In many cases, the composition of an austempered ductile iron (ADI) casting differs little from that of a conventional Ductile Iron casting. When selecting the composition, and hence the raw materials, for both conventional Ductile Iron and AUSTEMPERED DUCTILE IRON (ADI), consideration should be given first to limiting elements which adversely affect casting quality through the production of nonspheroidal graphite, or the formation of carbides and inclusions, or the promotion of shrinkage. The second consideration is the control of carbon, silicon and the major alloying elements that control the hardenability of the iron and the properties of the transformed microstructure. When determining the alloying requirements both the section size and type and the severity (or speed) of the austempering quench must be considered.
For a typical salt quench with agitation section sizes up to about 3/8 inch (10 mm) can be successfully through hardened without pearlite with even unalloyed Ductile Iron. For a highly agitated austemper quench with water saturation section sizes of up to ? inch (20 mm) can be through hardened with no additional alloying. For castings of heavier section size selective alloying is required to through harden the parts and avoid pearlite in the heat treated microstructure.
Effect of alloying elements
Increasing carbon in the range 3 to 4% increases the tensile strength but has negligible effect on elongation and hardness. Carbon should be controlled within the range 3.6-3.8% except when deviations are required to provide a defect-free casting.
Manganese can be both a beneficial and a harmful element. It strongly increases hardenability, but during solidification it segregates to cell boundaries where it forms carbides and retards the austempering reaction. As a result, for castings with either low nodule counts or section sizes greater than 3.4 in. (19mm), manganese segregation at cell boundaries can be sufficiently high to produce shrinkage, carbides and unstable austenite. These microstructural defects and inhomogeneities decrease machinability and reduce mechanical properties. To improve properties and reduce the sensitivity of the AUSTEMPERED DUCTILE IRON (ADI) to section size and nodule count, it is advisable to restrict the manganese level in AUSTEMPERED DUCTILE IRON (ADI) to less than 0.3%. The use of high purity pig iron in the AUSTEMPERED DUCTILE IRON (ADI) charge offers the twin advantages of diluting the manganese in the steel scrap to desirable levels and controlling undesirable trace elements.
Silicon is one of the most important elements in AUSTEMPERED DUCTILE IRON (ADI) because it promotes graphite formation, decreases the solubility of carbon in austenite, increases the eutectoid temperature, and inhibits the formation of bainitic carbide. Increasing the silicon content increases the impact strength of AUSTEMPERED DUCTILE IRON (ADI) and lowers the ductile-brittle transition temperature. Silicon should be controlled closely within the range 2.4-2.8%.
Up to 2% nickel may be used to increase the hardenability of AUSTEMPERED DUCTILE IRON (ADI). For austempering temperatures below 675 F (350 C) nickel reduces tensile strength slightly but increases ductility and fracture toughness .
Molybdenum is the most potent hardenability agent in AUSTEMPERED DUCTILE IRON (ADI), and may be required in heavy section castings to prevent the formation of pearlite. However, both tensile strength and ductility decrease as the molybdenum content is increased beyond that required for hardenability. This deterioration in properties is probably caused by the segregation of molybdenum to cell boundaries and the formation of carbides. The level of molybdenum should be restricted to not more than 0.2% in heavy section castings.