Answer: Option D
nodular iron, obtains its special properties through the addition of magnesium into the alloy.
The presence of magnesium causes the graphite to form in a spheroid shape as opposed to the flakes of gray iron.
Composition control is very important in the manufacturing process. Small amounts of impurities such as sulfur and oxygen react with the magnesium, affecting the shape of the graphite molecules.
Different grades of ductile iron are formed by manipulating the microcrystalline structure around the graphite spheroid.
This is achieved through the casting process, or through heat treatment, as a downstream processing step.