Numerical modelling of solidification of thin walled hypereutectic ductile cast iron

Numerical simulation of solidification of ductile cast iron is normally based on a model where graphite nodules are surrounded by an austenite shell. The two phases are then growing as two concentric spheres governed by diffusion of carbon through the austenite shell. Experiments have however shown the presence of austenite dendrites even in hypereutectic castings.

In thin-walled castings the presence of austenite dendrites is even more pronounced, which increases the risk of shrinkage porosities. This off-eutectic austenite is therefore an important part that should be taken into account during simulation of thin-walled ductile iron castings. Simulations have been performed with a 1-D numerical solidi¬fication model that includes the precipitation of non-eutectic austenite during the eutectic stage.

Results from the simulations have been compared with experimental castings with wall thick¬nesses between 2.8 and 8 mm. The experiments have shown that the solidification of thin-walled castings is divided into two stages where the first stage is relatively small with little or no reca¬lescence and the second stage is the main stage with a larger recalescence.

In the 8 mm plates the eutectic solidification had only one main stage. The simulations reveal that the first stage of solidification can be explained by precipitation of off-eutectic austenite