Austempered ductile iron (ADI) is finding an ever increasing worldwide market in the automotive and other sectors. It offers a range of mechanical properties superior to those of other cast irons, and shows excellent economic competitiveness with steels and aluminum alloys.
ADI is a heat treated form of as-cast ductile iron. The heat treat process, austempering, was developed with the intent of improving on the strength and toughness of ferrous alloys. Ductile iron, with its relative low cost and ease to manufacture, has been one of the largest beneficiaries of the austempering process.
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Austempered ductile iron (ADI) is finding an ever increasing worldwide market in the automotive and other sectors. It offers a range of mechanical properties superior to those of other cast irons, and shows excellent economic competitiveness with steels and aluminum alloys.
ADI is a heat treated form of as-cast ductile iron. The heat treat process, austempering, was developed with the intent of improving on the strength and toughness of ferrous alloys. Ductile iron, with its relative low cost and ease to manufacture, has been one of the largest beneficiaries of the austempering process. As a result, ADI has burst onto the scene in recent years with a host of creative and innovative casting solutions.
The term "cast iron" designates an entire family of metals with a wide variety of properties. Cast iron contains more than 2% carbon, present as a distinct graphite phase. In ductile cast iron the graphite occurs as spheroids or spherulites rather than as individual flakes as in gray iron. Ductile iron exhibits a linear stress-strain relation, a considerable range of yield strengths, and, as its name implies, ductility.
"Austempering" is a high performance heat treatment for ferrous alloys which produces an engineered, tailorable matrix structure. This austempered matrix structure gives tensile strength, toughness, impact strength and fatigue properties that are comparable to heat-treated steels. Figure 1 shows the tensile elongation of steel and austempered ductile iron ADI.
Figure 1: The tensile elongation between steels and austempered ductile iron
The ADI casting requires a precisely controlled heat-treatment (heat, old, quench, austemper, and cool) to develop the desired microstructure (acicular ferrite and carbon-stabilized austenite) and mechanical properties.
Properties of ADI compared to steel:
Austempered ductile iron is produced by heat-treating cast ductile iron to which small amounts of nickel, molybdenum, or copper have been added to improve hardenability. Specific properties are determined by the careful choice of heat treating parameters. Austempering involves the nucleation and growth of acicular ferrite within austenite, where carbon is rejected into the austenite. The resulting microstructure of acicular ferrite in carbon-enriched austenite is called ausferrite. Even though austenite in austempered ductile iron is thermodynamically stable, it can undergo strain-induced transformation to martensite when locally stressed. The result is islands of hard martensite that enhance wear properties.
Advanced Cast Products (ACP) uses salt baths for austenitizing, quenching, and austempering in order to achieve close dimensional control. Times and temperatures are tightly controlled throughout the entire process.
Steps in the ACP Austenitizing Process
1. Heat castings in a molten salt bath to austenitizing temperature.
2. Hold at austenitizing temperature to dissolve carbon in austenite.
3. Quench quickly to avoid pearlite.
4. Hold at austempering temperature in molten salt bath for isothermal transformation to ausferrite.
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1. Initial austenitizing times and temperatures ( to °F) are controlled to ensure formation of fine grain austenite and uniform carbon content in the matrix. The precise temperature is grade dependant.
2. Quench time must be controlled within a few seconds, to avoid the formation of pearlite around the carbon nodules, which would reduce mechanical properties. Quench temperatures (450 to 750°F) must stay above the point of martensite formation (except for ASTM A 897 Grade 5).
3. In the austempering step which follows austenitizing, the temperature of the final salt bath must also be closely controlled. The austempering step is also precisely time-controlled, to avoid over- or under-processing. By the end of this step, the desired ADI ausferrite structure has developed.
Austempered ductile iron provides:
The properties of ADI coupled with the cost and flexibility benefits of ductile iron castings means the potential for ADI applications is vast:
Table 1: British Standards Specification for ADI EN :
Table 2: Standard ADI Grades (USA) ASTM 897-90 (ASTM 897M-90)
Some of the desirable properties of ADI Grade 1 include:
The heat treat process, austempering, was developed with the intent of improving on the strength and toughness of ferrous alloys. Ductile iron, with its relative ease of use and low cost of manufacture, has been one of the largest beneficiaries of the austempering process.
Because it can be cast like any other ductile iron, ADI offers all the production advantages of a conventional ductile iron casting. Consequently it is subjected to the austempering process to produce mechanical properties that are superior to conventional ductile iron, cast and forged aluminum and many cast and forged steels.
Carson Castings (Foundry-Source.com) represents the best austempered ductile iron foundries in the USA and backs it up with professional and ongoing service. We deal strictly in the sourcing of castings and machined castings. We can provide raw castings or complete assemblies and everything in between (painting, heat treat and machining).
Our years of experience in the casting field ensure we place your work at a facility suited to your project&#;s unique needs. Our tooling options include no bake and green sand molding. We also offer floor, manual, squeezer, cope and drag, Hunter, Sinto and Disa molding processes.
Want more information on Austempered Ductile iron casting? Feel free to contact us.