Trying to Get By Without a Riser

Stator housings are produced at the ITT foundry in Emmaboda, Sweden. In one housing, shrinkage porosity occurred at the top of the castings slightly below the surface. Because this area would be machined for a tightening ring, no porosity could be allowed.

With the grey iron GJL-250 alloy used, it was expected that graphite expansion would compensate for the austenite shrinkage during solidification and lead to a sound casting. What was the reason for this porosity? The MAGMASOFT® simulation showed shrinkage in the problem zone, but also showed why the graphite expansion did not compensate for the solidification shrinkage. The reason was that the liquid shrinkage in the castings before solidification was not compensated by the gating system. This is typically a requirement for riser-less casting. One remedy for the problem was to attach a small riser to the gating system, which would deliver feed metal to compensate for the liquid shrinkage. Simulation showed no porosity for the two castings where this riser is active. For the two not being fed in this way, the original problem remains. Casting trials in the foundry have proven that this solution works.

Porosity prediction for original riser-less layout shows problems near the top surface (left) and porosity prediction for a layout shows that a small riser compensates liquid shrinkage (middle). The photo right shows the pattern plate for the original layout.

* Courtesy of ITT, Sweden

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