Effect of the Starting Microstructure in the Formation of Austenite at the Intercritical Range in Ductile Iron Alloyed with Nickel and Copper

Intercritical austenitizing is a key step on the production of dual-phase austempered ductile iron. Therefore, understanding the formation of austenite at the intercritical range should provide critical information for the future development of this family of alloys. In this work, a ductile iron alloyed with copper and nickel (3.4 C, 2.6 Si, 0.9 Ni, 0.6 Cu, wt%) was studied. The as-cast alloy was submitted to ferritic annealing and normalizing in order to obtain fully ferritic and fully pearlitic microstructures, respectively. The effect of microsegregation, initial microstructure (ferrite or pearlite) and nodule count on the formation of austenite in the intercritical range under continuous heating was studied using electron probe micro-analyzer-EPMA- high-resolution dilatometry, optical microscopy and scanning electron microscope-SEM-. The results showed that silicon, copper and nickel segregate around the graphite nodules and manganese segregates to the last freezing zones. Also as nodule count increases the segregation level decreased. Regarding the rate of austenite formation, the results showed that it increases as nodule count increases. Additionally, austenite formation is faster when the starting microstructure is pearlitic and it increases as the pearlite interlaminar spacing decreases. Finally, the results showed that the critical temperatures for austenite formation depend mainly on the starting microstructure (ferrite or pearlite).