Quick Facts

Austempered Ductile Iron (ADI) is the name for a family of heat-treated ferrous materials having excellent toughness, strength, and wear properties.  However, its principal advantage is that it can be easily cast.  Compared with steel, ADI provides a relatively simple route to sound, high-yield, low-cost, intricate castings that can be used where very high strength or wear resistance is required. 

Microstructure Comparison

During austempering, acicular ferrite nucleates and grows within the austenite, rejecting carbon, with the result that the remaining material is carbon-enriched.   ADI's characteristic ausferrite microstructure is a combination of acicular ferrite and thermodynamically stable high-carbon austenite.  This combination is both tough and wear resistant.  Individual ladle chemical analysis and computer-controlled heat treatment give superior metallurgical consistency to every ADI part at Advanced Cast Products. 

The Process

ADI 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 of the material are determined by careful choice of heat treating parameters.  Austenitizing is accomplished in a high temperature salt bath or a controlled atmosphere furnace, and austempering is done in liquid salt or oil.  The temperatures and process times selected are a result of the desired grade of material and must be carefully controlled.  During the austempering process, acicular ferrite nucleates and grows within the austenite, rejects the carbon and results in a carbon enriched material.  The characteristic "ausferrite" microstructure of ADI is a combination of acicular ferrite and thermo-dynamically stable high-carbon austenite.

ADI Advantages

Initially, buyers and product engineers are attracted by ADI's lower cost due to its inherent manufacturing advantages.  To receive equal strength and wear properties from cast steel, expensive alloy additions or heat-treating processes must be used.  In addition, since steel is cast at higher temperatures than ductile iron, a sound casting requires extra feed metal, increasing the price of the part.  Although low cost attracts first time buyers, it is ADI's engineering features that continue to impress them.  Beyond ADI's high strength and wear potential, the material has other significant advantages.  ADI weighs 8-10% less than steel.  In a cost comparison with steel forgings, a foundry pattern for an ADI casting is a fraction of the price of forging dies, and it has a longer life.  In addition, the casting process requires less of a draft angle and can produce more complex shapes, providing the engineer additional design freedom.  New opportunities for casting to near net shape result from the superior castability of ductile irons over steel.

ADI Disadvantages

Materials specifiers should look beyond limited mechanical property data when replacing steel parts with ADI.  For example, welding is not recommended for ADI parts.  In some instances, the stiffness of the design must be increased to compensate for ductile iron's lower modulus of elasticity.  Also, larger fillet radii are required than for steel to avoid stress concentrations.  To take maximum advantage of ADI when substituting for forgings, some designs should be modified.  Another desirable property of ADI is its work-hardening, which provides better rolling and sliding wear properties.  However, because of this characteristic, consideration must be given to the sequence of operations when machining is required.  The lower hardness grades can be machined after heat-treatment, but the higher hardness grades must be machined before heat treatment.