Volume Fractions of Proeutectoid Ferrite/Pearlite and Their Dependence on Prior Austenite Grain Size in Hypoeutectoid Fe
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I.
INTRODUCTION
PROEUTECTOID ferrite and pearlite are the most common structures composing hot-rolled sheets and forging steels. In these steels, volume fractions of proeutectoid ferrite/pearlite strongly influence the strength and formability of the products. It is thus of great importance to predict their volume fractions and then to optimize their structures. It is well known that pearlite forms when alloys with eutectoid compositions are cooled below the Ae1 temperature. In addition, for hypoeutectoid or hypereutectoid alloys, pearlite can also form directly without primary precipitation of ferrite or cementite, as long as the alloys are located within the region enclosed by the extrapolated Ae3 and Acm lines (i.e., the shaded region in Figure 1). It is because of this that alloys within this
ZHEN-QING LIU, Ph.D. Student, is with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, P.R. China, also with the Department of Metallurgy, Tohoku University, Aoba-ku, Sendai 980-8577, Japan. GORO MIYAMOTO, Associate Professor, and TADASHI FURUHARA, Professor, are with the Institute for Materials Research, Tohoku University. Contact e-mail: [email protected] ZHI-GANG YANG, Professor, is with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University. Manuscript submitted April 12, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
region are simultaneously supersaturated with respect to both ferrite and cementite.[1,2] For hypoeutectoid alloys quenched to points outside the shaded region, such as X, proeutectoid ferrite will be precipitated first. During the precipitation of proeutectoid ferrite, carbon is rejected into untransformed austenite and then the carbon concentration of untransformed austenite increases. The untransformed austenite can be decomposed into pearlite only when its average carbon content reaches the extrapolated Acm line, since the Acm line is generally supposed to be the limit below which pearlite cannot form.[1–3] Thus, the average carbon content of the resultant pearlite should correspond to the Acm composition, i.e., cc/h (see Figure 1). The volume fraction of proeutectoid ferrite with respect to pearlite can then be predicted by the lever rule, the expression of which is presented in Section II. As far as the authors are aware, the above-mentioned classic understanding on isothermal formations of proeutectoid ferrite and pearlite in hypoeutectoid alloys has been widely accepted and adopted in textbooks[1,2] and published papers.[4–9] For the present, the classic understanding itself is usually unrecognized as a problem worthy of study since one has assumed it must be true. But, in practice, few studies have been carried out to verify it,[4] and it is yet to be examined fundamentally and experimentally even now. Again, take for an instance the hypoeutectoid alloy quenched to point X. A carbon diffusion field will build
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