Development of high strength austempered ductile iron (ADI) from conventional pig iron

An alloy ductile iron can be transformed into high strength austempered ductile iron (ADI) by transforming ferritic-pearlitic matrix into ausferrite after subjecting to austempering heat-treatment process. ADI can be used for making complex shapes components for high strength applications owing to its castability. The present work aims to develop ductile iron containing Ni and Mo using low cost conventional pig iron rather than expensive Sorel pig iron by ladle sandwich method in a commercial high-frequency induction furnace. Effect of austempering time ranging from 30 to 120 min at austempering temperatures of 265 degrees C and 330 degrees C on microstructure and mechanical properties of resulting ADI was evaluated. Results showed that ADI developed from the cost-effective ductile iron produced by pig iron have mechanical properties comparable to ADI developed using Sorel pig. Optical and scanning electron microscopy revealed that large size graphite nodules were formed in the matrix of ausferrite and retained austenite at 265 degrees C austempering for 30 min. Increasing austempering time resulted in an increase in ausferrite fraction with the decrease in retained austenite fraction. ADI developed by austempering at 330 degrees C resulted in a small fraction of ausferrite and a large fraction of retained austenite compared to that developed at 265 degrees C. Austempering heat treatment process considerably improved mechanical properties of ductile iron at both tempering temperatures (265 and 330 degrees C). The tensile strength, elongation and hardness of ADI produced by austempering at 265 degrees C for 90 min soaking time increased by 42, 83 and 26% compared to that produced at 330 degrees C for 90 min.