Nodular graphite
Spheroidally shaped carbon nodules in ductile iron (GJS) achieved by treating the melt with magnesium. The rounded geometry avoids stress concentration, dramatically improving ductility.
Nodular graphite forms when you add magnesium to the iron melt. The magnesium binds the sulfur and influences the carbon so that it is deposited as round nodules during solidification instead of sharp flakes. This round shape is why ductile iron is tougher and less prone to cracking. When welding GJS, you have to be careful: the high heat can harden the area around the weld and cause cracks. The trick is to keep the heat input low and not heat the workpiece too much. When treating the melt, the right amount of magnesium is crucial – too little and you won't get nodular graphite, too much and the material becomes poorly castable.