To achieve superior nodularity in our ductile iron casting process, we must pay attention to pure pig iron, adding proper alloy, good inoculation and temperature controlling.
1. Starting off Strong With Purer Pig Iron
Pig iron forms the foundation of a ductile cast iron casting and the overwhelming percentage of its composition, so it's imperative we start off right.
2. Adding Magnesium
Nodule formation is achieved by adding nodulizing elements, most commonly magnesium (magnesium boils at 1100 °C and iron melts at 1500 °C) and, less often now, cerium (usually in the form of mischmetal).
3. The Intricacies of Inoculation
As molten iron is poured over the master alloy, fine magnesium particles are infused into the melt, a process known as inoculation.
The size of the inoculation chamber, the rate at which we pour the molten iron, and the dissolution properties of the magnesium alloy can be adjusted depending on the properties we want in the finished casting.
4. Temperature Phasing
If getting the graphite molecules together is step one to a more nodular casting; step two is getting strong networks (crystalline matrices of iron) to form around them.
We can manipulate which microstructures form by phasing the casting through various temperature ranges as the part cools. We also might apply any of the heat treatments of ductile iron, such as austempering, after production.
5. Nodularity Standards
The ultimate objective of the ductile iron casting process is to achieve a 100% nodularity rating — meaning the graphite nodules are perfectly spheroidal. When a graphite particle's length is more than twice its diameter, it is classified as a floccule and disqualified from the nodule count. |
