Difference Between Ductile Iron & Cast Iron

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Ductile iron was developed as an improvement over cast iron. Although both kinds of iron alloys are still in use and are closely related, there are a lot of differences between the structure and properties of ductile and cast iron. These differences are essential information for structural engineers, builders and all other professionals who have to make a variety of structures using iron.

Related yet Different Irons

Cast iron is a generic term for any kind of high-carbon iron alloy that can be molten and poured into a cast as opposed to being forged or molded. Cast iron is differentiated into various kinds depending on the kind or structure of carbon present. The most common kind of carbon found in common cast iron (gray iron) is graphite. Ductile iron is a kind of cast iron that also has graphite, but its graphite is differently structured from that of regular cast iron. While both ductile and cast iron are created by introducing carbon, ductile iron differs in that it is made by adding magnesium into the hot molten iron bath. The process causes the graphite to form a characteristic nodular shape upon solidification.

Graphite Formation

The graphite that is found in ductile iron is shaped like nodules or spheres while the graphite in the usual cast iron or gray iron has a rodlike or random flaky pattern. The shape of the graphite in the iron lends to a lot of the properties of ductile or cast iron. The spherical nodules of graphite in ductile iron create fewer discontinuities that enable ductile iron to be stronger than cast iron and have better ductility, the ability of metal to be shaped into wire.

Tensile and Yield Strength

Cast iron (gray iron) has a tensile strength of approximately 20,000 psi and no measurable yield strength. Ductile iron has far greater tensile strength that ranges from 60,000 to 100,000 psi and has a minimum yield strength of 40,000 psi.


Ductility is an important property in assessing the impact resistance of metals such as iron. The most ductile of ductile irons is the A395, which has a ductility of 18 to 30 percent elongation. The usual cast iron (gray iron) does not have any recognizable elongation.

Impact Resistance

Ductile iron is more resistant to impact than cast iron and can be tested using the Charpy impact test, a test that measures impact resistance by applying an impact load to a notched specimen. A ductile iron pipe can resist a minimum of 7 foot-pounds of impact. A cast iron pipe (gray iron) can only survive up to 2 foot-pounds of impact.