Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys
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I.
INTRODUCTION
THIS investigation was
undertaken primarily to assess the role of growth kinetics in producing the bay in isothermal time-temperature-transformation (TTT) diagrams and especially in engendering the incomplete transformation or stasis phenomenon. Iron-carbonmolybdenum alloys were employed as the vehicle for this research mainly because sufficient concentrations of Mo have long been known to produce a conspicuous bay region tm] but also because Mo readily restricts the formation of pearlite to temperatures high above that of the bay. t31 As the investigation progressed, however, it was found that extreme degeneracy of the ferritic component of the microstructures effectively prevented reliable measurements on growth kinetics from being made at temperatures significantly below that of the bay, Tb. If the TTT diagram consists of two independent C-curves,
G.J. SHIFLET, formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA, is Professor, Department of Materials Science, University of Virginia, Charlottesville, VA 22903. H.I. AARONSON, formerly Professor, Michigan Technological University, is R.F. Mehl Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 Word 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. METALLURGICALTRANSACTIONSA
Tb is the asymptotic upper limit of the lower C-curve. When these C-curves are connected, as in the present alloys, it is convenient to take Tb as the reaction temperature at which the time for initiation of transformation is longest. The investigation was thus refocused primarily upon evaluation of growth kinetics and overall transformation kinetics at temperatures above Tb, with the new objective of ascertaining whether or not incomplete transformation occurs above Tb in these alloys. In a subsequent research program, whose results are summarized in the next paper, Reynolds et al. t41 used a number of the present Fe-C-Mo alloys to study in detail overall reaction kinetics at temperatures below Tb. As forecast by the present work, Reynolds et al. were able to make only quite limited measurements of growth kinetics below Tb. Hence, they employed the growth and overall reaction kinetics data acquired during the present study in combination with their own kinetics data in order to interpret the stasis phenomenon. In the third paper in this series, Reynolds et al. tS] investigated the occurrence of stasis in a number of other Fe-C-X systems. Inasmuch as the present work served as the initiator of this sequence of studies on the stasis phenomen
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