Microstructure Formation During Solidification and Solid State Transformation in Compacted Graphite Iron
Abstract: Compacted graphite iron (CGI) is rapidly becoming an attractive alternative material for engine components in the automotive industry, replacing lamellar graphite iron (LGI) in applications where high mechanical strength is desired. However, the gain in mechanical strength comes with a cost; thermal conductivity, process control and machining are three areas that are more challenging for CGI. This generates a need for research regarding various aspects concerning CGI. In this thesis the microstructure formation during solidification and solid state transformation will be the focus of interest.The phase transformations relevant for microstructure formation of importance to properties in CGI were studied. Experiments were performed in an industrial foundry giving this research direct relevance to regular production of CGI castings. Solidification of the grey (graphite/austenite) eutectic will be discussed, focusing on some relevant aspects influencing the graphite morphology of CGI. The formation of graphite nodules has been investigated by studying colour-etched microstructures. In a material containing mainly CGI cells it was found that nodules form either early during solidification as a consequence of high undercooling or late in the solidification sequence due to a combination of high undercooling and segregation of nodularising elements. Solidification of the white (cementite/austenite) eutectic was studied using chill wedges and the influence of some alloying elements on the amount of carbides was examined. To further enhance the understanding of carbide formation in CGI a commercial casting simulation software was used to correlate real castings to simulations. It was found that the alloying elements investigated influence the carbide formation in a similar way as in other graphitic cast irons and that high nodularity CGI is more prone to chill formation than low nodularity CGI. The solid state transformation was studied and a deterministic model was developed. The model divides a eutectic cell into layers, in order to take into account segregation of alloying elements, which was observed to be influential for the ferrite growth. Moreover, the effect of alloying elements on mechanical properties (tensile properties and hardness) was evaluated. Properties were correlated to microstructural features originating from both solidification and solid state transformations. The trends found generally confirmed previous results regarding properties in graphitic cast irons.
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