High-temperature tensile ductility in WC-Co cemented carbides
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I.
INTRODUCTION
THE compounds WC-Co have extensively been used for wear and cutting tools at elevated temperatures. Thus, information on high-temperature strength and plastic flow behavior in cemented carbides is very important for practical use. However, it is not so easy to find a rate controlling mechanism for plastic flow in this material, because the flow stress at elevated temperatures depends on various factors such as carbide grain size, binder content, addition of other carbides, and so on.[1–12] It has been clarified by creep testing or compression testing that the flow stress-strain rate relationship in cemented carbides is given by a sigmoidal curve at a constant temperature as in superplastic metals[9,11,13–16] Three different mechanisms must be operating at each temperature depending on strain rate. Judging from the experimental data, it is expected that the superplastic flow occurs in region II. The present article aims to report the high-temperature tensile ductility in WC-Co at temperatures between 1150 7C and 1250 7C. II.
EXPERIMENTAL PROCEDURES
The chemical composition and carbide grain size of the materials used in the present study are given in Table I. The WC powders with an average grain size of 0.88 mm supplied by Japan New Metals Co., Ltd. (Toyonaka City, Osaka, Japan) were used for starting materials. The alloys were prepared by ball milling the powders of WC, Cr3C2, VC, and Co in ethanol together with cemented carbide balls for 24 to 48 hours. The slurry was dried and granulated. These granules were pressed into bars in a cemented carbide die under a pressure of 20 MPa, and then cold isostatically pressed under a pressure of 130 MPa in a rubber tube. Sintering was carried out at temperatures between 1380 7C and 1430 7C for 1 hour in a pressure range of 1021 to 1022 torr. The carbide grain size was controlled by choosing the sintering temperature. The bulk density of sin-
I.C. LEE, Lecturer, is with the Department of Metallurgical Engineering, Kyungpook National University, Taegu-Si 702-701, Korea. T. SAKUMA, Professor, is with the Department of Materials Science, The University of Tokyo, Tokyo 113, Japan. Manuscript submitted October 18, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
tered bodies was measured by the Archimedes method. The tensile specimens with 2 3 2 3 13.3-mm3 gage length were prepared from the sintered samples by cutting and grinding. High-temperature tensile tests were made using an Instron-type mechanical testing machine SHIMADZU AG5000C equipped with a high-temperature furnace. Hexagonal BN powders were sprayed on the test specimen in order to avoid sticking of the specimen with SiC rods. The specimen was set inside a quartz tube in Ar atmosphere and heated with an infrared image furnace ULVAC RHLE45. The tests were carried out in a temperature range of 1150 7C to 1250 7C and in an initial strain rate range of 6.3 3 1026 to 6.3 3 1023 s21. To examine the effect of the testing atmosphere on the tensile deformation behavior, several samples were deformed at 120
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