In situ observations of graphite formation during solidification of cast iron

Grey cast irons are a group of alloys with a unique combination of properties in terms of mechanical performance and castability. These properties are strongly related to their composite structure where graphite precipitates are embedded in a metallic matrix. The graphite precipitates form during solidification and growth continues throughout solid state cooling and the eutectoid transformation.

Years of research have greatly improved the understanding of the basic mechanisms that control graphite growth as well as the ability to control graphite morphology during industrial production of cast components. This is important since the shapes of the graphite precipitates play a determining role for the properties of grey cast irons.

However, to reach the full potential of cast irons and enable high-performance light-weight designs, more in-depth knowledge of the mechanisms controlling graphite growth and morphological developments is required. It is the aim of the present thesis to contribute with new insights within these fields.

In ductile cast irons graphite precipitate as spheroids which result in a material in which the mechanical properties are similar to those of steel. To predict the mechanical properties of ductile cast iron it is important to estimate the density of nodules as well as the distribution of nodule shapes and sizes at room temperature.

This emphasises the importance of models which can correctly describe the nucleation and growth of spheroidal graphite during solidification. In this thesis, the solidification of cast iron is studied with focus on formation and growth of spheroidal graphite. To this end, an experiment is conducted at the Diamond Light Source synchrotron facility in Harwell, UK: Employing an environmental cell devel-oped at the Manchester X-ray Imaging Facility at the University of Manchester, a small cylindrical sample of ductile cast iron is melted.

During re-solidification, the sample is continuously imaged. As a result, the first time resolved imaging of graphite formation in three dimensions is presented in the present thesis. A comparison of a one dimensional model for spheroidal graphite growth to experi-mental observations showed that the model can describe the observations relatively well despite its simplicity.

The investigation also showed that a gradually decreasing growth rate towards the end of solidification is not reflected in the model in spite of an extension to solid state growth presented in the present thesis. From the analysis it is clear that the presented data is of an unprecedented quality and that it represents a solid basis for validation of future models.

Solidification simulations of a ductile cast iron component highlights the importance of the nucleation model for the correct prediction of the final nodule density as well as the cooling curve. The tomographic data showed that nucleation within the studied sample is initiated at very high undercoolings and that it accelerates rapidly as eutectic solidification takes off.

Experimental data can be reasonably described by two different models both emphasising the importance of taking into account the fraction solid in nucleation models. Since very limited graphite particle movement is observed during the course of solidi-fication, the particles must be anchored in austenite and most likely also encapsulated while they grow.

Simultaneously, spherical graphite particles undergo significant morpho-logical changes and in many cases develop irregular features. Furthermore, the particles which are the most irregular after solidification grow significantly faster than their regular counterparts and it seems that fast growth is associated to the development of irregular features.

These observations are slightly surprising and highly interesting as it is usually assumed that irregular graphite shapes develop when the graphite is in contact with the liquid melt. These results have important implications for how degeneracy of spheroidal graphite should be understood and theoretically described in the future.