2, Yangzhou University, Yangzhou, Jiangsu, China
3, University of Windsor, Windsor, Ontario, Canada
Ductile iron (DI) is a unique engineering material due to its low production costs and attractive mechanical properties. However, compared with forged steel, the relatively poor toughness still imposes limitations on its widespread use. Inspired by a quenching and partitioning process (Q&P), and the formation of a nano super bainite at low temperatures in very high strength steels, a commercial unalloyed ductile iron has been heat treated to produce a multiphase matrix microstructure consisting of lenticular prior martensite (PM), feathery upper bainite (UB) and a nano super bainite composed mainly of nano-scaled lath bainitic ferrite (BF) and a carbon-enriched austenite (RA) film. Multi-step thermal treatments composed of heating the DI to 890°C for 20min followed by rapidly quenching in a patented water-based liquid at 190°C, which is a slighter lower than the starting temperature of martensite transformation (Ms) for controlling amount of PM, have been developed. The DI is then reheated to 220°C for times between 5 and 240min in an electric furnace, rather than a typical salt bath treatment used for most high strength DI, and subsequently air-cooled to room temperature. A very high tensile strength of more than 1.6 GPa, a hardness of HRC54, and an elongation in excess of 5%, are achieved. This is attributed to a synergistic multi-phase strengthening effect. The developed nano super bainite exhibits a good balance between strength and toughness. The presence of martensite formed during the quenching process prior to the isothermal treatment, accelerates the kinetics of subsequent nano super bainitic transformation by bainitic laths nucleating quickly at the martensite-austenite interfaces. This design methodology potentially broadens the application of DI to components that experience in a more demanding service environments such as heavy loads.