Spheroidal graphite compaction
The targeted spheroidization of free carbon via magnesium treatment to generate stress-reducing round particles in the cast iron microstructure.
Nodular graphite forms when you add magnesium to cast iron before pouring. The magnesium causes the carbon to precipitate as round nodules instead of flakes. This makes the material much tougher and stronger than normal gray cast iron. When welding, it's important to keep the heat low. If the part gets too hot, the nodular graphite breaks down back into flakes, and the weld becomes hard and brittle. So you need to weld in short sections and let the part cool down in between. When making a weld on GGG, you need special filler material containing nickel or nickel base. This gives you a soft, easily machinable weld. Normal steel electrode material would otherwise become too hard and crack.