The optimisation and evaluation of VC-WC-Co hardmetals.
Date
2011-10-05
Authors
Whitefield, David James.
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Abstract
The use of vanadium carbide in hardmetals is not new. The original use of vanadium carbide
was as a grain refiner in ultra-fine and fine-grained hardmetals in amounts up to lwt%. These
additions of VC were made to prevent discontinuous grain growth, thereby maintaining a fine carbide
microstructure. In this work, the vanadium carbide is used as a substitute for the tungsten carbide.
The amount of vanadium carbide used in this work, 10wt%, was meant to show that VC could be
used as a partial substitute for WC. The partial replacement of WC with VC is a novel approach in
producing a new type of hardmetal alloy for industrial and mining applications. This work relates to
the optimisation and evaluation of this new WC-VC-Co hardmetal. The production route of the
material is by powder metallurgy techniques due to the high melting points of the tungsten carbide
and vanadium carbide. Critical process operations such as milling and sintering have been closely
monitored and optimised. After sintering, the properties of the hardmetal were evaluated using
standard hardmetal quality control procedures, as well as additional techniques, such as field emission
scanning electron microscopy and transmission electron microscopy. Field testing was also
undertaken to determine how this material would compare to other commercially available materials.
The materials produced possess some unique properties. The(W,V)C phase was found to
contain Co and should therefore be more correctly called (W,V,Co)C. Initially, the material was
prone to crack initiation from and propagation through the mixed carbide phase. This problem was
overcome by using starting powders with a finer grain size or by refining the powders by extended
milling times. This reduced the grain size of the (W,V,Co)C in the sintered material to less than the
critical value for crack initiation and growth.
The hardness of the WC-VC-Co materials was found to be superior to conventional
Nanograde (approximately SOOum) WC-Co materials with the same cobalt content. The toughness
of the WC-VC-Co materials was found to be similar or greater than Nanograde WC-Co alloys with
approximately the same hardness. The density of the materials produced with 1 Owt% Co and
10wt% VC are 2 g.cm"3 less dense than for conventional Nanograde WC-Co grades with the same
cobalt content. This would give significant weight savings in most applications.
Field testing was done at two different mines, one mining iron ore and the other silica. On
the silica mine, the results were disappointing as the WC-VC-Co material was worn away quickly in comparison to the commercial grade of carbide currently being used. It must be noted that the grade
used currently has a nickel binder providing significant improvement in corrosion resistance over the
WC-VC-Co with only a cobalt binder. For a fair comparison, a Ni binder should have been used for
the VC alloy.
On the iron ore mine, the WC-VC-Co materials performed well and gave the same life as the
grade of material currently used. Due to the worry about chipping and damaging the conveyor belt
the WC-VC-Co grade had 12wt% cobalt binder, whereas the grade currently used is WC-10wt% Co
without nickel as the environment does not have as low a pH as the silica mine. If a WC-VC-10wt%
Co material were to be used, the wear resistance would have been superior to the grade currently
used.
A brief cost analysis has also been conducted to determine whether there is any cost
advantage to these WC-VC-Co materials. Since only the raw materials differ and processing is the
same, the cost of production would be equal, but overall there is a slightly greater expense for the
WC-VC-Co materials as the VC is of significantly higher cost than the WC.