ADI is the abbreviation of Austempered Ductile Iron, which is produced by an isothermal heat treatment know as austempering. The preponderance of information on the austempering of steel and the superficial similarities between the austempering heat treatments applied to steels and ADI, have resulted in comparisons which are incorrect and damaging to the understanding of the structure and properties of ADI. ADI is sometimes referred to as “bainitic Ductile Iron”, but correctly heat treated ADI contains little or no bainite. Bainite is a matrix of acicular (plate- like) ferrite and carbide. ADI’s ausferrite matrix is a mix of acicular ferrite and carbon stabilized austenite. This ausferrite may resemble bainite metallographically, however it is not because it contains few or none of fine carbides characteristic in bainite. An ausferrite matrix will only convert to bainite if it is over tempered.

ADI is austempered by being:
    •   Heating the casting to the austenitizing temperature in the range of 815 to 927 °C (1500 to 1700 °F)
    •   Holding the part at the austenitizing temperature for a time sufficient to get the entire part to temperature and to saturate the austenite with carbon
    •   Quenching (Cooling) the casting rapidly enough to avoid the formation of pearlite to the austempering temperature in the range of 232 to 400 °C (450 to 750 °F). (The temperature is above the martensite start temperature (Ms) for the material).
    •   Austempering the part at the desired temperature for a time sufficient to produce a matrix of ausferrite in which content of stabilized austenite is about 2% carbon.
    •   Cooling the part to room temperature.

Advantage in microstructures and process make ADI offer the best combination of design flexibility, good machinability and good toughness, wear resistance and fatigue strength. For example, compared to the conventional Ductile Iron, ADI delivers twice the strength for a given level of elongation (Fig. 1). Especially, ADI provides a high strength-to-weight ratio material at a component price that is typically 20% less than that of steel or aluminum. It may surprise you to know that in some applications, ADI has replaced aluminum as a weight savings. ADI components are very competitive with steel forgings, castings, and weldments, as well as aluminum castings and forgings. Austempering can also replace induction and flame hardening as a cost savings. Because of the advantage properties above, ADI has widely application in industrial field, such as Heavy Truck and Bus Components, Light Auto and Truck Components, Construction and Mining Components and so on.

SAMT is the strategic partner of Applied Processing in China to provide the innovative solution of ADI technology for metalworking industries.