy and X-ray microanalysis techniques were used to identify the nucleating particles: they were found to have a duplex structure consisting of a sulphide core surrounded by an oxide shell. Graphite can grow epitaxially on these duplex nuclei. It is concluded that the efficiency of nodule formation is controlled by the kinetics of formation of the duplex nuclei and also by the nucleation rate of graphite on the nuclei. A model for the nucleation process, which accounts qualitatively for all the experimental results, is presented and discussed.