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Ferrite (α-iron, δ-iron) |
| Steel classes |
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Carbon steel (≤2.1% carbon; low alloy) |
| Other iron-based materials |
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Cast iron (>2.1% carbon) |
Ductile iron, also called ductile cast iron, spheroidal graphite iron, or nodular cast iron, is a type of cast iron invented in 1943 by Keith Millis.[1] 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.[2]
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Metallurgy
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.
Graphite iron having spherical graphite are produced from grey cast iron by a malleabilizing heat treatment. The heat treatment consists of heating grey cast iron at around 900 °C for almost 24 hours and cooling slowly or moderately in air. This treatment converts the graphite from flakes to a spherical shape. Cooling slowly gives a ferrite matrix while cooling moderately gives a pearlitic matrix.
A recent development in ductile iron metallurgy is austempered ductile iron where the metallurgical structure is manipulated through a sophisticated heat treating process.
Composition
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
Much of the annual production of ductile iron is in the form of ductile 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 with pipe made from other materials. In difficult terrain it can be a better choice than PVC, concrete, polyethylene, or steel pipe.
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
References
- ^ Modern Casting, Inc
- ^ US2,485,760 (1949-10-25) Keith Millis, Cast Ferrous Alloy.
External links
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