The defining feature of SG cast iron is its nodular graphite structure. Unlike grey cast iron, where graphite exists in flake form, SG iron contains graphite in spherical nodules. This structure is achieved through controlled magnesium treatment during molten metal processing.

These graphite nodules interrupt crack propagation, allowing the metal matrix to deform rather than fail suddenly. As a result, SG cast iron exhibits a rare combination of high strength, ductility, and toughness, making it suitable for both static and dynamic loading conditions.