Magnesium treatment
Magnesium treatment is the key step in ductile iron production: just before pouring, a little magnesium is added to the melt. The magnesium prevents the carbon from growing as brittle flakes – it solidifies as spheroidal graphite, and the casting becomes tough instead of brittle.
What magnesium treatment does
Adding a small amount of magnesium to a cast iron melt just before pouring changes how the carbon solidifies. Without treatment, the graphite grows as brittle flakes—that's gray iron. With magnesium, it's forced to grow as spheres—that's ductile iron.
Why magnesium
Magnesium binds sulfur and oxygen in the melt and disrupts flake growth so carbon solidifies as spheres. The spheres don't act as notches, making the material tough and ductile.
The catch
Magnesium evaporates quickly and partially burns during addition. Treatment must therefore happen right before pouring, or the effect is lost.
Note when welding
In the weld area, the spheroidal graphite structure is locally remelted. If it cools too quickly, brittle structure forms—the area loses its toughness. So weld ductile iron warm and allow slow cooling, using matching filler metals per manufacturer's specs.