Compacted graphite iron is rapidly becoming an attractive alternative for engine applications in the automotive industry. The improved process control now available that allows CGI components to be cast in a reproducible and reliable way has been the driving force for this development. To be able to optimise the properties in a CGI component it is crucial to have extensive knowledge of the factors influencing microstructure formation in the material.

To understand microstructure formation and how the microstructure influences the mechanical properties a series casting trials were performed. A hemispherical sampling cup was used enabling temperature measurements as well as evaluation of the microstructure and a geometry consisting of cylinders of different diameters was used to obtain tensile test bars. A range of cooling rates and chemical compositions were studied in the casting experiments, resulting in substantial differences in both graphite morphology and matrix structure, suggesting that mechanical properties will vary accordingly. Nineteen trials were cast investigating the influence of varying: nodularity treatment level, Cu-content, Si-content, Sn-content and varying content of carbide promoting elements (Cr, Mn and Mo).

It was found that the majority of the investigated alloying elements (Cu, Mg, Si and Sn) affect the graphite morphology. Similar to other graphitic cast irons Cu, Sn and carbide promoting elements promote pearlite formation in CGI, while Si promotes ferrite formation. It was also found that higher nodularity favours a pearlitic matrix structure. Mechanical properties are generally raised by increasing nodularity and increasing pearlite content, but a contribution from solution hardening of ferrite was found at high Si-contents. The influence of carbides on mechanical properties was negligible, for the investigated alloying contents and cooling conditions.

From the microstructure investigation it was found that CGI is prone to develop a ferritic matrix. It was also clear that segregation of pearlite and ferrite promoting elements influenced the ferrite content. A deterministic model was developed to describe the ferrite growth in CGI. The growth rate of the ferrite border was assumed to be determined by an interface reaction at the ferrite-graphite interface, and that specifically the amount of carbon atoms that can be added to the graphite played a dominant role.

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.