Microstructural development and austempering kinetics of ductile iron during thermomechanical processing
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I. INTRODUCTION
AUSTEMPERED ductile iron (ADI) offers a better combination of strength, ductility, toughness, fatigue and wear resistance, and design flexibility at low cost than any other material, allowing the manufacturer to obtain a wide range of properties in a component. It offers this superior combination of properties because it can be cast like any member of the ductile iron family and then subjected to an austempering heat treatment, which produces mechanical properties that are superior to all cast irons and many cast steels.[1] Austempering treatment consists of fully austenitizing ductile iron of appropriate chemical composition, quenching to an austempering temperature and holding for an appropriate length of time to permit isothermal transformation, and air cooling to room temperature. The austempering reaction in ADI occurs in two stages. Stage I: g0 (C 0g) → a 1 gS (C Sg) (formation of ausferrite) Stage II: gS (C Sg) → a 1 carbide (formation of bainite) In stage I, the matrix austenite g0 with carbon content C 0g transforms isothermally to ausferrite, which is a mixture of acicular ferrite and carbon-enriched stabilized austenite gS with carbon content C Sg. In stage II, the stabilized austenite gS decomposes to ferrite and carbide, decreasing the stabilized austenite phase fraction. The austempering reaction in ADI differs from the analogous reaction in steels, where austenite decomposes uniformly into the ferrite-carbide aggregate called bainite in a relatively shorter period of time. Thermodynamics of Austempering The thermodynamics of austempering transformation depends on the chemical composition of the iron used and can be analyzed using the appropriate phase diagram. The
JAGADEESHA ACHARY, formerly Graduate Student, Materials Department, University of Wisconsin, is Metallurgist, Karl Schmidt Unisia, Fort Wayne, IN 46803. DEV VENUGOPALAN, Professor, is with the Materials Department, University of Wisconsin, Milwaukee, WI 53211. Manuscript submitted May 24, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
matrix carbon content C 0g at the austenitizing temperature is obtained from g /g 1 Gr (austenite/austenite 1 graphite) phase boundary at the temperature. Since the phases in the ausferrite phase mixture at the austempering temperature are in a metastable state, the compositions of these phases can be obtained from a projection of the a 1 g (ferrite 1 austenite) region from above the eutectoid temperature down to the region of austempering temperature. The extension of the a 1 g field down to the austempering temperature in the equilibrium diagram suggests that with decreasing austempering temperature, both ferrite and austenite phases dissolve increasing amounts of carbon, as much as 0.2 pct carbon in the former and more than 2 pct in the latter.[2] The driving force for the reactions occurring at the austempering temperature can be understood from a free energy–composition diagram of the iron at the temperature.[3] A schematic free energy–composition diagram along with corresponding phas
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