Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron and SG iron, is a type of

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

-rich

cast iron Cast iron is a class of iron–carbon alloys with a carbon content of more than 2% and silicon content around 1–3%. Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its car ...

discovered in 1943 by Keith Millis. While most varieties of cast iron are weak in tension and

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...

, ductile iron has much more impact and fatigue resistance, due to its ''nodular graphite'' inclusions. Augustus F. Meehan was awarded in January 1931 for inoculating iron with calcium silicide to produce ductile iron subsequently licensed as Meehanite, still produced . In October 1949 Keith Dwight Millis, Albert Paul Gagnebin and Norman Boden Pilling, all working for INCO, received on a cast ferrous alloy using magnesium for ductile iron production.

Metallurgy

Ductile iron is not a single material but part of a group of materials which can be produced with a wide range of properties through control of their microstructure. The common defining characteristic of this group of materials is the shape of the graphite. In ductile irons, graphite is in the form of ''nodules'' rather than flakes as in grey iron. Whereas sharp graphite flakes create stress concentration points within the metal matrix, rounded nodules inhibit the creation of cracks, thus providing the enhanced ductility that gives the alloy its name. Nodule formation is achieved by adding ''nodulizing elements'', most commonly

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

(magnesium boils at 1100 °C and iron melts at 1500 °C) and, less often now,

cerium Cerium is a chemical element; it has Chemical symbol, symbol Ce and atomic number 58. It is a hardness, soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it ...

(usually in the form of mischmetal). Tellurium has also been used.

Yttrium Yttrium is a chemical element; it has Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost a ...

, often a component of mischmetal, has also been studied as a ''possible nodulizer''. '' Austempered ductile iron'' (ADI; i.e., austenite tempered) was discovered 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.

Composition

Elements such as copper or tin may be added to increase tensile and yield strength while simultaneously reducing ductility. Improved corrosion resistance can be achieved by replacing 15–30% of the

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

in the alloy with varying amounts of nickel, copper, or chromium. Other ductile iron compositions often have a small amount of sulfur as well. Silicon as a graphite formation element can be partially replaced by aluminum to provide better oxidation protection.

Applications

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

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

ic materials such as PVC, HDPE, LDPE and

polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer Propene, propylene. Polypropylene belongs to the group of polyolefin ...

, which are all much lighter than steel or ductile iron; being softer and weaker, these require protection from physical damage. Ductile iron is specifically useful in many automotive components, where strength must surpass that of aluminum but more expensive steel is not necessarily required. Other major industrial applications include off-highway diesel trucks, class 8 trucks, agricultural tractors, and oil well pumps. In the wind power industry ductile iron is used for hubs and structural parts like machine frames. Ductile iron is suitable for large and complex shapes and high (fatigue) loads. Ductile iron is used in many

piano A piano is a keyboard instrument that produces sound when its keys are depressed, activating an Action (music), action mechanism where hammers strike String (music), strings. Modern pianos have a row of 88 black and white keys, tuned to a c ...

harps (the iron plates which anchor piano strings). Ductile iron is used for vises. Previously, regular cast iron or steel was commonly used. The properties of ductile iron make it a significant upgrade in strength and durability from cast iron without having to use steel, which is expensive and has poor castability.

References

Bibliography

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External links