Investigation of Austenite-to-Ferrite Transformation in Ultralow and Low-Carbon Steel Using High-Speed Quenching Dilatom
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THE decomposition of austenite can give birth to a variety of industrially important morphologies such as ferrite, bainite, and martensite. Therefore, the kinetics of austenite decomposition is of immediate relevance to the metal processing industry. The kinetics is dependent on the phase transformation mechanisms, which play a pivotal role in microstructure-forming processes that tune these morphologies. The austenite (c)–to–ferrite (a) transformation is one of the most complex of these processes. Commonly, long-range diffusion-controlled as well as martensitic and massive transformation mechanisms are operative in steels. Long-range diffusion-controlled transformation occurs at low cooling rates, because these rates allow for equilibrium partitioning of (interstitial) alloying elements, such as carbon (C), during phase separation. Bhadeshia[1] and Christian[2] characterize the austenite-to-ferrite transformation as a part of reconstructive transformation mechanisms. Martensitic transformations occur at high cooling rates, because substantial diffusion during phase separation is practically suppressed. Experimentally, these high cooling rates are obtained by rapidly quenching the austenite to F. IMTIAZ is with the Institute of Materials Science and Technology, TU Wien, 1060 Wien, Austria, and also with Materials Division, Directorate of Technology, PINSTECH, Islamabad 45650, Pakistan. E. KOZESCHNIK is with the Institute of Materials Science and Technology, TU Wien. Contact e-mail: farhan_92saifi@yahoo.com Manuscript submitted September 22, 2016. Article published online April 6, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
room temperature. At intermediate cooling rates, either the massive or the two-stage transformation mechanism is operative. The massive transformation also does not involve any partitioning of alloying elements. The product phase (a) inherits the chemical composition of the parent phase (c) and is characterized by a microstructure with the typical appearance of ‘‘massive grains.’’ In the two-stage transformation observed in the present work, phase separation proceeds in two stages. Either the first stage can proceed without partitioning or vice versa, depending on the cooling rates. Massalski[3] investigated the massive transformations in iron and its alloys at intermediate cooling rates, i.e., at cooling rates high enough to avoid long-range diffusion-controlled transformations and, at the same time, not high enough to allow for martensitic transformations. Bibby and Parr[4] studied the effect of cooling rate on the mechanism of austenite-to-ferrite transformation in ultralow carbon (ULC) steel using a gas quench unit with supersonic gas velocities for obtaining cooling rates in excess of 35,000 K/s. They plotted the transformation start temperature as a function of cooling rate and observed that the trend follows a two-plateau behavior. The plateau, which was obtained at medium cooling rates up to 5000 K/s and higher transformation start temperature, was attributed to the massive transfo
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