Overall kinetics and morphology of the products of austenite decomposition in a Fe-0.46 Pct C-5.2 Pct Cr alloy transform
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BETWEEN 1926 and 1930, Hultgren,[1] Robertson,[2] and Bain and Davenport[3] showed that ferrite plus carbide aggregates formed directly from the austenite of steels at all temperatures above Ms, not from intermediate metastable phases; at the same time, they established the isothermal heat-treatment procedure for studying austenite decomposition. Bain showed that ferrite plus lamellar pearlite came from the decomposition at high temperatures, while acicular ferrite products containing dispersed carbides (later labeled upper and lower bainite) came from the decomposition at low temperatures. He also proposed the timetemperature-transformation (TTT) curve as the standard diagram for displaying austenite decomposition kinetic data, showing the characteristic C shape of plain Fe-C steels and the formation of two different C curves separated by a bay in steels containing carbide forming alloying elements. Two years later, Wever and Lange[4] discovered the incomplete transformation phenomenon (stasis) in the upper bainite range, linking this phenomenon to the existence of a bay and separated C curves in the TTT diagram. In 1939, Jolivet and Portevin[5,6] noted that it was not only below the bay that a wide diversity of ferrite plus carbide aggregate morphologies appeared: nickel, chromium, and molybdenum containing alloys with increasing undercooling (but still above the bay) presented granular eutectoid colonies with ragged interfaces and reentrant angles, preferred growth directions, and finally arborescent (dendritelike) shapes; at even lower temperatures, the eutectoid became acicular. Figure 1, reproduced from his article, schematically depicts Jolivet’s eutectoid morphologies. H. GOLDENSTEIN, Associate Professor, and J.A. CIFUENTES, formerly Graduate Student, are with the Metallurgical and Materials Engineering Department, Escola Polite´cnica da Universidade de Sa˜o Paulo, Sa˜o Paulo, SP, Brazil. Contact e-mail: [email protected] 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
Lyman and Troiano[7] studied the austenite decomposition in a series of 3 pct Cr alloys with increasing C contents; for higher C contents, they found the same morphologies as Jolivet. Using X-ray diffraction of extracted carbides, they showed that the M3C carbide in the eutectoid microconstituent was gradually replaced by M7C3 with increasing times at the isothermal temperature. Hultgren described similar structures in Cr and Mo alloyed steels as ‘‘wrinkled pearlite’’ containing ‘‘shots’’ and ‘‘veins.’’ Wever and Koch[9] and Schrader,[10] in 1953, made the first TEM observations of arborescent structures in Fe-Cr-C alloys using replicas and extracted carbides to show a transition from a nodular high-temperature lamellar structure (‘‘u¨bere perlitstuffe’’) to lower temperature acicular ramified eutectoid st
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