How to make Tungsten Carbide Tool

How to make Tungsten Carbide Tools

Tungsten carbide, often simply “carbide,” is a familiar material around the industry. This compound of tungsten and carbon has revolutionized the metal-cutting world over the decades, enabling increased speeds and feeds and providing longer tool life.

It was first theorized as a tool for cutting material in 1925, by Dr. Samuel Hoyt, a scientist of the Research Laboratory of the General Electric Company at Schenectady. He also developed Carboloy, an alloy of tungsten, carbide, and cobalt which later GEC opened as the Carboloy division to produce tungsten carbide cutting tools. In the late 1930s, Philip M. McKenna, founder of Kennametal, found out that by adding a titanium compound to the mix, it made the tools work better at higher speeds. This began the forward march toward today’s lightning-fast cutting speeds.

Tungsten carbide (WC) is an inorganic non-natural compound composed of equal parts of tungsten (W) and carbon (C) atoms. Tungsten Carbide exhibits a hexagonal structure lattice made of a grid of those tungsten and carbon. It’s most striking properties, that make it useful for making tools, are a high density, a very high melting point of 2600 °C. It is also a very hard compound and also has the metal-like, high values of electric and thermal conductivity. Tungsten carbide is approximately twice as strong as steel.  

Prior to 1945, tungsten carbide was prepared by melting tungsten, carbon black and metal oxides at a temperature of 2000 °C but the tools made from this metal were found to be far too brittle for industrial use.

Making of Tungsten Carbide Tools

Mining & Processing

There are several tungsten ores that can be mined and refined into tungsten or made into tungsten carbide. Wolframite is the best known. Scheelite and/or wolframite are commonly located in narrow veins which are very slightly inclined and often widen with the depth.

The ore is then crushed, heated and treated with chemicals. The result: tungsten oxide. Then, the fine particles of tungsten oxide are carburized, turning them into tungsten carbide. In one method, the tungsten oxide is mixed with graphite (carbon). This mixture is then processed continuously and heated to over 1200˚ C (2200˚ F) which causes a chemical reaction that removes the oxygen from the oxide and combines the carbon with the tungsten to yield tungsten carbide.

 Grain Size and Grading

Once processing is done, the carbide grain sizes will determine the mechanical properties of the final product. This is key in its processing. The size of the tungsten oxide particles, and how long and at the temperature that the oxide/carbon mixture may have been processed decided or as such creates the size of the carbide grain.

So, as consequences go, if the size of the grain is too large and the percentage of binder used is much higher in comparison, the carbide deforms under this pressure. Now, one of the major advantages of carbide to be alloyed with tungsten is its proficiency to cope under pressure or compressive force. If the alloy now is too soft it loses that ability.

The tungsten carbide particles processed are received at a fraction of the size of a grain of sand. A series of sieves are used to sort out the different grain sizes: less than one micron, one and one half microns, and so forth.

Now, the tungsten carbide grains, after sieving, are ready for blending into “grade powder.” In the tungsten carbide industry, one speaks of grades rather than alloys, but they mean the same thing.

The story of Tungsten Carbide (WC) and its powder metallurgy, and particularly that of the hard metal industry, is depicted by a progressively widening range of available grain sizes; whilst simultaneously narrowing the grain size distribution for each grade of WC powder.

The tungsten carbide grains go into a mixing vessel with the other components of the grade. Powdered cobalt metal is made to act as the “glue” to hold all the components. The other materials, such as titanium carbide, tantalum carbide and niobium carbide are added to improve the properties of the material when cutting.

Pressing

Once the mixing process is completed, the liquid must now be removed. This generally happens in a spray dryer, that looks similar to a stainless steel silo. An inert, drying gas, one of nitrogen or argon, is blown into the spray dryer from the bottom up. Once all the liquid is removed, the sand like remaining dry material is the “grade powder”

The grade powder then goes into insert shaped molds that are specially designed and made to allow for the shrinkage that will happen later on in the process to create the cutter inserts. Just as pharmaceutical tablets are formed, in a process similar to that, the powder is compressed into the molds.

Heating

After pressing is completed, it is fairly delicate and the form looks like oversized inserts. These inserts are then removed from the like-shaped molds and are placed on graphite or molybdenum trays, and go into a sintering furnace where they are heated in a low-pressure hydrogen atmosphere to 1100-1300˚ C (about 2000- 2400˚ F). Any cobalt present melts. The insert in the furnace consolidates into a solid, smaller size.

The inserts are removed from the furnace and are now cooled. On cooling, they become dense and hard. A quality control check is performed. The inserts, after the check, are usually ground or honed to achieve the correct dimensions and cutting edge. Honing, done to a radius of 0.001″ is typical, though some of these parts receive a cutting-edge radius of half a thousandth or as large as 0.002″, and some of these are left extremely sharpened as sintered in the furnace. 

The next step is grinding and other such operations. Even still, some of the types and designs of inserts retrieved from the sintering furnace are in their final shape and have the correct edge as well. Hence, they do not need further grinding.

Introducing Tungsten

Tungsten occurs naturally in nature as Wolframite, FeMnW04, and Scheelite, Ca W04.

The ore is digested in hydrochloric acid to remove the calcium and to precipitate tungstic acid, H₂W0₄, which is then dissolved in ammonia and crystallized as ammonium tungstate, (NH₄)₂W0₄. Ammonium tungstate is evaporated by boiling to give ammonium paratungstate, or APT, 5(NH₄)₂O.12W0₃.5H₂0. At temperatures below 50⁰C, fine APT is obtained. By calcining APT or tungstic acid, tungsten oxide, W0₃, is obtained. The lower the temperature, finer the W0₃ produced. When W0₃ is reduced in hydrogen, tungsten, W, is produced: because oxygen forms a more stable.

Coating

Cutting conditions may not be optimal and hence the tools’ lives need to be prolonged so as to face these conditions. Many types and combinations of coatings have been developed. They can be applied in two ways: by chemical vapor deposition (CVD) or physical vapor deposition (PVD). Both types are applied in furnaces.

Chemical vapor deposition

For CVD, the coating is usually 5- 20 microns thick. Milling and drilling inserts usually receive 5–8  icrons, as these operations require better surface finish, and they encounter more impact, so they require greater edge toughness. For turning applications, the coatings tend to be in the range of 8–20 microns. In turning, heat and abrasion tend to be more of a concern.

In its simplest explanation, CVD involves flowing a precursor gas or gases into a chamber containing one or more heated objects to be coated. Chemical reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface.

Physical vapor deposition

PVD coatings are typically about 2-4 microns thick. Different manufacturers use different numbers of layers. These PVD coatings are well-suited to applications cutting high temperature, nickel-based, cobalt-based or titanium-based materials, and sometimes steel and stainless steel.

Tool manufacturers are meeting the pressures for ever increasing feeds and speeds, and the need for longer tool life and lower costs, by continually improving the designs of tungsten carbide cutting tools and developing better and better coating technologies.