Spheroidal graphite
Nodular graphite is a carbon structure in cast iron formed by magnesium treatment. It has a globular shape and improves toughness and elongation.
Physical-Chemical Mechanism of Action
Nodular graphite forms in cast iron through controlled magnesium treatment. Magnesium reacts with carbon in the molten iron to form magnesium carbide, which then remains as nodules in the structure.
Metallurgical Relevance & Welding Risks
Nodular graphite improves toughness and elongation of the cast iron. It prevents sharp notches and reduces cracking susceptibility. However, excessive magnesium treatment can soften the structure, reducing strength.
Shop Practice & Action Items
To produce nodular graphite in cast iron, add magnesium to the melt. The optimal magnesium content is about 0.1 to 0.2 %. Cooling must be controlled to maintain the nodular shape. When welding nodular cast iron, careful preheating is needed to avoid cracks and hot cracking.