On the Solidification and Feeding of a Ductile Iron Casting
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Mater. Res. Soc. Symp. Proc. Vol. 1243 © 2010 Materials Research Society
On the Solidification and Feeding of a Ductile Iron Casting Eudoxio A. Ramos Gómez1, Marco A. Ramírez-Argáez2, Carlos González-Rivera2. 1 PCeIM, Universidad Nacional Autónoma de México, Mexico City, Mexico, 2 Facultad de Química, Universidad Nacional Autónoma de México, Mexico City, Mexico ABSTRACT Solidification of a simple casting made of ductile iron is mathematically modeled in this work. The model is able to numerically simulate the cooling rate and solidification of the whole casting system composed by a cubic piece, a blind riser connected through a rectangular neck, and immersed in a green sand mold. The center of the neck acts as a valve that allows the flow of liquid metal between the casting and the riser based on the feeding technique known as Pressure Control Risering (PCR). The developed model couples the energy conservation equation and the solidification kinetics of ductile iron, through the statement of proper nucleation and growth laws. This model is satisfactorily validated by comparing the thermal histories predictions with experimental cooling curves obtained in the foundry laboratory for the same casting. According to a process analysis developed in this work, the pouring temperature is the variable that affects the most the solidification and the feeding behavior, since it increases significantly the solidification times in all regions of the casting system. INTRODUCTION Porosity due to contractions is still one of the most frequent rejecting causes of ductile iron sand castings. Despite these phenomena are not new, mechanisms involved in the appearance of these defects in ductile iron differ significantly from other metals. From experimental evidence, it is currently accepted that ductile iron suffers an expansion during solidification that builds up a pressure high enough to plastically deform the sand mold to yield swollen castings that contain contraction defects [1, 2]. The implementation of a risering method is a way to avoid the presence of these defects since it controls the degree of pressure due to expansion during solidification of the casting. The risering technique called Pressure Control Risering (PCR) controls pressure in a way that the liquid iron is always under a positive pressure relative to the atmosphere, but never reaches high values of pressure that would plastically deform the green sand mould [3]. THEORY The casting studied in this work consists of a simple iron cube (casting-neck-feeder) contained in a green sand mould that transfers heat to the surroundings by free natural convection (see Figure 1). It is assumed that initially the entire casting plus the feeder are filled with liquid iron of eutectic composition being all initially at the pouring temperature. It is also assumed that conduction controls heat transfer and thus convection is neglected in liquid iron, volume changes are not considered in the calculations. The system is assumed to be continuous and therefore the gap formed between
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