In our past article, we provided an overview of the material known as Tungsten Carbide. In this follow up, we’ll tell you how this key material is made.
Tungsten has the highest melting point of all metals, at 3410oC (6170oF). In order to create parts made from tungsten or tungsten alloys like tungsten carbide, the typical process of melting and blending of the metal and alloying elements is cost prohibitive.
One method that eliminates the need for completely melting the key metals is called powder metallurgy.
Powder metallurgy starts with powder forms of the key metals and usually at least one secondary metal that has a much lower melting point. These raw materials are blended using energetic methods such as ball milling, shaped into a close representation of their final form, then a more reasonable level of thermal energy is used to consolidate the particles into a matrix or composite of the materials, at a much lower temperature than the melting point of the key metal.
A general step-by-step description of the process is shown below:
Approximately 85% of the world’s tungsten comes from China and is extracted from various ores. Tungsten ore is refined to form tungsten oxide or pure tungsten powder.
The process to combine tungsten metal with carbon to form tungsten carbide (abbrev.WC) can be accomplished several different ways. One method to create the powder that is unique to Kennametal is by a chemical reaction using extremely high temperatures – greater than 2,200 °C – generated from burning aluminum to react tungsten with carbon. The WC powder forms in a crystal mass as it cools; then further cleaned and processed to extract the WC powder. The unique high-temperature carburization process produces perfectly stoichiometric one atom of tungsten to one atom of carbon in a relatively large crystal size on the order of 100 microns or bigger.
The coarse WC powder is wet milled into a finer particle size. The size of the WC particles is tailored for specific applications since it significantly affects the physical properties of the final product. The WC powder is also blended with a metal binder, such as cobalt and possibly other hard materials, as well as a soft wax lubricant used to temporarily hold the particles together after compaction.
The wet slurry of powders is dried using either a vacuum dryer or a spray dryer to remove most of the moisture. The resulting agglomerated powder particles may need to be reshaped into a better flowing particle through a pelletizing operation.
A number of different processes can be used to compact the powder into various shapes. A couple of the most common processes used for shaping WC powders include die pressing and injection molding. An emerging process to shape WC powders into components is additive manufacturing (aka 3D Printing). After this shaping step, the parts are not fully dense and considered to be in a “green” state held together by the wax binder.
The “green” WC shapes are thermally processed to remove the temporary wax binder and to allow the permanent metal binder to melt and flow around the hard particles. The parts are then cooled to freeze the binder and trap the hard particles in place.
As an analogy, think about Rice Krispy Treats. The hard WC particles are like the rice krispy pieces and the metal binder (eg cobalt) is the marshmallow. By increasing the amount of binder – marshmallow in our example – the final product has more “forgiveness” or toughness than with little binder which produces are harder, but more brittle product. We vary the amount of binder in the final product to meet the requirements of specific applications.
After the sintering process the very hard, fully dense parts receive final post treatments which may include a final grinding step to ensure the part meets final dimensional specifications. Also, often times, tungsten carbide components will receive a coating that will extend the useful life of the part in the customer’s process.
The result of this process is one of the most useful materials ever discovered, one that touches many of the other objects that we use every day. The ability to use powder metallurgy to produce parts in this material has provided the economic advantage that has caused it to flourish in the 100 years since tungsten carbide was first produced.
Learn More
Did you skip a step? Check out our first article of this series, where our IVG Experts (Dave Siddle and Mike Verti) answer the commonly asked question, “What is Tungsten Carbide?”
Or maybe you’re seeking quote on our wide range of Tungsten Powders, Tungsten Carbide Powders, and/or Ready-to-Press Powders? We can help you out here.
2 Comments
Thank you for the article. It is interesting. I saw this production at many facilities at many countries. But Kennametal uses word “Green”? What does it mean? Ecological? Without wastes?
Vladimir, the term “green” in this case refers to an early state of the powder metallurgy process where the shaped part is held together by an intermediate binder, normally a wax-based substance. The “green” binder is sometimes called lube (short for lubricant) as this waxy material provides lubrication during the shaping to minimize powder flow issues such as interlocking of the grains, bridging, variable density through the part, etc. The lubricant, added near the end of the milling & blending process, also provides protection against oxidation, allowing the powder to be stored for longer periods. The “green” part has sufficient strength to withstand normal handling without damage (but is susceptible to failure if handled improperly) allowing it to be moved from the shaping operation to the thermal processing step. While in this “green” state, if the part is not considered to be correct in geometry or any other performance criteria, it can be easily recycled back into the powder state by removing the wax, and the powder can be quickly reinserted into the process stream without too much expense. In normal processing, the “green” part goes through a thermal process that removes the lubricant completely prior to sintering to its final state.