Effect of the Austenitizing Temperature on Microstructure Evolution and Impact Toughness of a Novel Bainite Ductile Iron

The effect of austenitizing temperature on microstructure evolution and impact toughness of a newly developed Fe–3.0C–2.8Si–2.0Mn–0.9V–0.2Cr bainite ductile iron was investigated in this research. The ductile iron specimens were heat treated under different continuous cooling process, involving austenitizing between 900 and 980 °C and followed tempering at 200 °C. Optical microscopy, X-ray diffraction, scanning electron microscope and transmission electron microscope tests were conducted to investigate the microstructure evolution. Impact toughness and Rockwell hardness were measured. The results showed that the microstructure of the ductile iron mainly consisted of graphite, acicular bainite and retained austenite after continuous cooling process. The austenitizing temperature could change the volume fraction and size of bainite and retained austenite. There existed a C-area, where retained austenite accumulated near the graphite, except for specimen austenitized at 920 °C. The impact toughness of specimens increased first and then get worse with the increasing of austenitizing temperature. The impact toughness was related with the volume fraction of bainite and the morphology of retained austenite. The fracture mechanism of the bainite ductile iron belonged to cleavage fracture. Chunky graphite acted as the source of microcrack during the impact process. The bulky retained austenite behaved as a prior path for the microcrack propagation, while the bainite and thin filmy retained austenite limited its propagation. Graphic Abstract