Development of Austempered Ductile Iron for High Tensile and Fracture Toughness by Two-Step Austempering Process

摘要

During conventional austempering, austempered ductile iron showed a decrease in fracture toughness with increasing austempering temperature, while the tensile toughness increased. Thus high fracture toughness was associated with low tensile toughness. A two-step austempering treatment was then adopted where the samples were first austempered at 300℃ for short periods varying from 10 minutes to 60 minutes, and then subsequently transferred to a second furnace at 400℃ for further austempering for 2 hours. It was found that this resulted in fine ferrite grain size, high carbon content of the retained austenite together with increased stability of the austenite. Under such conditions, it was possible to achieve an excellent combination of high fracture toughness and high tensile toughness.