Three-stage model for nucleation of graphite in grey cast iron

Abstract

A three-stage model for the nucleation of graphite in grey irons has been proposed. Stage 1 involves formation of small oxide based sites (usually <2·0 μm) in the melt. Stage 2 involved precipitation of complex (Mn,X)S compounds (usually <5·0 μm) nucleated by stage 1 microinclusions. Finally, in stage 3, graphite nucleates on the sides of the (Mn,X)S compounds, which have low crystallographic misfit with graphite. Three groups of elements are important to sustain this sequence for effective graphite nucleation. These are strong deoxidising elements, such as Al and Zr, to form a high count of very small stage 1 microinclusions, Mn and S to sustain MnS type sulphide formation, and inoculating elements, which act in the first stage and/or in the second stage of the graphite nucleation sequence. Inoculating elements improve the capability of (Mn,X)S compounds to nucleate graphite. In inoculated irons, the (Mn,X)S compounds are more complex. They have a lower Mn/S ratio and higher capability to nucleate graphite, especially when preconditioning/inoculating elements contribute with a high count of effective stage 1 particles.