Influence of magnetic field on the kinetics of proeutectoid ferrite transformation in iron alloys
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I. INTRODUCTION
THE increasing availability of strong magnetic fields enables us to study magnetic field effects on various phase transformations in steel.[1,2] Compared to diffusionless martensitic transformation in iron alloys,[3–6] the studies of magnetic field effects on transformations that involve solute diffusion are comparatively few; Peter and Miodownik[7] reported the acceleration of ␥ → ␣ ⫹ ␥ transformation in Fe-Co alloys around 1000 ⬚C in a field of 19 kOe (⫽1.9T). An increase in hardness was reported to occur by magnetic aging of Fe-Cr alloys in the field of 14 T.[8] Recently, magnetic field effects were studied on continuous cooling ferrite transformation in iron alloys,[9] the reverse transformation in Fe-C alloys,[10] and pearlite transformation in an Fe-CCr alloy.[11] In parallel to these studies, it was shown by thermodynamic calculations that the driving force for ferrite and martensitic transformations is increased in the presence of a magnetic field.[4,6,7,12] In this article, the influence of an externally applied static magnetic field on the kinetics of proeutectoid ferrite transformation was studied in Fe-C base alloys in a temperature range that covers both the ferro- and paramagnetic states of ferrite. The Gibbs free energy of iron and its alloys in a magnetic field was calculated using experimental data on the magnetic susceptibility and Weiss molecular field theory.[13,14] The observed change in the nucleation and growth kinetics was discussed in terms of the change in the phase stability of ferrite and austenite due to an applied magnetic field. II. EXPERIMENTAL PROCEDURES The alloys were vacuum induction melted from electrolytic iron and high purity carbon, nickel, and boron. After
M. ENOMOTO, Professor, Department of Materials Science, H. GUO, Graduate Student, Faculty of Engineering, and Y. TAZUKE, Professor, Basic Sciences for Engineering, are with Ibaraki University, Hitachi 3168511, Japan. Y.R. ABE, Senior Research Scientist, is with Steel Research Laboratories, Nippon Steel Corporation, Futtu 293-8511, Japan. M. SHIMOTOMAI, Principal Researcher, is with Technical Research Laboratories, Kawasaki Steel Corporation, Chiba 260-0835, Japan. Manuscript submitted January 31, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
hot forging, they were homogenized at 1200 ⬚C for 2 days. The chemical compositions of alloys are shown in Table I. Alloy (a) containing 0.11 mass pct C was prepared for studying the transformation at a temperature above the Curie temperature (designated as Tc); boron was added to retard the transformation kinetics by suppressing the nucleation of ferrite.[15] In alloy (b), containing 0.39 mass pct C, the Ae3 temperature is close to Tc and nucleation and growth rates of ferrite were measured at temperatures 20 ⬚C to 30 ⬚C below Tc. In alloy (c), the transformation kinetics were studied at a temperature considerably below Tc without the concurrence of pearlite reaction. Cylindrical specimens of 4 mm in diameter and 12 to 18 mm in length were machined from homogeniz
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