The divorced eutectoid transformation in steel
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I.
INTRODUCTION
THE eutectoid transformation in steel causes austenite to transform into two product phases consisting of roughly 11 pct cementite and 89 pct ferrite. One can partition the growth modes of the eutectoid transformation into two types, cooperative and noncooperative. In the cooperative growth mode, the product phases grow as coupled pairs at the growth front, as shown in Figure 1(a), and produce the famous pearlite structure. When the two phases form in a noncooperative mode, it is appropriate to call the transformation a divorced eutectoid transformation (DET). The formation of upper bainite and lower bainite are two examples of the DET. Both of these microstructures give rise to unique sets of physical properties in the steels and they have been widely studied and well recognized. There is a third mode of the DET that also gives rise to steels with unique physical properties, which has not been widely recognized and studied despite an old literature data base. This mode is characterized by a transformation front, shown in Figure 1(b), in which a cellular-shaped austenite/ferrite boundary advances into an array of austenite plus fine cementite particles, which may or may not be large enough to be observed in an optical microscope. The eutectoid transformation occurs along this boundary in a divorced manner, producing a spherical array of cementite particles in a ferrite matrix. The pre-existing cementite particles are simply enlarged as the transformation front passes. This mode of the eutectoid transformation in steels has recently been studied by Sherby and co-workers[1,2,3] and has been shown to be very effective in high-carbon steels (C around J.D. VERHOEVEN, Professor, is with the Materials Science and Engineering Department and Ames Laboratory, Iowa State University, Ames, IA 50011. E.D. GIBSON, formerly Assoc. Metallurgist with Ames Laboratory, Iowa State University, is retired. Manuscript submitted April 24, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
1.5 pct) for production of very fine arrays of cementite particles, which, in turn, promote fine grain size and superplastic properties of the austenite. They refer to the transformation as the DET. There is an old literature base on this transformation, to be discussed later, which has established that this transformation occurs only at temperatures immediately below A1 and is replaced by the lamellar pearlite transformation at slightly lower temperatures. Sherby’s DET terminology will be adopted here, with the understanding that DET refers to the divorced eutectoid reaction occurring at temperatures just below A1 and excludes reactions occurring at much lower temperatures, i.e., the bainite reactions. It is well established in the literature that the eutectoid transformation in steel can produce a transformation product of spheroidal cementite and ferrite with a transformation front, as shown in Figure 1(b). In an article published in 1922,[4] Whiteley presented photomicrographs of the transformation front of the DET, showing that sphe
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