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12XV International Symposium on Self-Propagating High-Temperature Synthesis
HARDMETALS WITH HIERARCHICAL AND EXTRA HOMOGENEOUS STRUCTURE FOR A NEW GENERATION MINING TOOLS
E. N. Avdeenko*", A. A. Zaitsev", I. Konyashin", E. A. Levashov", and D. A. Sidorenko"
aNational University of Science and Technology MISiS, Moscow, 119049 Russia *e-mail: avdeenko.misis@mail.ru
DOI: 10.24411/9999-0014A-2019-10015
Development of promised mining tools with improved performance and properties is an important task. In addition to large reserves of oil and gas in the Arctic region there are about 10% world reserves of nickel, about 19% world reserves of the platinum group metals, 10% world reserves of titanium and more than 3% world reserves of zinc, cobalt, gold and silver. Climate conditions in the Arctic region are very tough. This causes a reduction in the ductility and fracture toughness of hardmetals and also excludes the use of water cooling. Mining tools are subjected to severe fatigue, compressive and bending loads, and intense abrasive wear. Therefore, increasing the mechanical and performance properties of the tools is a task of great importance, especially in the Arctic region. Mechanical and performance properties of hardmetals can be varied over a wide range either by changing the cobalt content or by affecting the grain size of the carbide phase. However, these approaches do not allow a simultaneous increase of the wear-resistance and fracture toughness of WC-Co hardmetals.
A promising approach allowing one to improve the mechanical properties of coarse-grained hardmetals is the nano-modification of the binder phase due to the decomposition of the supersaturated solid solution of tungsten and other refractory metals in the binder phase. The solubility of TaC and WC in cobalt is similar and decreases when decreasing the temperature, which allows one to expect that it is possible to form dispersed tantalum-containing precipitates (e.g. Co3Ta, Co3(W, Ta) or Co3(W,Ta)3C phases) in the binder phase upon cooling from sintering temperatures, in analogy with the formation of nanoparticles in heat-treated WC-Co hardmetals with low carbon contents and consequently high concentrations of tungsten dissolved in the Co-based binder [1-5]. The proposed hypothesis was partially confirmed in the work [6] in which calculations by the electron-density functional method for the Co-Ta system was performed. The growth of fracture toughness and strength of hardmetals can also be achieved by obtaining hardmetals with a narrow WC grain distribution [7].
In this work, WC40 grade powder with an average particle size of 40 ^m was used to obtain a narrow fractional WC powder with a grain size of 5-15 |im [8]. The narrow fractional WC powder was mixed with a Co powder (extrafine grade, Umicore, Belgium) and a TaC powder (HGS grade, H.C. Starck, Germany) in a ball mill. Sintering was carried out in a Sinter-HIP furnace at a maximum temperature of 1420°C. The hardmetals obtained from the narrow fractional WC powder have a very uniform microstructure and comprise rounded tungsten carbide grains in the microstructure (Fig. 1b) as compared to the traditional coarse-grained hardmetals (Fig. 1a). It was established by transmission electron microscopy (TEM) and high resolution TEM (HRTEM) that the cobalt binder of a hardmetal sample comprises nanocrystalline precipitates consisting of (TaxWyCoz)Ck with a mean size of around 5 nm (Fig. 2). The binder nano-hardness of the hardmetals with the hierarchical structure is found to be 7.5 ± 0.2 GPa, while the binder nano-hardness of conventional coarse-grained hardmetals is less than roughly 5 GPa. The hardmetals with hierarchical and ultra-uniform microstructure are characterized by a unique combination of transverse rupture strength (2490 MPa), hardness (HV = 11.7 GPa) and fracture toughness (K1C = 15.9 MPam1/2).
ÏSHS2019
Moscow, Russia
a 6
Fig. 1. Microstructures of coarse-grained hardmetals obtained from a conventional coarse-grain WC powder (a) and using the narrow fractional WC powder (b).
Fig. 2. Structure of the binder phase of a hardmetal sample: (a) TEM image and (b) HRTEM image.
This work was supported by the Ministry of Science and Higher Education of the Russian Federation in the frame of the Federal Target Program «Investigations and Developments over Priority Directions of the Scientific and Technology Complex of Russia for 2014-2020», agreement no. 14.575.21.0156, project RFMEFI57517X0156.
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