Effect of Austempering Process on Microstructure and Mechanical Properties of QT800‐5 Ductile Iron

This study systematically investigated the effects of austempering process parameters on the microstructure evolution, crystalline structure, and mechanical properties of QT800‐5 ductile iron. The material was characterized using optical microscopy (OM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results indicate that the austempering process significantly influences the matrix structure, including the size of acicular ferrite, the volume fraction of retained austenite, and its carbon concentration, which collectively determine the mechanical response. Optimal properties were achieved by austenitizing at 890°C for 1 h, followed by austempering at 360°C for 2 h. Under these conditions, the austempered ductile iron (ADI) exhibited an excellent combination of strength and toughness, with an ultimate tensile strength (UTS) of 1235 MPa, elongation of 16%, and impact energy of 12 J. The corresponding microstructure consists primarily of fine acicular bainitic ferrite and ∼34.9 vol.% carbon‐enriched retained austenite. Furthermore, EBSD analysis revealed the coexistence of both Kurdjumov–Sachs (K‐S) and Nishiyama–Wasserman (N‐W) orientation relationships between the bainite and austenite in nearly equal proportions. A preferential formation of specific variants was observed in samples austempered at higher temperatures.