Austempered Ductile Iron Castings (ADI)
The mechanical properties of ductile iron and ADI are primarily determined by the metal matrix. The matrix in conventional ductile iron is a controlled mixture of pearlite and ferrite.
The properties of ADI are due to its unique matrix of acicular ferrite and carbon stabilized austenite; called Ausferrite. The austempering process is neither new nor novel and has been utilised since the 1930’s on cast and wrought steels. The austempering process was first commercially applied to ductile iron in 1972 and by 1998 worldwide production was approaching 100,000 tonnes annually.
In many cases, the composition of an ADI casting differs little from that of a conventional ductile iron casting, the alteration in properties being achieved by the addition of alloys like nickel, copper and molybdenum at quite low levels followed by the austempering heat treatment. The effects of altering the alloys in ADI are summarised below.
The presence of stable, carbon enriched austenite accounts for the wear resistant property of ADI. While thermodynamically stable, the enriched austenite can undergo a strain-induced transformation when exposed to high, normal forces. This transformation, which gives ADI its remarkable wear resistance, is more than mere “work hardening”. In addition to a significant increase in flow stress and hardness (typical in most metallic materials), this strain induced transformation also produces a localized increase in volume and creates high compressive stresses in the “transformed” areas. These compressive stresses inhibit crack formation and growth and produce significant improvements in the fatigue properties of ADI when it is machined after heat treatment or subjected to surface treatments such as shot peening, grinding or rolling.
|Carbon||Increasing carbon in the range 3 to 4% increases the tensile strength but has a 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.|
|Silicon||Silicon is one of the most important elements in 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 ADI and lowers the ductile-brittle transition temperature. Silicon should be controlled closely within the range 2.4-2.8%.|
|Manganese||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 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 ADI to section size and nodule count, it is advisable to restrict the manganese level in ADI to less than 0.3%. The use of high purity pig iron in the ADI charge offers the twin advantages of diluting the manganese in the steel scrap to desirable levels and controlling undesirable trace elements.|
|Copper||Up to 0.8% copper may be added to ADI to increase hardenability. Copper has no significant effect on tensile properties but increases ductility at austempering temperatures below 675°F (350°C).|
|Nickel||Up to 2% nickel may be used to increase the hardenability of ADI. For austempering temperatures below 675°F (350°C) nickel reduces tensile strength slightly but increases ductility and fracture toughness.|
|Molybdenum||Molybdenum is the most potent hardenability agent in 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.|
ADI is a group of materials whose mechanical properties can be varied over a wide range by a suitable choice of heat treatment. A high austempering temperature, 750°F (400°C), produces ADI with high ductility, a yield strength in the range of 500 MPa (72 ksi) with good fatigue and impact strength. These grades of ADI also respond well to the surface strain transformation previously discussed which greatly increases their bending fatigue strength. A lower transformation temperature, 500°F (260°C), results in ADI with very high yield strength (1400 MPa (200 MPa)), high hardness, excellent wear resistance and contact fatigue strength. This high strength ADI has lower fatigue strength as-austempered but it can be greatly improved with the proper rolling or grinding regimen. Thus, through relatively simple control of the austempering conditions ADI can be given a range of properties unequalled by any other material.
We make ADI to BS ISO 17804:2005 which superseded a Euronorm standard, EN 1564:1997. However, due to the international nature of the material, buyers may find other descriptions and standards on drawings which can cause confusion. All of the following terms can be used to describe this material:-
- Austempered Ductile Iron
- Ausferritic Spheroidal Graphite Cast Iron
There are also a range of local standards that can be used to specify ADI such as ASTM A897-02, ASTM A897M-02, EN 1564:1997, JIS G5503-1995 and SAE J2477 May 2004. This list is not exhaustive and if you have a casting that you think is made in ADI and you’re not sure, ring us and we will try to determine if it is.
The development and commercialization of Austempered Ductile Iron (ADI) has provided the design engineer with a new group of cast ferrous materials which offer the exceptional combination of mechanical properties equivalent to cast and forged steels and production costs similar to those of conventional Ductile Iron. In addition to this attractive performance:cost ratio, ADI also provides the designer with a wide range of properties, all produced by varying the heat treatment of the same castings, ranging from 10-15% elongation with 125 ksi (870 MPa) tensile strength, to 250 ksi (1750 MPa) tensile strength with 1-3% elongation. Although initially hindered by lack of information on properties and successful applications, ADI has become an established alternative in many applications that were previously the exclusive domain of steel castings, forgings, fabrications, powdered metals and aluminium forgings and castings.
ADI castings have been used successfully in heavy bus and trucks, light auto and trucks, mining and quarrying, construction, railways, agriculture and defence. In particular, it has proved to be a success in specific applications such as gears, crankshafts, crawler shoes and agricultural ground engaging parts. This list is by no means exhaustive.
Durham Foundry does not carry out any heat treatment on site. Our preferred supplier for the heat treatment of ADI castings is:-
ADI Treatments Limited, Doranda Way, West Bromwich, B71 4LE, England
Tel: 0121 525 0303
Fax: 0121 525 0404
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