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Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron[1] and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis.[2] While most varieties of cast iron are weak in tension and brittle, ductile iron has much more impact and fatigue resistance, 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] Augustus F. Meehan was awarded a patent in January 1931 for inoculating iron with calcium silicide to produce ductile iron subsequently licensed as Meehanite, still produced as of 2024 .
Metallurgy
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Ductile iron microstructure at 100× magnification, showing carbon islanding effect[4] around nodules. Another micrograph showing the carbon islanding effect, with nodules surrounded by areas depleted of carbonDuctile 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.[5] Nodule formation is achieved by adding nodulizing elements, most commonly magnesium (magnesium boils at 2012F and iron melts at 2732F and, less often now, cerium (usually in the form of mischmetal).[6] Tellurium has also been used. Yttrium, often a component of mischmetal, has also been studied as a possible nodulizer.
Austempered ductile iron (ADI; i.e., austenite tempered[7]) 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.[citation needed]
Composition
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Mass fraction (%) for ferritic ductile iron castings[8] Fe C Si Ni Mn Mg Cr P S Cu Balance 3.0–3.7 1.2–2.3 1.0 0.25 0.07 0.07 0.03 0.1 Balance 3.3–3.6 2.2–2.8 0.1-0.2 0.03–0.04 0.005–0.04 0.005–0.02 <0.40Elements 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 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.[9]
Applications
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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; 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 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 grand piano harps (the iron plates to which high-tension piano strings are attached).
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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.
See also
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References
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Bibliography
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Though ductile iron pipe has been in use since 1955, its predecessor – gray cast iron pipe – traces its lineage all the way back to the year 1455 in Siegerland, Germany. PVC pipe comes from the discovery of polyvinyl chloride in the 1800s and was deployed in Europe and America a few decades later.
So why offer the history lesson? The reason is simple. Ductile iron and PVC pipe comprise a large amount of the more than 1,400 miles of water pipe underneath Rutherford County. Your home’s water supply likely runs through one or both of them. Here is a quick list of the qualities of these pipes:
DUCTILE IRON
• Considered the choice for modern water and wastewater systems
• Offers great strength, durability, and reliability for transporting raw and potable water.
• Withstands demanding conditions, including frozen ground, deep trenches, heavy traffic areas, river crossings, rocky ditches, and unstable soils.
• Virtually maintenance free
• Minimum strength requirements of 60,000 psi tensile strength, 42,000 psi yield strength, and 10 percent minimum elongation.
• Ability to bend under load greatly increases its resistance to beam load
• Needs no external corrosion protection.
• Easy to install in the field
• Wide variety of joints and standard fittings available for every application
PVC PIPE
• Resistant to almost all types of corrosion—both chemical and electrochemical—that are found in underground piping systems
• Cannot be damaged by aggressive waters or corrosive soils
• No lining, coating, cathodic protection, or plastic encasement required when installed
• Nearly totally resistant to attack from microorganisms (fungi or bacteria) or macroorganisms (grass roots, termites, or rodents)
• High strength-to-weight ratio and is a good electrical and thermal insulator
• Allows for maximum water pressure flow, which helps stop the potential stoppages in sewage and wastewater systems
Sources:
McWane Ductile: https://www.mcwaneductile.com/products/ductile-iron-pipe/
Ductile Iron Pipe Research Association: https://downloads.ctfassets.net/e4roza01bro8/5nncNovA06d1im1meDExvV/553017f20dedf78586e5793bbc0c1939/General-DuctileIronPipe.pdf
AWWA Manual M23: https://www.awwa.org/portals/0/files/publications/documents/samples/30023_excerpt.pdf
International Polymer Solutions: https://www.ipolymer.com/pdf/PVC.pdf
AccurateLeak.com: https://www.accurateleak.com/the-importance-of-pvc-pipes/
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