Mechanical Stability of High-Carbon Retained Austenite and Corresponding TRIP Effect in Austempered Ductile Iron (ADI)

Abstract The mechanical stability of austenite and its corresponding TRIP effect varies among different ADI grades, mainly due to its different volume fraction, morphology, and size. Despite the extensive research on ADI, a detailed quantitative evaluation of austenite stability using both experimental techniques and mathematical approach remains limited. This study addresses this gap by investigating the mechanical stability of austenite in two ADI grades austempered at 300 and 370 °C. Experimental observations, including interrupted tensile tests and XRD analysis, were integrated with two well-established mathematical approach to quantify austenite stability and its impact on the TRIP effect. The results revealed that ADI austempered at 300 °C exhibited greater mechanical stability due to its smaller austenite size and lower fraction of blocky morphology. In both ADI grades, blocky austenite transformed into martensite at the early stages of tensile deformation, particularly in ADI treated at 370 °C. Remaining austenite at later deformation stages was more enriched in carbon, with a more significant effect in ADI austempered at 300 °C. The mathematical models closely matched the experimental findings, further confirming the superior mechanical stability of austenite in ADI austempered at 300 °C. These insights highlight the crucial role of microstructural characteristics in optimizing the mechanical performance of ADI.