Effects of austempering time on the microstructure and properties of austempered ductile iron

The effects of austempering time on the structure and properties of high-strength austempered ductile iron were studied by using optical microscopy (OM), X-ray diffractometer (XRD) and scanning electron microscope (SEM). The results show that the matrix structure of austempered ductile iron (ADI) consists of acicular ferrite and retained austenite. With the increase of austempering time, the content of acicular ferrite increases and the content of retained austenite first increases and then decreases, which results in tensile strength, elongation and impact toughness increase whereas hardness and wear resistance decreases. The fracture characteristics of the ADI specimens change from brittle fracture to ductile fracture with the increase of austempering time. ADI has excellent comprehensive mechanical properties after austenitizing at 900 °C for 90 min and then austempering at 250 °C for 120 min.