Microstructure Formation and Mechanical Properties in Compacted Graphite Iron
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.