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 cast iron invented in 1943 by Keith Millis. While most varieties of cast iron are brittle, DI has much more impact and fatigue resistance, due to its nodular graphite inclusions.
Much of the annual production of DI is in the form of ductile iron pipe, used for water and sewer lines, but there many ductile iron castings applications in multiple market segments. It competes with polymeric materials such as PVC, HDPE, LDPE and polypropylene, which are all much lighter than steel or ductile iron; being more flexible, these require protection from physical damage.
Ductile Iron Uses
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. In wind power industry nodular cast iron is used for hubs and structural parts like machine frames. Nodular cast iron is suitable for large and complex shapes, commonly castings, and high fatigue loads. Ductile iron castings are thus used in a wide variety of industries.
Ductile Iron Chemical Make Up
DI 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, the rounded nodules in ductile iron 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 (note magnesium boils at 1100°C and iron melts at 1500°C) and, less often now, cerium (usually in the form of Mischmetal). Tellurium has also been used.
One of the occurrences that must be monitored when producing nodular iron castings is “magnesium fade,” Nodularity is lost with time, and so the timing of when the inoculant is added to the molten metal to when the metal is cast is critical to maintaining the necessary material properties.
“Austempered Ductile Iron” (ADI) was invented 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. The “aus” portion of the name refers to austenite.