Austempering Ductile Iron

Austempering Ductile Iron

Austempered ductile iron castings, or more commonly ADI castings, is a ductile iron casting that is processed by a special heat treatment. The austempering process, with ductile iron’s superior castability, results in a material which can be used to cast complex shapes, with a greater consistency of quality, and often at less cost.  The heat treatment requires an interrupted quench, usually into a salt bath, to cool the cast part at a planned rate and then hold isothermally for long enough to convert the microstructure to Ausferrite. The resulting material has a combination of exceptional strength and toughness, meeting and often exceeding those of alloy steels.

Benefits

Advantages of Austempered Ductile Iron: ADI provides high strength, good fatigue properties, superior wear qualities, excellent toughness, and cost-effectiveness.

Tensile and Yield Strength: ADI’s tensile and yield strengths are at least twice those of standard ductile irons.

Fatigue Strength: ADI’s fatigue strength is typically 50% higher than that of standard ductile irons. It can be further increased by shot peening or fillet rolling. The lower hardness grades of ADI work well in structural applications.

Toughness: ADI’s excellent impact and fracture-toughness properties make it ideal for applications such as ground-engaging tools.

Wear Characteristics: The higher hardness grades of Austempered ductile iron are excellent for wear applications. Unlike case-hardened materials, typically the ADI is uniformly hardened throughout the part. Also, ADI work hardens when stressed. This produces a surface of very hard martensite where wear resistance is most needed.

Cost-effectiveness: ADI is usually 15% to 20% less costly than steel forgings or castings. It is the most economical way of obtaining tensile, yield, or fatigue strength. Austempered ductile iron often competes favorably with heat-treated and alloy steels for heavy-duty applications where reliability is crucial. It is a useful upgrade from standard grades of ductile iron. In some cases, it replaces manganese steel and nickel-chrome iron. Because of ADI’s high strength-to-weight ratio, it has even replaced aluminum where the design allows reduced section sizes.

Properties of ADI Compared to Steel:

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

Application

Austempered ductile iron (ADI) is often used where high strength is needed and where excellent wear resistance and fatigue strength are required. For applications like gears, ADI has been used with great success. This tough work-hardening material has proved to be an excellent replacement for hardened steels. The use of austempered ductile iron can result in less weight, reduced number of components, and quieter running. The unique structure of Austempered ductile iron allows it to work harden which adds to the contact fatigue strength. The superior tribological properties of ADI have resulted in the elimination of bronze-bearing bushings and will allow the gears to run temporarily without lubrication. Due to the type of matrix structure, the softer grades of ADI can be shot-peened to double the root fatigue strength.

Another common application of austempered ductile iron castings has been crankshafts and axles. Axle applications benefit from the material’s lack of notch sensitivity, good fatigue strength, and reasonable machinability.

The railroad industry has an immense application both in retarders and rolling stock. Austempered ductile iron is very popular for retarder brake shoes, where its superior quietness and wear resistance are well received in urban semi-residential communities. ADI brake beams have also been shown to outlast steel beams and withstand the cold weather, at more than 20% less cost.

The military is also becoming a major user of austempered ductile iron for shells and projectiles. Track shoes in Austempered ductile iron have also been very successful both in construction, and earth-moving equipment.