An analysis of retained austenite in austempered ductile iron
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THE remarkable combination of strength and ductility attained in austempered ductile iron (ADI) has been described by many researchers.[1–7] The heat treatment includes two steps: austenization at temperatures around 850 ⬚C to 950 ⬚C, the exact temperature controlling the carbon concentration of austenite, which is in equilibrium with graphite and followed by rapid cooling to the bainite temperature range (250 ⬚C to 450 ⬚C) for 1 to 3 hours before cooling to ambient temperature. The isothermal formation of bainitic ferrite is termed the austempering process. Austempering leads to a final microstructure, which is a mixture of highcarbon retained austenite with bainitic ferrite or ferrite-carbide platelets distributed throughout, depending on temperature employed. The choice of austempering time is critical; the isothermal heat treatment should stop within a process window[1,4,8] to allow bainitic ferrite to maximize, but before the precipitation of carbides. Steels alloyed with more than 2 wt. pct of silicon, which can retard cementite precipitation,[9] producing carbide-free austenite in bainitic microstructures. Cast iron also contains more than 2 wt. pct of silicon and austenite is always free of carbide. Apart from graphite nodules, bainite in ADI and high silicon steel are essentially identical;[9,10] the established bainite transformation mechanism for high silicon steel therefore can be applied to ADI. Bainite forms in two distinct stages,[12,13] the first involving the growth of bainitic ferrite by displacive transformation, and the second the partitioning of excess carbon into the residual austenite. At elevated temperatures, if all excess carbon escapes into the residual austenite, upper bainite is obtained with a microstructure of bainitic ferrite and carbonenriched austenite. Cementite may eventually precipitate from the carbon-enriched residual austenite. As the reaction temperature is reduced, some carbon precipitates as fine carbides within the ferrite, with the remainder partitioning into austenite and precipitating as carbides. The bainite reaction ceases when the carbon concentration of the austenite L.C. CHANG, Associate Professor, is with the Department of Mechanical Engineering, Kuang Wu Institute of Technology, Taipei, Taiwan 112, Republic of China. Contact e-mail: [email protected] or hml118@ ms7.hinet.net Manuscript submitted April 6, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
reaches a T0 value. The loci of points at which austenite and ferrite of the same chemical compositions have identical free energy are named the T0 curve.[14] It was modified to T ⬘0[15] in order to account for the strain energy associated with the phase transformation in ferrite. The ADI’s mechanical properties are determined largely by its main constituents: bainitic ferrite and retained austenite. The carbon concentration in retained austenite affects its chemical and mechanical stability, and the volume percent of retained austenite is vital to ductility and toughness.[7,16,17] The aim of the presen
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