Influence of heat treatment time on microstructure evolution of austempered nodular cast iron evaluated by image segmentation

This study aimed to investigate the microstructural characteristics of a pearlitic austempered nodular cast iron (ADI) prepared with different austempering times. In austempering of ductile irons, the material is austenitiezed and rapidly cooled to temperatures typically between 420 and 290°C in a salt bath to allow the formation of a microstructure composed of graphite nodules dispersed in a residual austenitic matrix, stabilized with a high carbon content, and acicular ferrite, known as “ausferrite”. Presently, microstructure characterization by optical and scanning electron microscopy were carried out after different austempering times. Neural network image segmentation using ImageJ® software was employed to perform quantitative phase analysis, and the results were compared with volume fractions obtained by XRD measurements, one of the traditional methods for determining the residual austenite content in ferrous alloys. It was found that both carbon content in austenite and austenite fraction increase with austempering time and that graphite nodule geometry becomes gradually more irregular. The proposed methodology for quantitative analysis allowed classification of microstructure constituents with high accuracy in comparison to the XRD results.