Influence of microstructure on tensile properties of spheroidized ultrahigh-carbon (1.8 Pct C steel
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INTRODUCTION
IN the mid-1970s, it was demonstrated that a class of steels now known as ultrahigh-carbon steels (UHCSs) could be made to behave superplastically, tl,2,3] The steels are plain-carbon steels containing between 1.0 and 2.1 wt pct carbon (15 to 32 vol pct cementite, Fe3C). Traditionally, steels of such a high-carbon content have been neglected by industry because of a belief that they are inherently brittle. Steels of this composition, however, do have a rich and fascinating history; that is, they are possibly a rediscovery of Damascus steel.t4] In addition to its attribute of superplasticity, UHCSs have promising room-temperature mechanical characteristics. 15,6,7] These properties, especially the tensile ductility, however, have not been studied extensively. A search of the literature surprisingly revealed that the influence of high-carbon content on the room-temperature tensile elongation was studied over 100 years ago. Figure 1 illustrates the influence of carbon content (from 0.04 to 1.7 wt pet carbon) on the tensile ductility of steel. This graph is reproduced from the book The Metallurgy of Steel by the American metallurgist Henry Marion Howe, published in 1891) 8] As can be seen in the figure, the tensile elongation decreases dramatically with an increase in carbon content, although there is a large scatter in the elongation data for a given carbon content. In the composition range for UHCSs, from 1.0 to 1.7 pct carbon in the figure, the typical elongation is roughly constant decreasing only slightly from about 3 pct to about 2 pct, although two points are above this C.K. SYN, Engineer/Materials Scientist, and D.R. LESUER Engineer/Materials Scientist and Group Leader, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. O.D. SHERBY, Professor, is with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted September 20, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
scatter band at 1.31 and 1.39 pct carbon. Subsequently, similar graphs have been reproduced in many contemporary books, without data points, to illustrate the same tensile ductility-carbon content correlation given by Howe. 19J~ A generally accepted explanation of the trend observed is that, with increasing carbon content, in the hypereutectoid composition range, an increase in the amount of the brittle proeutectoid carbide network will occur, and this leads to a natural decrease in tensile ductility. It is, by now, well established that when a UHCS is processed in a way that avoids the formation of a carbide network, the steel can exhibit tensile elongations as high as 30 pct. 151The tensile ductility and strength can be varied enormously, depending on the microstructural condition, for example, spheroidized, pearlitic, or tempered martensitic states. A major motivation of the present study is to investigate potential sheet applications in the automotive and related industries for a spheroidized UHCS. In sheet applications, it is the cold-forming
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