Metalcasting: All about iron

Engineering Manager RJ Hawkins explores the main differences and applications between gray and ductile iron.

From a microstructure standpoint, the primary difference is the shape of the graphite. Gray irons have flake graphite, where ductile iron has a nodular or spherical shape. This graphite shape is what drives the basic 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 for gray iron, only tensile strength is listed because there is no elongation. Still, for ductile irons, the specifications call out tensile strength, yield strength, and elongation.

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 (but not limited to) 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 would include, but again not be limited to, structural or other highly stressed components, high-speed gearboxes, gears, wheel hubs, air/gas cylinders, and other pressurized components.

Lawton’s flexibility in meeting customer’s needs goes far beyond adjusting the various grades of iron. If standard grades of iron do not meet the needed specifications, contact Lawton today to provide the necessary properties. Lawton will make adjustments that may include specific chemistry needs, material properties, special hardness requirements, etc.