Effect of austempering temperatures on mechanical properties of dual matrix structure austempered ductile iron

Abstract This study investigates the effect of the austempering temperatures on the microstructures and mechanical properties of austempered ductile iron with dual-matrix structure (DMS-ADI). An unalloyed ferritic ductile iron bar samples were intercritically austenitized at 810 °C for 60 min and austempered at different austempering temperatures (300 °C, 325 °C, 350 °C, and 375 °C) for 120 min. Microstructural and mechanical characterization were examined using optical microscopes, scanning electron microscopes (SEM), X-ray diffraction (XRD) analyses, hardness, and tensile tests. Experimental results showed that the DMS-ADI samples exhibit a dual-phase structure consisting of proeutectoid ferrite + ausferrite structures. The volume fraction of ausferrite (42.5–47.5 %) was almost stable in all austempering temperatures in DMS-ADI samples. The high-carbon retained austenite volume fraction, and its carbon content increased with increased austempering temperatures. At austempering temperatures up to 350 °C, strength and hardness increased, while energy at break and total elongation decreased. The strength and hardness decreased, while energy at break and total elongation increased at 375 °C austempering temperature. All DMS-ADI samples exhibited ductile fracture surfaces except for those austempered at 350 °C. The highest ( σ UTS = 590 MPa and ε T = 18.8 %) and lowest ( σ UTS = 434 MPa and ε T = 28.5 %) mechanical properties were obtained in DMS-ADI samples austempered at 350 °C and 300 °C, respectively.