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Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spherulitic graphite cast iron^[1] and SG iron, is a type of cast iron invented in 1943 by Keith Millis.^[2] While most varieties of cast iron are brittle, ductile iron is much more flexible and elastic, due to its nodular graphite inclusions.

On October 25, 1949, Keith Dwight Millis, Albert Paul Gagnebin and Norman Boden Pilling received US patent 2,485,760 on a Cast Ferrous Alloy for ductile iron production via magnesium treatment.^[3]

Metallurgy[link]

Ductile iron microstructure at 100×. Note carbon islanding effect^[4] around nodules.

Ductile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the morphological structure of the graphite. In ductile irons, the graphite is in the form of spherical nodules rather than flakes (as in grey iron), thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name.^[5] The formation of nodules is achieved by addition of nodulizing elements, most commonly Magnesium (note Magnesium boils at 1100C and Iron melts at 1500C) and, less often now, Cerium (usually in the form of Misch metal), into the melt.^[6] Tellurium has also been used. Yttrium, often a component of Misch metal, has also been studied as a possible nodulizer.

Besides the requirement that the graphite be manipulated into the spheroidal shape, the ferrite and pearlite ratios can be controlled through alloying, shakeout temperature control or post-casting heat treatment to vary the relative amounts pearlite and ferrite from 0% pearlite and 100% ferrite, to 100% pearlite and 0% ferrite. The control of the pearlite and ferrite ratio manipulates the tensile, yield and elongation characteristics of the ductile iron to produce numerous standard grades of material.

"Austempered Ductile Iron" (ADI) was invented in the 1950s but was commercialized and achieved success only some years later. In ADI, the metallurgical structure is manipulated through a sophisticated heat treating process. The "aus" portion of the name refers to austenite.^[7]

Composition[link]

A typical chemical analysis of this material:

• Iron
• Carbon 3.3 to 3.4%
• Silicon 2.2 to 2.8%
• Manganese 0.1 to 0.5%
• Magnesium 0.03 to 0.05%
• Phosphorus 0.005 to 0.04%
• Sulfur 0.005 to 0.02%

Other elements such as copper or tin may be added to increase tensile and yield strength while simultaneously reducing elongation. Improved corrosion resistance can be achieved by replacing 15% to 30% of the iron in the alloy with varying amounts of nickel, copper, or chromium.

Applications[link]

Much of the annual production of ductile iron is in the form of ductile iron pipe, used for water and sewer lines. It competes with polymeric materials such as PVC, HDPE, LDPE and polypropylene, which are all much lighter than steel or ductile iron, but which, being flexible, require more careful installation and protection from physical damage.

Ductile iron is specifically useful in many automotive components, where strength needs surpass that of aluminum but do not necessarily require steel. Other major industrial applications include off-highway diesel trucks, Class 8 trucks, agricultural tractors, and oil well pumps.

See also[link]

• Malleable iron

References[link]

Bibliography[link]

• Smith, William F.; Hashemi, Javad (2006), Foundations of Materials Science and Engineering (4th ed.), McGraw-Hill, ISBN 0-07-295358-6. 

External links[link]

• Media related to Ductile iron at Wikimedia Commons
• Ductile Iron Society
• Ductile Iron Pipe Research Association