Ductile ironv • d • eIronalloyphases
Austenite (γ-iron; hard)
Cementite (iron carbide; Fe3C)
Ledeburite (ferrite - cementite eutectic, 4.3% carbon)
Ferrite (α-iron, δ-iron; soft)
Pearlite (88% ferrite, 12% cementite)
Ductile iron, also called ductile cast iron or nodular cast iron, is a type of cast iron invented in 1943 by Keith Millis. While most varieties of cast iron are brittle, ductile iron is much more ductile, due to its nodular graphite inclusions.
In 1949, Keith Millis, Lee Aunkst, Albert Gagnebin and Norman Pilling received U.S. Patent 2,485,760 on ductile iron production via magnesium treatment.
Grey iron was the original "cast iron", and is an iron alloy characterized by its relatively high carbon content (usually 2% to 4%). When molten cast iron solidifies some of the carbon precipitates as graphite, forming tiny, irregular flakes within the crystal structure of the metal. While the graphite enhances the desirable properties of cast iron (improved casting & machining properties and better thermal conductivity), the flakes disrupt the crystal structure and provide a nucleation point for cracks, leading to cast iron's characteristic brittleness. In ductile iron the graphite is in the form of spherical nodules rather than flakes, thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. The formation of nodules is achieved by addition of "nodulizers" (for example, magnesium or cerium) into the melt. Yttrium has also been studied as a possible nodulizer.
A recent development in ductile iron metallurgy is austempered ductile iron where the metallurgical structure is manipulated through a sophisticated heat treating process.
A typical chemical analysis of this material:
- Carbon 3.3 to 3.4%
- Silicon 2.2 to 2.8%
- Manganese 0.1 to 0.5%
- Sulfur 0.005 to 0.02%
- Phosphorus 0.005 to 0.04%
- Magnesium 0.03 to 0.05%
- Iron (remainder)
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 and/or chromium.
Much of the annual production of ductile iron is in the form of ductile cast iron pipe, used for water and sewer lines. Ductile iron pipe is stronger and easier to tap, requires less support and provides greater flow area compared to pipe made from other materials. In difficult terrain it can be a better choice than PVC, concrete, polyethylene or steel pipe.
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