High-speed quenching dilatometer investigation of the austenite-to-ferrite transformation in a low to ultralow carbon st
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sformation of fcc iron (austenite) to bcc iron (ferrite) has attracted the interest of researchers for many decades. This is mainly due to the fact that a number of morphological variants of austenite decomposition products are observed. Depending on the amount of carbon redistribution, interface coherence, and undercooling, morphologies such as allotriomorphic ferrite (grain boundary ferrite), Widmannsta¨tten ferrite, massive ferrite, bainite, or martensite can develop from the parent austenite. Each of these variants has characteristic mechanical properties and applications (e.g., References 1 and 2). In the last decades, intensive theoretical and experimental work has been undertaken to investigate the interface migration mechanisms and kinetics of the austenite-toferrite transformation. The present work is focused on the austenite-to-ferrite transformation kinetics in a low-alloyed very low-carbon steel. In the experiments, the specimens are austenitized and quenched at maximum rates to various annealing temperatures employing a high-speed quenching dilatometer. The holding temperatures were chosen to be close to the equilibrium phase boundary of ferrite to keep the chemical driving force low. These conditions promote formation of ferrite starting at grain boundaries and avoid intergranular nucleation. Depending on the chemical composition of the specimens and the available driving force, interface migration during growth of the ferrite phase can be either diffusion controlled or interface controlled. In the latter case, the product phase is commonly denoted as ‘‘massive ferrite’’, and the parent and product phases have identical chemical composition. From the measured sample dilatation, information on the ferrite fraction transformed over time is obtained and conclusions on the prevailing interface migration mechanism are drawn. Particular focus of this work is placed on the E. KOZESCHNIK, Assistant Professor, is with The Institute for Materials Science, Welding and Forming, Graz University of Technology, A-8010 Graz, Austria, and The Materials Center Leoben, A-8700 Leoben, Austria. ¨ GER, Researcher, Contact e-mail: [email protected] E. GAMSJA is with the Institute of Mechanics, Montanuniversita¨t Leoben, A-8700 Leoben, Austria. This article is based on a presentation made in the ‘‘Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,’’ December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden. METALLURGICAL AND MATERIALS TRANSACTIONS A
exploration of the transition from an initially very rapid interface mobility-controlled reaction to a slower carbon diffusion-controlled reaction. Finally, the interface kinetics is analyzed quantitatively. From the first, fast stage of the g–a transformation—the interface mobility-controlled growth of ferrite—the intrinsic mobility of the interface is estimated.
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EXPERIMENTAL
Specimens of pure Armco iron were machined into tubes of 10-mm length, 5-mm outer diameter, and variable w
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