Influence of stepped austempering process on the fracture toughness of austempered ductile iron

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 conventio nal upper- and lower-ausferrite austempered ductile irons (ADIs). The resul ts 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. Thi s provided a unique combination of high toughness with good hardness (stren gth) properties for the ADI with a two-step austempering. Optical microscop y, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural featu res.