An investigation of the generality of incomplete transformation to bainite in Fe-C-X alloys
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I.
INTRODUCTION
THE bainite
reaction has been the subject of extensive research since the 1930s. Because early studies II,2] of the structure of bainite in steel were hampered by difficulties encountered in resolving the fine structure of the transformation product with optical microscopy, much emphasis was soon placed upon characterizing the kinetic features of the bainite reaction. This approach led to the view that the overall reaction kinetics of bainite are characterized by several salient features. 13m Bainite has its own C-curve on a time-temperature-transformation (TTT) diagram distinct from the C-curve for the pearlite reaction. The highest temperature of the bainite C-curve, or the Bs (bainite start) temperature, is the maximum temperature of bainite formation; this temperature is usually well below that of the eutectoid temperature. At reaction temperatures just below the Bs, isothermal transformation of austenite to bainite is incomplete. The fraction of the parent austenite which transforms to bainite increases with decreasing reaction temperature from zero at the Bs to unity at the By (bainite finish) temperature. Below the W.T. REYNOLDS, Jr., formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Assistant Professor, Department of Materials Engineering, Virginia Polytechnic Institute, Blacksburg, VA 24061-0237. S.K. LIU, formerly Visiting Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Professor and Head, Department of Mechanical Engineering, Southwestern Jiaotong University, Emei, Sichuan, People's Republic of China. F.Z. LI, Visiting Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. S. HARTFIELD, formerly Undergraduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is Graduate Student, Department of Materials and Metallurgical Engineering, University of Michigan, Ann Arbor, MI 48109. H.I. AARONSON, R.F. Mehl Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213-3890. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
Bf, austenite can transform to 100 pct bainite. Early work on the bainite reaction based upon this view is reviewed in Reference 5; another generation of work is included in the review of Reference 4; more recent summaries are available in References 6 and 7; and brief current reviews are offered in the two preceding papers in these proceedings, t8,9] with a more comprehensive overview being provided in the first paper published in this group, t~~ It should be noted that the foregoing
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