Cast Iron Model
Cast iron solidification is fundamentally different from most other casting alloys. While aluminum and steel contract during solidification, cast iron undergoes a complex interplay of shrinkage and expansion driven by the formation of different microstructural phases. Getting this balance right is critical: too much shrinkage leads to porosity, while unconstrained expansion can cause mold distortion or surface defects.
The Cast Iron Model in FLOW-3D CAST describes the solidification of hypo- and hyper-eutectic iron-carbon-silicon alloys, predicting the formation of graphite, austenite, and carbide phases during the eutectic reaction. By coupling microstructure evolution with volumetric changes and heat release, the model gives foundry engineers accurate predictions of shrinkage, expansion, and porosity risk across the full range of gray and ductile iron alloys.
How the Model Works
The Cast Iron Model computes the solidification path from a realistic iron-carbon phase diagram using the initial carbon and silicon composition of the melt. As the metal cools through the eutectic region, the model determines the local formation rates of graphite, austenite, and carbide at each point in the casting. The latent heat release is computed as a direct function of temperature along this freezing path, rather than using a simple fixed latent heat value.
The key to accurate cast iron simulation is the chilling susceptibility criterion, which controls how much carbide forms relative to graphite at each location. In well-inoculated gray iron, graphite dominates and the iron expands during eutectic solidification. Near mold walls and in thin sections where cooling is rapid, the chilling tendency increases and carbide formation dominates, producing local shrinkage and harder white iron zones. The model captures this spatial variation, enabling prediction of chill zones without requiring separate analysis. The actual value of the chilling susceptibility criterion must be determined experimentally, for example, from an ASTM chill-wedge test.
The Cast Iron Model works in conjunction with the Simple Shrinkage Model in FLOW-3D CAST, coupling the density changes from phase formation directly to porosity and shrinkage predictions. The model can also be used together with the general solidification model with flow. However, volume changes associated with the formation of different phases are only coupled to the simple shrinkage model that does not include flow.
Modeling Capabilities
The Cast Iron Model in FLOW-3D CAST predicts:
- Graphite, austenite, and carbide phase fractions throughout the casting as solidification progresses
- Local chilling tendency based on eutectic front velocity, enabling spatial prediction of white iron zones near mold walls and in thin sections
- Latent heat release as a function of the realistic iron freezing path, derived from carbon and silicon composition
- Liquid shrinkage during cooling from pouring temperature to the start of solidification
- Coupling with the Simple Porosity Model to predict porosity risk from net volumetric changes
- Sensitivity to inoculation state through the chilling susceptibility parameter, which can be calibrated from standard chill wedge tests
Applications
The Cast Iron Model is used in FLOW-3D CAST for:
- Predicting shrinkage porosity in gray and ductile iron castings based on alloy composition and cooling conditions
- Identifying chill zones in iron castings and evaluating the effect of inoculation on microstructure
- Optimizing riser design for iron castings, where the expansion/shrinkage balance varies across the casting
- Evaluating the effect of pouring temperature on graphite and carbide formation
- Supporting alloy composition selection to achieve the desired balance of graphite and carbide
- Assessing the risk of hard spots and machinability issues from local carbide formation