tungsten carbide

 

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Tungsten carbide
Tungsten carbide milling bits with carbon and tungsten samples
General
Molecular formula	WC
Molar mass	195.86 g·mol−1
Appearance	grey-black solid
CAS number	[12070-12-1]
Properties
Density and phase	15.8 g·cm−3, solid
Solubility in water	Insoluble
Melting point	2870 °C, 5198 °F (3143K)
Boiling point	6000°C, 10832 °F (6273K)
Thermal conductivity	84.02 W·m−1·K−1
Tensile strength	0.3448 GPa
Mohs hardness	9
Structure
Coordination geometry	?
Crystal structure	Hexagonal
Thermodynamic data
Standard enthalpy of formation ΔfH°solid	? kJ·mol−1
Standard molar entropy S°solid	? J·K−1·mol−1
Hazards
EU classification	not listed
NFPA 704
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Tungsten boride Tungsten nitride
Other cations	Molybdenum carbide Titanium carbide Silicon carbide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Monotungsten carbide, WC, or Ditungsten Carbide, W₂C, is a chemical compound containing tungsten and carbon, similar to titanium carbide. Its extreme hardness makes it useful in the manufacture of cutting tools, abrasives and bearings, as a cheaper and more heat-resistant alternative to diamond. Tungsten carbide is also used as a scratch-resistant material for jewelry including watch bands and wedding rings.

The term ‘Tungsten Carbide” typically refers to WC and more specifically to materials in which tungsten carbide grains are ‘cemented” together with cobalt as a binder much as concrete is gravel cemented together with cement.

This version of tungsten carbide was the first of the very hard materials known as “Hardmetals” and Cermets. The derivation of the term hardmetal is obvious. The term “Cermet” comes from “metal based ceramic.” Tungsten carbide grains are often considered a ceramic depending on what definition of ceramic you prefer.

While tungsten carbide is technically a cermet the term more commonly refers to a Titanium based material such as TiC, TiN, TiCN and similar. Allegedly this is due to a translation error from English to Japanese.

The International Hardmetal Directory, the unofficial bible of the industry, lists about 5,000 grade of tungsten carbide. Since then an estimated 1,000 to 2,000 grades have been added.

Tungsten carbide comes in more than 5,000 grades plus maybe 500 advanced grades before you even get into materials that are largely titanium carbide but still based on tungsten carbide technology.

Under great heat and pressure the carbon atoms are actually packed inside the tungsten atom lattice as interstitials instead of being joined side by side as with ordinary compounds.

Interstitial carbides, such as tungsten carbide (WC), form when carbon combines with a metal that has an intermediate electronegativity and a relatively large atomic radius. In these compounds, the carbon atoms pack in the holes (interstices) between planes of metal atoms. The interstitial carbides, which include TiC, ZrC, and MoC retain the properties of metals.

Cemented tungsten carbide is made by sintering (at high temperature) a combination of tungsten carbide powder with powdered cobalt (Co), a ductile metal that serves as a "binder" for the extremely hard tungsten carbide particles. The heat of the sintering process does not involve a reaction of the two constituents, but rather causes the cobalt to reach a near-liquid state and become like an encapsulating glue matrix for the WC particles (which are unaffected by the heat). Two parameters, namely the ratio of Cobalt to WC and the WC particle size, significantly control the bulk material properties of the resulting "cemented tungsten carbide" piece.

The high solubility of WC in cobalt at high temperatures and a very good wetting of WC by the liquid cobalt binder result in an excellent densification during liquid phase sintering and in a pore-free structure. As a result of this, a material is obtained which combines high strength, toughness and high hardness.

Actually Tungsten and most metals arrange themselves in a lattice which is like a 3 D version of a chain link fence.

This is hard to do because you are dealing with two things. First, you need to pack the carbon atoms into the tungsten lattice. In a piece of Carbide 1 inch by ½ inch by 1/8 inch you need to pack about 975,000,000,000,000,000,000,000 (975 septillion) atoms in to that many holes. Second, the tungsten is trying to grow into a single big crystal but you want millions of small crystals.

