This research studied the effect of a two-step austempering process on the fracture toughness of ductile iron and compared it to that of the conventional upper-and lower-ausferrite austempered ductile irons (ADIs). The results showed that such a two-step austempering heat-treatment process yielded a fracture-toughness value equivalent to that of the upper-ausferrite ADI, while the hardness was maintained at the level of lower-ausferrite ADI. This provided a unique combination of high toughness with good hardness (strength) properties for the ADI with a two-step austempering. Optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural features.
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