Austempered ductile iron (ADI) is ductile iron which undergoes an isothermal heat treatment. The optimal microstructure of ADI consists of graphite nodules surrounded by acicular ferrite and high carbon content austenite, which is called ausferrite. The excellent mechanical properties of ADI have led it to becoming a good alternative to steel castings and forgings and even aluminum in diverse applications, especially in the automotive area. The production of ADI requires stringent control because the final ausferritic morphology can be influenced by chemical composition, holding time of heat treatment, cooling rate, etc. Standard ADI grades have been specified in terms of material properties .
The ADI heat treatment is comprised of two controlled steps: austenitizing and austempering. The austempering step can be subdivided into two continuous reaction stages depending on holding time. The unique ausferritic matrix can be achieved only within the first stage. In the second stage, the high carbon content austenite will be decomposed into ferrite and carbon will be precipitated in the form of carbide. In this case, some of the mechanical properties of ADI will be degraded with the formation of bainite in the matrix.
In industry, tempering has been utilized as an effective heat treatment to reduce brittleness and relieve the internal stress of quenched and normalized materials. The ductility and toughness of materials can be improved by tempering along with a decrease in hardness. The resulting material properties are dependent on the tempering temperature and time and alloying elements and their percentages. However, the hardness may not be changed by the tempering process for some materials containing Mo and S, and high speed steel after tempering is also an exception, which becomes harder due to completed martensite formation.
The hardness of austempered low carbon equivalent ductile iron decreased with increasing tempering temperature. The microstructure of austempered ductile cast iron had an austenite-free ferritic matrix after being tempered at 484 °C for 2 h. The hardness of tempered ADI samples could be increased or decreased, dependent on the original microstructure of non-tempered ADI.
The hardness of ADI samples decreases with increasing austempering temperature at the same holding time, and also decreases with increasing tempering temperature for each austempering condition.
The ADI samples austempered at 373 °C have thick feather-like ausferrite, but thin needle-like ausferrite is formed for 276 °C and 321 °C austempering temperatures.
The ausferritic structure is gradually decomposed into dispersive cementite particles with higher tempering temperatures. There is very little needle-like component which still exists at and above 538 °C tempering temperature for ductile iron samples austempered at 276 °C and 321 °C. Also, there is only a small amount of feather-like component existing at and above 482 °C tempering temperature for ductile iron samples austempered at 373 °C.
The above results related to hardness and microstructure are important for selecting proper tempering temperatures and should also be considered when applying case hardening processes, such as nitriding treatment (500–600 °C) and nitrocarburizing treatment (530–600 °C).