These grains are combined with Cobalt powder and mixed in a ball mill. Tungsten carbide balls are mixed with grains allowed to run for several days to get even dispersal of the grains and the cobalt powder. This powder is then dried and wax is added as a binder. The wax holds the powder together and makes it somewhat slippery so it presses into shapes well. The shapes are presintered in an atmosphere-controlled furnace at temperatures of 1,000 - 1,500F. The wax melts out and leaves the pieces sort of like a soft chalk. These chalk pieces can be easily machined although they are also easy to break and can be chipped here if handled improperly.

The final step is another sintering step that can take place in a special atmosphere, a vacuum or both. The temperature is typically 2,500 - 2,700 f. During final sintering the parts will shrink up to 15% in any dimension and up to 35% in volume. Typically 15 to 30 tons of pressure is used to form the tungsten carbide into a tool shape such as a saw tip.

The parts are typically pressed one of three ways. They are rammed in a mold before sintering. They are isostatically pressed. Isostatic pressing means they are surrounded by a liquid or a gas and the pressure is applied to the liquid. This transfers the pressures to the surface of the parts uniformly. The third pressing method is hot pressing during sintering.

It starts with at least four powders and can have eight or more powders. These are extremely fine and hard to work with. If you have ever had to work with toner powder you have some idea. It wants to stick to itself and everything else.

Once the powder is mixed then it is pressed into shape. The wax was added to keep the powder together for pressing. After pressing the wax is melted out.

If you make carbide right you get a nice even distribution of the same sized grains (left). If you are sloppy and / or use cheap materials then you get carbide like that on the right which has odd bits of the basic materials sort of like lumps in gravy.

C grades - Traditional C grades of carbide are only marginally relevant in any kind of use The original concept was to rate tungsten carbides according to the job that they had to do. If you had a particular job you would specify a "C" grade of tungsten carbide and you could buy from anybody. This has led to a situation where a C-7 tungsten carbide can be almost anything as long as it does C-7 style work. According to Machinery's Handbook it can range from 0 - 75% tungsten carbide, 8 to 80% titanium tungsten carbide, 0 - 10% Cobalt and 0 - 15% Nickel. The problem is that two C-7 tips from two manufacturers will almost certainly work very differently in two different applications.

A common misconception is that there is a straight progression from C-1 to C-14 or wherever. A common view is that each higher grade has less cobalt in the binder and is therefore harder and more likely to break. Following this line of thought is the belief that the higher C number is harder and better for wear resistance. This is like classifying automobiles by size from a moped to an eighteen-wheel semi. This is clear and handy but unfortunately it is not true.

C grades classification C-1 to C-4 are general grades for cast iron, non-ferrous and non-metallic materials

• C-1 Roughing
• C-2 General Purpose
• C-3 Finishing
• C-4 Precision

Steel and steel alloys - these grades resist pitting and deformation

• C-5 Roughing
• C-6 General Purpose
• C-7 Finishing
• C-8 Precision

Wear Surface

• C-9 No shock
• C-10 Light shock
• C-11 Heavy shock

Impact

• C-12 Light
• C-13 Medium
• C-14 heavy

Miscellaneous

• C-15 Light cut, hot flash weld removal
• C-15A Heavy cut, hot flash weld removal
• C-16 Rock bits
• C-17 Cold header dies
• C-18 Wear at elevated temperatures and/or resistance to chemical reactions
• C-19 Radioactive shielding, counter balances and kinetic applications

Uses in machine tools

Carbide cutting surfaces are often useful when machining through materials such as carbon steel or stainless steel, as well as in situations where other tools would wear away, such as high-quantity production runs. Sometimes, carbide will leave a better finish on the part, and allow faster machining. Carbide tools can also withstand higher temperatures than standard high speed steel tools. The material is usually tungsten-carbide cobalt, also called "cemented carbide", a metal matrix composite where tungsten carbide particles are the aggregate and metallic cobalt serves as the matrix.

Machining with carbide can be difficult, as carbide is more brittle than other tool materials, making it susceptible to chipping and breaking. To offset this, many manufacturers sell carbide inserts and matching insert holders. With this setup, the small carbide insert is held in place by a larger tool made of a less brittle material (usually steel). This gives the benefit of using carbide without the high cost of making the entire tool out of carbide. Most modern face mills use carbide inserts, as well as some lathe tools and endmills.

