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. While most varieties of cast iron are brittle, ductile iron is much more flexible and elastic, due to its nodular graphite inclusions.
In 1949, Keith Millis, Lee Aunkst, Albert Gagnebin and Norman Pilling received a US patent on ductile iron production via magnesium treatment.
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
, 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
) 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:
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.
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 with pipe made from other materials. In difficult terrain it can be a better choice than PVC
, or steel
Ductile iron is specifically useful in many automotive components, where strength needs surpass that of aluminum but do not necessarily require steel. Other major industries off-highway diesel trucks, class 8 trucks, agricultural tractors, and oil well pumps.