Microstructural evolution in austempered ductile iron during non-isothermal annealing

This work investigates the thermal stability of ausferritic microstructures such as those found in NiCuMo austempered ductile iron (ADI). Typical ADI microstructures consisting of acicular ferrite and high carbon austenite were produced by heat treating NiCuMo ductile iron for 2 h at 315 °C and 370 °C. It was found that low temperature austempering led to the precipitation of a hexagonal ε-carbide phase. Hence, differential thermal analysis was used to closely follow the active phase transformations found in the experimental ADI during non-isothermal annealing. From these tests, it was apparent that the ausferrite stability is strongly influenced by the austempering conditions and heating rates. In particular, the experimental outcome shows that the ADI microstructures decompose at temperatures of 428°C and 459°C for irons austempered at 315°C and 370°C, respectively. Further evidence for the decomposition reactions involving acicular ferrite and high carbon austenite was provided by TEM observations. The TEM results indicate that ausferrite decomposes into ferrite-cementite during non-isothermal annealing via the precipitation of transition carbides. In the iron austempered at high temperature, orthorhombic η-Fe₂C was identified as the transition carbide, whereas tricilinic silicon-carbide precipitates became dominant in ADI treated at 315°C.