CC BY
33NEAR-NANO AND COARSE-GRAIN WC POWDERS OBTAINED BY THE SHS AND CEMENTED CARBIDES ON THEIR BASIS
A. A. Zaitsev*", E. A. Levashov", V. I. VershinnikovA, I. Konyashin"c, and E. I. Patsera"
aNational University of Science and Technology MISiS, Moscow, 119049 Russia bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia cElement Six GmbH, Burghaun, 36151 Germany *e-mail: aazaitsev@bk.ru
DOI: 10.24411/9999-0014A-2019-10196
Tungsten carbide (WC) is a chemical compound used for many applications due to its unique combination of physical and chemical properties. In particular, WC is mainly employed in cutting and drilling tools in the form of cemented tungsten carbide. The properties of cemented carbides depend primarily on binder content and grain size of WC. Methods for producing WC are quite diverse. WC is usually fabricated on a large scale either by the direct carburization of tungsten oxides or the carburization of tungsten metal powders. Such methods as the rapid carburization process, thermochemical precursor reduction, vapor phase reaction process, mechano-chemical synthesis, and high energy milling are also used to obtain near-nano or nano-WC powders [1-6]. WC powders can also be produced by self-propagating high-temperature synthesis (SHS). This method is now employed for the fabrication of different refractory compounds [7, 8]. The reaction of elemental synthesis of WC: W + C = WC is characterized by a relatively low heat effect (AH = -40.6 kJ/mol, the calculated adiabatic flame temperature ~1083 K [9]), which makes it difficult to produce WC in the combustion mode without adding heat to the system (e.g., by passing electric current [10, 11] or by employing induction heating [12, 13] of tungsten-carbon mixtures). Reactions involving the reduction stage with tungsten compounds (oxides, halides, etc.) used for the SHS synthesis instead of tungsten and highly active metals used as reducing agents, show promise for producing WC powders. In the present study magnesium is used as a reducing agent so SHS reaction can be summarized as follow:
WO3 + Mg + C + R ^ WC-MgO-Mg + R' + Q
where R is a regulating additive, Q is the thermal effect, and R' is other products of the reaction. In the general case R' is a result of interaction Mg + C + R.
The morphology of the near-nano and medium-coarse WC powders obtained by the SHS is shown in Fig. 1.
Fig. 1. Morphology of the near-nano (a) and medium-coarse WC powder (b) obtained by the SHS method.
■SHS 2019 Moscow, Russia
The near-nano WC powder (Fig. 1a) is uniform and does not comprise abnormally coarse WC grains; the mean grains size was found to be about 200 nm. The chemical analysis showed that the synthesized near-nano tungsten carbide contained 6.13 wt % Cbound, 0.08 wt % Cfree, and 0.08 wt % oxygen. The coarse-grain WC powder (Fig. 1b) is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 |im and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 |im to 8 |im in size.
The microstructure of the submicron grade with 5 wt % Co made of near-nano WC powder obtained by the SHS method and the standard submicron grade is similar with slightly coarser WC grains in the grade made of SHS WC powder (Fig. 2a). The mechanical properties of the submicron WC-5% Co grade made of near-nano WC powder obtained by the SHS method and its wear-resistance (TRS = 2130 MPa, Kic = 9.5 MPam1/2, HV30 = 18.1 GPa, Wear = 9.110-6 mm/rev) are comparable with those of the standard ultrafine grade, except for the TRS, which is slightly lower for the grade of SHS WC powder. The microstructure of the medium-coarse WC-6 wt % Co cemented carbide made of medium-coarse WC powder obtained by the SHS method and its properties (TRS =1900 MPa, K1C = 13.5 MPam1/2, HV30 = 13.1 GPa) are comparable with those of the standard medium-coarse grade for percussive drilling (Fig. 2b).
Fig. 2. The microstructure of the submicron (a) and coarse-grain (b) WC-6 wt % Co cemented carbide made from the near-nano and coarse WC powder obtained by SHS.
Results of laboratory performance tests on percussive drilling of the medium-coarse WC-6% Co grade obtained from both the SHS WC powder indicate that its wear-resistance is very similar to that of the standard grade for percussive drilling produced from the conventionally fabricated WC powder. Thus, it is established that high-quality WC-Co cemented carbides with different WC grain sizes varying from submicron to medium-coarse can be produced from the WC powders fabricated by the SHS method.
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