Defect Formation during Solidification in Grey Iron Castings

Doctoral thesis, 2009

The presence of defects is of major concern in the production of grey iron castings. Some defects are merely an aesthetic problem while others can be detrimental for the performance of the component. Among the several possible defects, shrinkage porosity is one of the more challenging to understand and avoid. This type of defect can penetrate through the casting as a three-dimensional network. The purpose of this work has been to develop knowledge and understanding about the foundry process in order to increase product quality by eliminating or at least minimising the presence of shrinkage porosity in the production of vital engine components. This was done by developing fundamental knowledge concerning factors known to influence the presence of shrinkage porosity. Initially, an investigation was carried out in order to characterize shrinkage porosity commonly occurring in cylinder heads. Factors found to be important and contributing to the formation of shrinkage porosity were identified. It was found that this type of defect is formed at hot spots that have migrated during solidification. Knowledge of the migrating hot spot was used as a design criterion in the development of a geometry with the purpose of deliberately generating porosity. Based on this geometry a mechanism for the formation of this type of shrinkage porosity was proposed. The formation of shrinkage porosity depends on gases dissolved in the melt. In this work, their levels and variations in the melting process were investigated. It was shown how the solubility of oxygen depends on the temperature but not on the method used to produce the iron. On the other hand, the total amount of oxygen depends on the process but not on temperature. The discussion about gases also included hydrogen and nitrogen. The solidification of grey cast iron is complex. It is during the solidification that not only the final properties of the component but also the conditions for defect formation are established. Shrinkage porosity is formed due to volumetric changes followed by an inability to supply material to the contracting areas. The solidification of grey iron starts with nucleation and growth of the primary austenite followed by nucleation of eutectic cells. It was found that the microstructural constituents are interconnected. The importance of the macrostructure was also discussed and it was shown that shrinkage porosity can be found not only between eutectic cells but also between equiaxed crystals.