What factors cause shrinkage porosity in ductile iron castings?

General rules affecting the shrinkage porosity rate of ductile iron castings;

1. The modulus of ductile iron castings. If the casting modulus is greater than 2.5, it is easy to achieve riserless casting. However, some experts are skeptical about this limit. Generally speaking, thick castings can easily be cast without shrinkage porosity due to the expansion of graphite. At this point, the carbon equivalent should not exceed 4.5% to avoid floating graphite. Fine castings with dispersed hotspots are prone to shrinkage porosity, which can be addressed through intense cooling, chromite sand, or localized internal risers. Special attention should be paid to the feeding and shrinkage of the pouring riser system. Generally, hot risers should be used whenever possible to avoid cold risers.

2. Proper attention to the rigidity of the sand box and the hardness of the sand mold. The setup should not be too full in terms of sand box rigidity and sand mold density.

3. Reasonableness of the pouring and riser design. It is preferable to use hot risers and intense cooling, as cold risers have poor feeding effects.

4. Cooling rate of the mold.

5. Reasonable selection of pouring temperature and speed. For some thick castings, appropriately increasing the pouring temperature and extending the pouring speed can solve shrinkage problems. This also helps in the floating out of secondary oxidized slag from the casting, increasing defect detection compliance.

6. Reasonably choosing the chemical composition and maintaining appropriate residual magnesium and rare earth content.

7. Under the cooling conditions of the sand mold, striving for more graphite spheres is beneficial in reducing shrinkage porosity and improving mechanical properties.

8. To obtain better raw materials and good metallurgical quality of molten iron, special attention should be paid to not keeping the molten iron at high temperatures for too long before pouring. Pre-treatment should be done before pouring to increase the graphite nucleation in the molten iron, which can increase the number of graphite spheres and reduce shrinkage porosity.