Ductile iron is more impact and fatigue resistant than gray iron due to the spheres of graphite that form with added magnesium.
The science and strength of ductile iron
From a microstructure standpoint, the primary difference between gray and ductile iron is the shape of the graphite. Gray irons have flake graphite, whereas ductile iron has a nodular or spherical shape. This graphite shape drives the primary difference in each from a material properties specifications perspective.
The flake graphite shape in gray iron, which creates fracture planes, is brittle and has less strength. On the other hand, with its nodular graphite shape, ductile iron, as the name implies, has ductility, which allows for elongation under tension and better impact resistance. In the material properties of gray iron, only tensile strength is listed because there is no elongation. For ductile irons, the specifications call out tensile strength, yield strength, and elongation.
When should ductile iron be used?
Gray iron is typically used in applications where strength is not the primary requirement. Due to the graphite shape, it is more effective at damping vibration than ductile iron. Typical part families are compressors, pumps, counterweights, low-speed gearboxes, and machine bases, to name a few.
On the other hand, ductile iron will be used in engineered applications due to its superior strength and elongation. Typical parts include structural or other highly stressed components, high-speed gearboxes, gears, wheel hubs, air/gas cylinders, and other pressurized components.
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