Spheroidal graphite morphology

Ductile cast iron is made by adding magnesium or cerium to the melt. They make the carbon solidify as spheres instead of flakes. This makes the material tougher and less sensitive to notches. When welding, you have to be careful not to get too much of that stuff. If you over-treat the melt, you get weak spots in the structure. So keep the addition tight and controlled. The melt must be hot enough for the reaction to start cleanly, and then the weld should cool slowly. Only then do you get a uniform structure without cracks.