5 Interesting Facts About Tungsten

July 9, 2015 Joshua Jablons Ph.D.

How Tungsten Is Unique, from Its Name to Its Properties

With Metal Cutting’s long history of supplying and manufacturing high-grade tungsten materials, we’ve developed a special appreciation for the element and what it can do. So just for fun, this week we thought we’d take a look at some of the more interesting — and sometimes curious — facts about tungsten.

1. What’s in a name?

The derivation of the name “tungsten” — and the story of why the element’s symbol on the periodic table is W — is an internationally paradoxical tale. The element itself was discovered back in 1783 by two Spanish chemists, brothers Juan José and Fausto Elhuyar, in samples of the mineral called wolframite. Today, in many countries around the world, the element is referred to by the Germanic name “wolfram,” after the mineral wolframite. So, it’s easy to see where the chemical symbol W came from — although if you’re like us, you might wonder why the element wasn’t named “Elhuyarite” or if there is a Spanish word for wolframite.

However, the name “tungsten” is what is used today in the U.S. and in English, French, and various other languages. This term came from the Swedish words tung and sten, meaning “heavy stone,” and is the old Swedish name for the mineral scheelite, another source of tungsten ore. But ironically, “tungsten” is NOT used as the name of the element in Sweden or the other Nordic countries. There, as well as in most Germanic and Slavic languages, they use the name “wolfram” or “volfram”.

Tungsten/Wolfram/Volfram is still extracted primarily from wolframite and scheelite. Of all metals in pure form, tungsten has the highest melting point (3422°C, 6192°F), lowest vapor pressure (at temperatures above 1650°C, 3000°F), and the highest tensile strength. So, whether you call it tungsten or any other name, this range of properties means tungsten is used in many industries and products around the world, from high-speed cutting tools and jet turbine engines, to ammunition, lighting, and even fishing weights.

2. Tungsten wire diameter is expressed in milligrams.

You might hear people describe tungsten diameter as 14.7 mg, 3.05 mg, 246.7 mg, and so on. It’s because in the old days, lacking tools for accurately measuring very thin wires — say, from .001” up to .020” in diameter — the convention was to take 200 mm (about 8”) of tungsten wire, weigh it, and plug the weight into a mathematical formula to determine the diameter.

To calculate the diameter (D) of tungsten wire based on the weight per unit length, the formula is:

D = 0.71746 x square root (mg weight/200 mm length)

The standard diameter tolerance is ± 3% of the weight measurement, although tighter tolerances are available, depending on the application for the wire product. This method also assumes the wire has a constant diameter, with no significant variation, necking down, or other conical effects anywhere on the diameter.

3. Most tungsten wire comes doped — whether you need it that way or not!

Once again, the practice goes back to the old days — specifically, a time when the main purpose of tungsten wire was in filaments for light bulbs. The trouble was, light bulbs throw off white-hot temperatures that caused early filaments to sag, leading to lamp failure. Through experimentation, someone came up with the idea of adding alumina, silica, and potassium to alter the mechanical properties of the tungsten wire. The dopants were added at the powder mixing stage and then, in the process of hot swaging and hot drawing the tungsten wire, the alumina and silica would out-gas and the potassium would remain, giving the wire non-sag properties under white-hot temperatures.

Adding these dopants at the powder mixing stage of tungsten wire manufacturing dates to the time when there were no other significant uses for the wire besides light bulb filaments. And while there are many other uses of tungsten wire today — not to mention incandescent light bulbs are becoming a thing of the past — the use of dopants in tungsten wire manufacturing continues. However, it happens that Metal Cutting can also offer 99.999% pure, undoped tungsten wire. All you need to do is ask.

4. Tungsten and tungsten carbide are NOT interchangeable.

Tungsten carbide is famous for its wear resistance; in fact, it can only be cut using diamond tools. But while tungsten carbide does have a lot of tungsten in it, the addition of cobalt to make it a cemented carbide gives tungsten carbide properties that are very different from those of pure tungsten.

Pure tungsten has many properties that make it useful; however, it is also known for being notoriously difficult to machine. Try to use diamond tools, and pure tungsten will simply load, or “gum up,” the diamond wheel. Metal Cutting specializes in methods that are very effective for cutting pure tungsten, but interestingly, those same methods are not useful when trying to cut tungsten carbide.

For example, a customer may tell us they have or want a tungsten tube, but upon further investigation we may find out they actually have or need tungsten carbide. Pure tungsten simply cannot be made into a tube, aside from gun drilling final sizes — and that assumes the part has a favorable length to ID ratio and the customer has lots of money and doesn’t want a lot of these painstakingly produced parts. Tungsten carbide, on the other hand, can be pressed and sintered into tubular shapes, but that, too, is not a cheap or high-volume process. And unlike other metals, neither tungsten nor tungsten carbide can be drawn into tubes.

5. In theory, you can have liquid tungsten — but in practice …

People sometimes ask if you can have liquid tungsten, and the answer is … well, it’s complicated. Having the highest melting point of any known metal, at 3422°C, obviously tungsten would be very difficult to melt. In theory, anything can be melted if you apply enough heat. However, for commercial purposes, tungsten’s melting point makes liquid tungsten an insurmountably impossible proposition.

For instance, in what type of container would you hold a large amount of liquid tungsten? In all practicality, anything you’d try to use would be melted by the high temperature of the tungsten. That is why tungsten is manufactured in a non-liquid state, using powder metallurgy. Commercially, tungsten products, such as heavy alloys, copper tungsten, and electrodes, are made through pressing and sintering in near net shape. For wrought products, the pressing and sintering, followed by swaging and a repeated schedule of repeated drawing and annealing, produces the characteristic elongated grain structure, which carries over to the finished product, whether it is a large rod or a very thin wire.

The only known element with a higher melting point than tungsten is carbon, at 3550°C. However, even carbon cannot be used to hold liquid tungsten, because at high temperatures the two will react to form tungsten carbide. Experimentally, liquid tungsten has been produced, using super-conductive copper crucibles in which the heat is pulled away from the surface of the crucible so that it remains intact. But again, this is impractical for commercial volumes — and that means, at least for the present time, you just can’t buy liquid tungsten products.

How do facts 1-5 add up?

With all of its unique and interesting properties, tungsten is one of the most widely used of the refractory metals. (And no, it is not a rare earth element, even though for political reasons tungsten can be grouped with those elements that make the headlines — but that is a topic for another time.) As a company that has been delivering specialized tungsten wire and rod products since 1967 — and, as the exclusive North American distributor for Nippon Tungsten Co., Ltd. — Metal Cutting can help you untangle the mysteries of tungsten for your applications.

 

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