To increase the life of carbide tools, they are sometimes coated. Four such coatings are TiN (titanium nitride), TiC (titanium carbide), Ti(CN) (titanium carbide-nitride), and TiAlN (Titanium Aluminum Nitride). (Newer coatings, known as DLC (Diamond Like Coating) are beginning to surface, enabling the cutting power of diamond without the unwanted chemical reaction between real diamond and iron.) Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool. The coating is usually deposited via thermal CVD. However if the deposition is performed at too high temperature, an eta phase of a Co₆W₆C tertiary carbide forms at the interface between the carbide and the cobalt phase, facilitating adhesion failure of the coating.

Military use

Tungsten carbide is often used in armor-piercing ammunition, especially where depleted uranium is not available or not politically acceptable. The first use of W₂C projectiles occurred in Luftwaffe tank-hunter squadrons, which used 37 mm autocannon equipped Ju-87G Stuka attack planes to destroy Soviet T-34 tanks in WWII. Owing to the limited German reserves of tungsten, W₂C material was reserved for making machine tools and small numbers of projectiles for the most elite combat pilots, like Hans Rudel.

Tungsten carbide ammunition can be of the sabot type (a large arrow surrounded by a discarding push cylinder) or a subcaliber ammunition, where copper or other relatively soft material is used to encase the hard penetrating core, the two parts being separated only on impact. The latter is more common in small-caliber arms, while sabots are usually reserved for artillery use.

Tungsten carbide is also an effective neutron reflector and as such was used during early investigations into nuclear chain reactions, particularly for weapons. A criticality accident occurred at Los Alamos National Laboratory on 21 August 1945 when Harry K. Daghlian, Jr. accidentally dropped a tungsten carbide brick onto a plutonium sphere causing the sub-critical mass to go critical with the reflected neutrons.

In sports

Hard carbides, especially tungsten carbide, are used by athletes, generally on poles which impact hard surfaces. Trekking poles, used by many hikers for balance and to reduce pressure on leg joints, generally use carbide tips in order to gain traction when placed on hard surfaces (like rock); such carbide tips last much longer than other types of tips. Rocks along many popular hiking trails, such as the Appalachian Trail and Pacific Crest Trail, are scratched and pockmarked from hundreds or thousands of impacts from pole tips. ^{[citation needed]}

While ski pole tips are generally not made of carbide, since they do not need to be especially hard even to break through layers of ice, rollerski tips usually are. Roller skiing emulates cross country skiing and is used by many skiers to train during warm weather months. Because skiers require traction on bitumen (asphalt) carbide tips are used in the sport. ^{[citation needed]}

In fiction

In the Halo video game series, Magnetic Accelerator Cannon projectiles are coated in a hard outer layer of tungsten carbide.

In Monty Python's Flying Circus, in a sketch involving a dispute between a famous playwright and his son, a coal miner. The son mentions that they have started using some new 'tungsten carbide drills,' to which the father's response is 'Tungsten carbide drills?! What the bloody 'ell's tungsten carbide drills?!'

In the videogame EVE Online tungsten carbide Armor Plates are used as a main component for the Amarr Ships and other pieces of technology.

Domestic use

Tungsten carbide can now be found in the inventory of some jewelers, most notably as the primary material in men's wedding bands. When used in this application the bands appear with a lustrous dark hue often buffed to a mirror finish. The finish is highly resistant to scratches and scuffs, holding its mirror-like shine for years.

Many manufacturers of this emerging jewelry state that the use of a cobalt binder may cause unwanted reactions between the cobalt and the natural oils on our skin. Skin oils cause the cobalt to leach from the material. This is said to cause possible irritation of the skin and permanent staining of the jewelry itself. Many manufacturers now advertise that their jewelry is "cobalt free". This is obtained by substituting the cobalt with nickel as a binder.

The validity of these statements have not been verified by any impartial source and more research is needed.

External links

• International Chemical Safety Card 1320
• NIOSH Pocket Guide to Chemical Hazards
• Memsnet: Properties of tungsten carbide
• Tungsten carbide rings
• http://www.therightcarbide.com/
• Tungsten Carbide Powder

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