The incomplete transformation phenomenon in steel
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scovered by Wever and Lange[1] in 1932, has been described as the formation of significantly less than the Lever rule proportion of ferrite in the absence of carbide precipitation.[2] The overall reaction kinetics definition of bainite[3,4] ascribes to this transformation product its own C-shaped time-temperature-transformation (TTT) curve for the initiation of transformation, with the transformation to ferrite becoming increasingly incomplete as the upper temperature limit of this curve is approached[3] (Figures 1(a) and (b)). Zener[5] has proposed that this limit is the To temperature. Figure 2 shows decisively that this is not so, at least in the Fe-C-Mo system.[6] Inasmuch as international agreement on the definition of bainite has yet to be reached despite approximately 75 years of research, the other two principal definitions currently in use should be noted. One is the surface relief definition, wherein any plate-shaped (or lath-shaped) transformation product, formed above the martensite temperature range, that exhibits an invariant plane strain surface relief effect may be described as bainite. (On this view, distinguishing between martensite and bainite can thus be problematic, H.I. AARONSON, formerly R.F. Mehl University Professor Emeritus, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213-3890, and Visiting Professor, School of Physics and Materials Engineering, Monash University, Victoria 3800, Australia, is deceased. W.T. REYNOLDS, Jr., Professor, is with the Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA, 24061-0237. Contact e-mail: [email protected] G.R. PURDY, University Professor Emeritus, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. This article is based on a presentation made in the ‘‘Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,’’ December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden. METALLURGICAL AND MATERIALS TRANSACTIONS A
particularly below the Md temperature, the highest temperature at which martensite can form in deformed austenite.) The original microstructural definition of bainite[7,8] is Widmansta¨tten ferrite plates or laths with carbide precipitation at the austenite:ferrite boundaries. This definition has been subsequently generalized as a nonlamellar, competitive eutectoid reaction in which the eutectoid phases can have any morphology other than the alternating plates characteristic of pearlite.[4,9] Although claims to the contrary have been made,[10] and refuted,[11,12] these three definitions are often in conflict. A microstructure that is bainite on one definition may thus not be bainite on either or both of the other two definitions.[11,12] In this article, we will first summarize the experimental generalizations that have been made about the incomplete transformation (ICT) phenomenon. An attempt will then be made to situate ICT within the spectrum of e
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