An investigation on the stability of austenite in Austempered Ductile Cast Iron (ADI)

Austempered Ductile Cast Iron (ADI) has emerged as an important commercial engineering material in recent years because of excellent properties such as high strength with good ductility as well as good fatigue strength and fracture toughness together with excellent wear resistance. These properties are as a result of a microstructure composed primarily of acicular ferrite (α) and high carbon austenite (γHC). The role of the austenite in ADI is a topic that has been discussed extensively in the literature. Several investigators in the past have reported that the austenite in ADI is both thermally and mechanically stable; while others have reported it is neither thermally nor mechanically stable. Moreover, it is not clear whether the austenite formed at lower austempering (or bainitic) temperatures is more mechanically and thermally stable than that produced in the upper austempering (or bainitic) ranges. In this investigation, a comprehensive study was carried out to examine both the thermal and mechanical stability of austenite in the upper and lower austempering (or bainitic) temperature ranges. Compact tension and cylindrical tensile specimens were prepared and austempered at five different temperatures ranging from lower to upper bainitic temperatures. A select number of samples were then cryogenically treated to examine the thermal stability of the high carbon austenite. In order to examine the mechanical stability of austenite the microstructure and mechanical properties of these materials were evaluated as a function of austempering temperature, cryogenic treatment, and mechanical testing. The results of this study show that the austenite formed at lower austempering temperatures (260 and 288°C) was neither thermally nor mechanically stable; while the austenite formed in the upper bainitic temperature range (371–399°C) was partially stable under both thermal and mechanical conditions. Both stress- and strain-induced austenite-to-martensite transformations were observed in the ADI samples during mechanical tests. The test results also showed that cryogenic treatment can improve the mechanical properties without compromising the fracture resistance of the ADI. This is hypothesized to be due to a threshold level for the amount of austenite transformed to martensite; below this level, the amount transformed is not sufficient to produce an observable change in the fracture toughness.