Microstructure-property relations in as-extruded ultrahigh-carbon steels
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I. INTRODUCTION
ULTRAHIGH-CARBON steels (UHCSs), which are hypereutectoid steels (1 to 2.1 pct C), have been studied extensively for their unique mechanical properties.[1,2,3] These steels are superplastic at elevated temperatures and exhibit ultrahigh strength and good ductility at room temperature. The basis of this success is elimination of deleterious proeutectoid carbide networks and the development of ultrafine carbides in spherical or pearlitic form.[3–6] Many processing routes have been developed for breaking up the proeutectoid carbide network, including hot-andwarm working, warm working, a divorced eutectoid transformation (DET), and a DET with associated deformation (DETWAD).[3,4,5] Although such routes have proved successful in achieving desirable microstructures and properties in laboratory experiments, they may not be readily amenable to current mass-production steel processing. For example, mechanical working at intermediate temperatures (700 8C to 850 8C), as required in DETWAD processing, is a drastic change in current production procedures. There remains a need for ultrahigh-strength steels in many high-production steel applications for the automotive and construction industries. Many of the experimental ultrahighstrength steels, such as dual-phase steels[7] and martensitic steels,[8] however, have been hampered in their extensive application to commercial products. This situation has resulted from several sources, including the lack of sufficiently high reproducibility of the desired product and the need for the added expense of special heat-treatment procedures subsequent to the mechanical working step. With these difficulties in mind, the present article considers the structure D.R. LESUER, Engineer/Materials Scientist and Group Leader, and C.K. SYN, Engineer/Materials Scientist, are with the Lawrence Livermore National Laboratory, Livermore, CA 94551. J.D. WHITTENBERGER, Materials Research Engineer, is with the NASA–Lewis Research Center, Cleveland, OH 44135. O.D. SHERBY, Professor Emeritus, is with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted January 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
and properties of UHCSs produced from a single-step mechanical working operation with no subsequent heat treatments. To simulate an industrial forming procedure, extrusion was selected as the mechanical working process for the present investigation. Extrusion is a well-established fabrication process for making rods, T-sections, tubes and other similar geometric-shaped products. The purpose of the present investigation is to assess the structure and mechanical properties of UHCS bars of several compositions in the as-extruded and air-cooled condition. The extrusions were done at temperatures in the single-phase austenitic region or in the two-phase austenite-plus-carbide region. II. MATERIALS, PROCESSING, AND EXPERIMENTAL PROCEDURES A. Composition Four different-composition UHCSs were selected for study, which are listed in
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