Factors Influencing the Formation of Shrinkage Porosity in Grey Cast Iron
Licentiate thesis, 2008
The presence of shrinkage defects is of major concern in the production of grey iron castings. This type of defect can penetrate through the casting as a three-dimensional network. The aim of the present work is to develop knowledge and understanding about the foundry process in order to increase product quality by minimising the presence of shrinkage porosity. The general reason for shrinkage defects are the contractions which arise during solidification, but in the case of grey iron the situation is further complicated as graphite, a phase having lower density compared to the iron, expands when it precipitates. Besides these volumetric changes, the evolution of gases dissolved in the melt is another consideration as their solubility decreases with temperature.
It will be 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 process but not on temperature. The hydrogen concentration depends on holding time, approaching equilibrium due to exchanges between the melt and its environment. Events inside the mould are important for the formation of porosity and it will be shown how the concentration of hydrogen increases after mould filling. The amount of dissolved nitrogen also increases inside the mould, but not to the same extent. The solidification of grey iron starts with the precipitation of primary austenite and Dendrite Arm Spacing (DAS) can be related to the volumetric number of equiaxed crystals. The subsequent eutectic solidification depends on the primary solidification as the eutectic cell size is found to increase with increasing DAS. The columnar zone on the other hand is related to the equiaxed crystals through Columnar-to-Equiaxed Transition (CET). Shrinkage porosity is normally considered to be found in the areas last to freeze, and inter-dendritically. However, this work shows that shrinkage porosity also can be confined between grains in the primary structure. It will also be discussed how the columnar zone is suppressed because of a moving thermal centre and how this weak surface will constitute ingates for atmospheric gases compensating for the volumetric contractions.