In-situ observations of lattice parameter fluctuations in austenite and transformation to bainite
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I. INTRODUCTION
THE role of carbon during the transformation of austenite to bainitic ferrite in steels is interesting from a fundamental as well as a technological viewpoint. It is possible that bainite forms without diffusion and carbon subsequently redistributes or precipitates as carbides.[1,2] An alternative interpretation is that the ferrite grows with its equilibrium carbon concentration.[3,4] Historically, it has also been speculated that the austenite becomes heterogeneous with carbon-enriched and carbon-depleted regions, so that ferrite formation initiates in the carbon-depleted regions.[5–11] In the present work, it has been possible to follow the lattice parameter changes associated with the austenite at any temperature and during the course of the bainite transformation. If the changes can be attributed to solute concentration, then they help interpret the role of carbon. II. EXPERIMENTAL The chemical composition of the steels used in this investigation is Fe-0.75C-1.63Si-1.95 Mn-0.29Mo-1.48Cr-0.1V-0.01Al0.003P-0.003S (wt pct). This steel was selected because the transformation rate is slow due to high carbon concentration and there are no other transformations including carbide precipitation that interfere with the formation of the bainitic ferrite.[12] The microstructure following isothermal transformation is a mixture of bainitic ferrite and carbon-enriched retained austenite. The silicon concentration is sufficiently high to prevent the precipitation of cementite from austenite, as would normally occur in the upper bainite transformation temperature range.[1]
S.S. BABU, Technology Leader, formerly at Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, is with the Edison Welding Institute, Columbus, OH 43017. E.D. SPECHT, Distinguished Research Staff, and S.A. DAVID, Corporate Fellow, Group Leader, are with the Metals and Ceramics Division, Oak Ridge National Laboratory. E. KARAPETROVA, Beam Line Scientist, and P. ZSCHACK, Group Leader, UNICAT, are with the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana–Champaign, Urbana, IL 61801. M. PEET, Graduate Student, and H.K.D.H. BHADESHIA, Professor, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB23QZ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted March 11, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
Rectangular samples (2 4 95 mm) were made from this high-strength steel bar, which was homogenized at 1200 °C for 48 hours, for the diffraction experiments. It is important to note that after this homogenization treatment, the microstructure of the samples was essentially martensitic, with small amounts of retained austenite. Room-temperature X-ray diffraction measurements also failed to show any texture in these samples. These homogenized samples were then heat treated in situ in a synchrotron beam line using the resistive heating method. The samples were heated to 1273 K and held at that temperature for 4 min
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