Compacted graphite iron : On solidification phenomena related to shrinkage defects

Abstract: An improved understanding of solidification of compacted graphite iron (CGI) is necessary to predict shrinkage related casting defects. Spheroidal graphite is not only found in ductile iron (SGI) but usually also in CGI, but its characteristics and role in this material is less understood. Uncertainties also remain regarding the segregation of alloying elements in cast irons. This is important because of its important role in the solidification process. The dendritic austenite structure has received little attention in cast iron research. A good understanding of the development of this structure is important to understand feeding of melt through the solidifying material.Nodularity is a measure of the amount of spheroidal versus compacted graphite in the microstructure. At a lower nodularity, the compacted graphite tips were found to grow in contact with the melt for a longer time before being encapsulated in the austenite. Moreover, as the nodularity was reduced, the subpopulation of larger spheroidal graphite gradually disappeared, reducing the bimodal size distribution to unimodal.Segregation of Si, Mn and Cu in SGI and CGI with a solidification time of near 10 min was found to be rather predictable under assumptions of no diffusion in austenite and complete mixing in the melt. Gradients of these elements contribute to a decrease in the driving force for diffusion of carbon into the austenite from the liquid, which is important for the growth of graphite which is separated from the liquid by austenite.During solidification of a near-eutectic CGI, the carbon concentration of austenite was found to deviate considerably from local equilibrium with graphite during solidification. This is important to consider in growth models for graphite by diffusion of carbon through a barrier of austenite.Micropores were shown to have displaced liquid from the solid structure at a late stage of solidification when solidification was slowing down and the temperature of the casting was falling at an increasing rate.The development of dendritic austenite in a near eutectic CGI was investigated. The contact area between liquid and the dendritic structure exceeded the contact area between liquid and eutectic cells through the dominant part of solidification. This highlights the importance of good understanding of the development of this structure in order to predict feeding of melt through the solidifying material. The coarsening of the structure was found to proceed at a higher rate compared to studies under isothermal condition. The dendritic structure continued to grow in parallel with the eutectic by a combination of thickening and dendritic growth.