Ductile iron Castings
—also referred to as spheroidal or nodular iron—is actually a group of irons that exhibit high strength, flexibility, durability, and elasticity due to their unique microstructure. Cast ductile iron normally contains over 3 percent carbon; it can be bent, twisted, or deformed without fracturing. Its mechanical properties are similar to steel, and far exceed those of standard cast irons. Ductile iron castings are solid metal objects that result from molten ductile iron being poured into voids in a mold. The ductile iron subsequently cools and solidifies in the shape of the void it occupies.
What is Ductile Iron?
Invented in 1943, ductile iron is a modern iteration of cast iron. Cast iron and ductile iron have contrasting physical properties caused by differences in their microstructures. The graphite and carbon contained in cast iron occur as flakes; cast iron exhibits positive compressive load capabilities, but tensile loading above its natural tensile strength can cause cracks to form and propagate rapidly out from stress points within the flaked graphite microstructure. As a result, cast iron has virtually no elongation. It is a brittle material, and therefore limited in its use in tensile and shock loading applications. The graphite within ductile iron occurs as spheroids, so it is sometimes referred to as spheroidal graphite. Similarly, the term nodular iron stems from the fact that the carbon contained in ductile iron is held in the form of nodules. This unique microstructure causes ductile iron to withstand bending and shock loading far better than traditional cast iron.
What is Ductile Iron made from?
Pig iron is used to produce ductile iron While ductile iron can be produced from steel or iron scrap, pig iron is the primary source of infeed for most modern ductile iron foundries. Pig iron refers to the primary iron production of a blast furnace that contains over ninety percent iron. The term “pig iron” arose from the old-fashioned method of casting blast furnace iron into molds arranged in sand beds in such a manner that they could be fed from a common runner. Because the groups of molds resembled a litter of suckling pigs, the individual pieces of iron were referred to as “pigs” and the runner was referred to as a sow. Modern pigs are produced by a continuous pig casting machines. Pig iron is used in ductile iron production as a primary source of pure iron. It offers a number of unique advantages: pig iron contains low residual or harmful elements, has a consistent chemistry, promotes optimal slag conditions, and improves process control by providing consistent charge properties. Demand for pig iron has increased in recent years as ductile iron foundries utilize its advantages over alternate iron sources such as iron scrap, or melting steel scrap and adding carbon.
Advantages of Ductile Iron Casting:
There are several advantages that ductile iron provides designers: Ductile iron can be easily cast and machined. It has an excellent strength to weight ratio. Ductile iron can be made for a much lower cost than steel. It has superior castability and machinability. Ductile iron provides a designer with an exceptional combination of toughness, low cost manufacturing, and reliability. Properties of Ductile Iron To create different grades of ductile iron, one must control the matrix structure around the graphite during the casting process or through subsequent heat treatment. The minor compositional differences between the different grades of ductile iron exist in order to create the desired matrix (microstructures).
Ductile iron parts can be considered as steel with graphite spheroids dispersed throughout its matrix. The qualities of the metallic matrix in which the graphite spheroids are suspended have a significant effect on ductile iron’s properties, but the graphite spheroids themselves do not.
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