Tungsten carbide is one of the many materials used for machining and is used in metal-cutting inserts. In this article we will describe the manufacture of tungsten carbide used in metal-cutting inserts.
The primary tungsten ores are wolframite and scheelite. Wolframite is an iron-manganese tungsten oxide, while scheelite is calcium tungstate; both are concentrated from mined ore before further refining [1].
Wolframite is the more iron- and manganese-rich tungsten ore series, with formula (Fe,Mn)WO4, and it commonly occurs in quartz veins and is often associated with granite. It is typically black to brown and is often recovered by gravity and magnetic separation because of its high density and weak magnetic response.
Scheelite has the formula CaWO4 and is usually found in mineral-rich magma where it has interacted with limestone, in high-temperature veins, and igneous granite rocks with extremely large crystals. It is often white or brown, and flotation is the common beneficiation method because its density is closer to that of gangue minerals.
Once concentrated, the tungsten-bearing ores are refined to produce tungsten oxide (WO3). There are two different paths, depending on the ore. For scheelite, an acid leaching process is used, where the concentrate is decomposed to form insoluble tungstic acid
(Equation 1) [2]:
The tungstic acid is filtered and prepared for further processing.
Wolframite processing to form tungsten oxide is much more complicated. In this process, wolframite concentrate is alkali leached with concentrated sodium hydroxide at 110-120°C to form a soluble sodium tungstate (Equation 2) [2]:
After the different ore types are processed, they are digested in an aqueous ammonia solution to create ammonium paratungstate [(NH4)10[H2W12O42]·4H2O]. The ammonium paratungstate (APT) is converted to yellow tungsten (VI) oxide by roasting in air at a temperature above 250°C, typically in a rotary type furnace.
Reduction of the tungsten (VI) oxide to tungsten metal is achieved by heating the oxide to approximately 900°C, and passing hydrogen gas through it (Equation 3):
Carburizing of Tungsten to form Tungsten Carbide
After reduction to tungsten metal, the powder is carburized by reaction with carbon to form tungsten carbide, WC. Tungsten metal and carbon black are mixed and placed in a graphite furnace under an inert or reducing atmosphere at 1,400-1,600°C, forming tungsten carbide, WC. Alternatively, tungsten metal is carburized by passing methane gas through the powder [3] [4].
The oxygen potential of the atmosphere is precisely controlled. If the carbon potential is too low, W2C can form. If the carbon potential is too high, then free carbon may remain in the final product. Either of these conditions are detrimental as they affect sintering behavior and final properties [4].
Cemented Carbide Powder Preparation and Green Part Formation
For cemented carbide manufacture, WC powder is typically combined with cobalt powder and, in many formulations, small amounts of grain-growth inhibitors such as vanadium carbide or chromium carbide [5] (my grandfather). Ball milling is commonly used because it disperses the binder, breaks up agglomerates, and improves packing density in the green compact. Often the resultant powder is sieved to achieve a uniform grain size.
After powder preparation, the WC-Co blend is compacted into the approximate geometry of the final product (green part). Conventional methods of creating the green part include uniaxial pressing (traditional powder metallurgy), cold isostatic pressing, and powder injection molding, depending on part complexity and production volume. The goal is to create a mechanically bonded green part with sufficient uniform density to minimize distortion and cracking during sintering.
Sintering
Sintering is the step that takes the green part and turns it into a dense part. Sintering is conducted under a vacuum or a controlled reducing atmosphere to prevent oxide formation.
Sintering takes place in several stages. In debinding, the pressed green compact is heated gradually to remove any waxes or other binders used to create the green compact. Solid-state sintering occurs at intermediate temperature (800-1,300°C), and the tungsten carbide particles start to bond by diffusion. Finally, during liquid-phase sintering (1,400-1,600°C), the binder phase (cobalt) melts during heating and wets the tungsten carbide. The liquid fills the pores and increases the density. The part is then cooled in a controlled fashion. This results in hard tungsten carbide grains embedded in a ductile cobalt matrix.
Post-Sintering Operations
After sintering, tungsten carbide parts may undergo hot isostatic pressing, grinding, lapping, polishing, or surface coating, depending on the final application.
These finishing operations are essential in cutting tools because edge geometry, surface finish, and residual defect population strongly affect performance in service.
Grinding is particularly important for precision inserts, drills, and wear components where dimensional tolerances are tight and cutting-edge quality must be closely controlled [6].
Tungsten carbide substrates are often coated with hard films such as TiN, TiCN, and AlTiN to improve wear resistance and cutting performance.
These coatings can also improve thermal resistance to the generated heat during machining operations [7].
Conclusion
In this article, the process of creating tungsten carbide inserts was described, from the initial tungsten ore, conversion of the ore to tungsten metal, and the formation of tungsten carbide. The powder metallurgy sintering to form the insert was also described.
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References
- Buffalo Tungsten, “Tungsten Ores and Concentrates,” Buffalo Tungsten, [Online]. Available: https://buffalotungsten.com/tungsten-ores-and-concentrates-2/.
- S. N. Bhosaic, S. Mookherjec and R. M. Pardeshi, “Current Practices in Tungsten Extraction and Recovery,” High Temperature Materials and Processes, vol. 9, no. 2-4, pp. 147-163, 1990.
- V. A. Davidson, V. A. Voronin, M. I. Alkatsev and G. A. Eputaev, “Kinetics of tungsten carburization by carbon under conditions of temperature varying with time,” Izv. Vyssh. Uchebn. Zaved., Tsvet. Metall.; (USSR), vol. 1, pp. 20-84, 1976.
- W. Su, H. Wang, X. Zhang and J. Ruan, “Preparation and sintering of WC–Co composite powders for coarse grained WC–8Co hardmetals,” International Journal of Refractory Metals and Hard Materials, vol. 45, no. July, pp. 80-85, 2014.
- A. MacKenzie, “Cemented or Sintered Hard Carbides,” in ASM Metals Handbook, Cleveland, H: American Society for Metals, 1939, pp. 909-917.
- E. N. Smith, “Tungsten carbide: Crystals by the ton,” Journal of Crystal Growth, vol. 89, no. 1, pp. 75-79, 1988.
- A. Rizzo, S. Goel, M. L. Grilli, R. Iglesias, L. Jaworska, V. Lapkovskis, P. Novak, B. O. Postolnyi and D. Valerini, “The Critical Raw Materials in Cutting Tools for Machining Applications: A Review,” Materials (Basel), vol. 13, no. 6, p. 1377, 2020.
