Accelerated Nano Super Bainite in Ductile Iron
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.440
Accelerated Nano Super Bainite in Ductile Iron Eric Jiahan Zhao1, Cheng Liu2, and Derek O. Northwood3 1
Beijing New Oriental Foreign Language School at Yangzhou, Yangzhou, Jiangsu, P R China
2
College of Mechanical Engineering, Yangzhou University, Yangzhou, Jiangsu, P R China
3
Mechanical, Auto and Materials Engineering, University of Windsor, Windsor, Ontario, Canada
ABSTRACT
A commercial unalloyed ductile iron has been developed to produce a multiphase matrix microstructure consisting of lenticular prior martensite, feathery upper bainite and a nanoscaled super bainite of lath bainitic ferrite and carbon-enriched film retained austenite. Multistep heat treatment composed of austenizing, rapidly quenching and isothermally holding at low temperature have been developed. A tensile strength of more than 1.6 GPa, a hardness higher than HRC 54, 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 prior to the isothermal treatment, accelerates the kinetics of subsequent nano super bainitic transformation by bainitic laths nucleating quickly at the martensite-austenite interfaces.
INTRODUCTION Ductile iron (DI) is a ferrous material with good castability, wear resistance, mechanical properties and low cost. Austempering, a heat treat process, when applied to DI produces a material called austempered ductile iron (ADI) has a strength-to-weight ratio superior to conventional DIs [1]. However, taking into account the requirement of more important structural application such as carburized automotive gears, the demand for ADIs with higher strength and ductility is still increasing [2, 3]. In the newly developed high strength steels, two types of heat treating processes have received considerable attention. One is the quenching and partitioning (Q&P) process, which consists of an initial quenching step to a temperature between Ms (a starting temperature of martensitic transformation) and Mf (a finishing temperature of martensitic transformation), and a second partitioning step to a temperature above the quenched temperature [4, 5]. Another is the low-temperature isothermal process, which comprises the formation of nanobainite by isothermally holding at a very low temperature close to Ms, such as 200ºC [6, 7]. The excellent properties of Q&P steel are related to the unique dual-phase microstructure that is composed of a martensite for high strength, and carbon
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stabilized retained austenite (RA) for good ductility. The steel with nano-scaled bainitic ferrite (BF) and carbon-enriched film austenite ex
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