The spheroidization rate is an important index of ductile iron castings. The spheroidizing agent (Nodulizer) is an additive added to the liquid cast iron in order to make the graphite crystal in the nodular cast iron become spherical. In ductile iron foundries, the main spheroidizing agents or nodolizers are magnesium, rare earth alloys (mainly including cerium and lanthanum-based light rare earths and yttrium-based heavy rare earths) and calcium.

Magnesium has the strongest spheroidizing ability and it is most widely used as spheroidizing agent. However, magnesium has a low boiling point, which is much lower than the temperature of molten iron. Therefore, when pure magnesium is used as a spheroidizing agent, the magnesium will be gasified at a high speed, which is not safe and economical, and will also pollute the environment. In nodular iron foundries, in order to slow down the gasification of magnesium, cast iron manufacturers generally use ferrosilicon-magnesium alloys with a mass fraction of magnesium not exceeding 10%, or rare earth ferrosilicon-magnesium alloys as spheroidizing agents.

The boiling point of cerium in rare earth is 1400℃, its effect is relatively stable, and it is not affected by other despheroidizing elements. However, the rare earth cerium is very expensive, and the roundness of graphite obtained after spheroidization with cerium is worse than that of magnesium, so its use is restricted. Heavy rare earth alloys mainly made of yttrium are mainly used for ball milled iron castings with thick and large sections in order to delay the spheroidization decline.

The boiling point of calcium is 1487°C, and the effect is stable as well, but the spheroidizing ability of calcium is weak, and the amount to be added is large. Calcium metal is easily oxidized and inconvenient to store, so calcium alone is not generally used as a nodulizer.

Moreover, magnesium, rare earth elements and calcium as spheroidizing agents have strong deoxidation and desulfurization effects in molten iron.