Austempered Ductile Iron

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

Austempered Ductile Iron (ADI) Advantages

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:

• ADI is much easier to cast than steel
• it is approximately 9% lighter than steel
• it has minimal draft requirements compared with steel forgings
• ADI loses less of its toughness than steel at sub-zero temperatures
• work hardens when stressed
• ADI has more damping capacity than steel.

The Austempering Process

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.

1. Initial austenitizing times and temperatures (1550 to 1700°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:

• Yield strength, toughness and impact resistance comparable to many cast/forged steels.
• Vibration dampening and heat transfer superior to other ferrous/non-ferrous alloys.
• Significant weight and cost savings over both aluminum and steel castings/forgings.
• Increased fracture and fatigue strength.
• Cost savings over aluminum and steel castings/forgings.

Applications & Examples

The properties of ADI coupled with the cost and flexibility benefits of ductile iron castings means the potential for ADI applications is vast:

• Agriculture - excellent resistance to soil wear
• Digger/Grab teeth - high strength and wear resistance
• Industrial - wear components, pumps, etc.
• Gears - for wear resistance and better vibration damping than steel
• Construction - crushing, grading and wear components etc.
• Food & feed milling - grinding, mixing, pelletizing etc.

Table 1: British Standards Specification for ADI EN 1564: 1997

Table 2: Standard ADI Grades (USA) ASTM 897-90 (ASTM 897M-90)