Austenite in Transformation-Induced Plasticity Steel Subjected to Multiple Isothermal Heat Treatments
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NTRODUCTION
EVER since the work of Papadimitriou and Fourlaris[1] on carbide-free bainite, it has been known that implementing two stages of isothermal transformation, both below the bainite-start temperature, leads to a refinement of microstructure. The amount of bainite that can form at a given temperature is limited by the T0 curve, which represents the locus of points on a temperature vs carbon concentration plot where austenite and ferrite of identical composition have the same free energy. A second step at a lower temperature therefore permits further transformation to bainite, thus reducing the quantity of retained austenite in the final structure. Bainite plates generated at a lower temperature are also finer[1,2] and hence contribute more to strength.[3,4] A two-stage process of this kind can be beneficial in reducing the overall transformation time if the reaction at the lower temperature is slow. The internal stresses generated during transformation at the higher temperature can stimulate the kinetics of transformation at a lower temperature.[5–7] Coarse regions of austenite are known to lead to poor toughness and ductility.[8–11] It is reasonable therefore to expect an improvement in the mechanical properties
VAN TUAN DUONG and YOU YOUNG SONG, Graduate Students, and DONG-WOO SUH, Associate Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Kyungbuk 790-784, Korea. Contact e-mail: [email protected] KYONG-SU PARK, Principal Researcher, is with the Technical Research Laboratories, POSCO, Pohang 790785, Korea. H.K.D.H. BHADESHIA, Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, and also with Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K. Manuscript submitted February 14, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
using a two-step heat treatment that allows sufficient time in each stage for the bainite reaction to stop. Using such heat treatments, a large increase in ductility and toughness was achieved in a high-carbon steel.[12] Similarly, a significant improvement in the tensile strength, without compromising ductility, has recently been reported for a medium carbon steel subjected to this kind of a two-step isothermal transformation, with the benefits attributed to the refinement of structure.[13] The detailed composition of the steel studied was Fe-0.3C-1.46Si-1.97Mn-1.5Ni-0.3Cr-0.96Cu-0.25Mo wt pct
The purpose of the present work was to study the role of two-step transformation into bainite in steels with a much lower substitutional solute content and two different carbon concentrations, and to assess specifically the role of the retained austenite during the course of deformation. As will be seen later, almost identical properties are possible to achieve with these simpler steels, and it will be shown that the two-step heat treatment does not always lead to improved properties relative to the single isothermal transformation experiment.
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