Introduction
H. Cross Co. is the leading specialty metal rolling manufacturer of precision
ribbons, sheets, foils and strips of tungsten and tungsten- base alloys. It also
supplies wire, rod and tubing.
H. Cross Co. sells no product from stock; all its products are made to your
exact specifications. This catalog is thus, of necessity, general in nature.
History
The word "tungsten" is an adaptation of the Swedish tung sten (heavy stone) and
was first applied to the mineral scheelite about 1758. The element was first
identified in 1781 by a Swedish chemist, K.W. Scheele, for whom the calcium
tungstate mineral, scheelite, was later named. Commercially, tungsten is
prepared from scheelite.
Further processing results in a yellow powder of tungsten tri-oxides that would
be considered extremely pure by most commercial standards. However, the desired
properties of tungsten wire are affected so adversely by minute quantities of
unwanted impurities that another purification is added. This consists of
re-dissolving the hydroxide, purifying the solution, and crystallizing the
tungsten out in the form of ammonium paratungstate.
Applications
Tungsten is practically the only material used for electron emitters. Even
though other more electropositive metals would have higher emission rates, the
advantage of tungsten is its extremely low vapor pressure even at high
temperatures.
This property is also important for electrical contact materials. While more
conductive metals like copper or silver evaporate under the conditions of
electric arc, tungsten withstands these.
The following illustrates some of tungsten's applications:
- Directly heated cathodes or heater coils for indirectly heated
cathodes in cathode ray tubes for TV sets or computer displays, X-ray
tubes, electron tubes, klystrons, magnetrons for microwave ovens.
- Thoria or rare earth oxide alloyed electrodes for inert gas welding,
as well as High Intensity Discharge (HID) lamps.
- Tungsten disks for substrate of high power semiconductor rectifying
devices; Electrical contacts.
- High temperature furnace parts such as tungsten heating coils,
reflectors and structural material.
- Tungsten / tungsten-rhenium thermocouples for measuring the
temperature in such furnaces.
- X-ray tubes for medical use are not only equipped with a tungsten
emitter coil, but also a static anode made of tungsten or
tungsten-rhenium. Important here are not only the low vapor pressure,
but also tungsten's good heat conductivity and the wavelength of the
resulting X-rays.
- Modern business machines, such as photocopiers, facsimile machines,
laser printers and air cleaners are equipped with tungsten charger
wires. Not only drawn tungsten wire, but also electro-polished,
gold-plated or platinum clad tungsten wires are used for this
application.
- Modern computer processors generate a heat output per square
centimeter similar to that of a household cook top. Tungsten-copper heat
sinks and the processor fan remove the heat.
Fabrication and Material Forms
Tungsten has such a high melting temperature (3410° C) that it is impractical to
transform it into ingot forms by conventional furnace melting techniques. Its
manufacture by powder metallurgy consists of three distinct phases: the
decomposition of the ore and its reduction to pure metal powder, the pressing of
this powder into bars and the sintering of these bars into solid ingots and the
swaging, rolling or drawing of these ingots into desired forms and sizes.
Alloys of Tungsten
In addition to commercially pure tungsten we offer several alloys that are used
for specialized applications. These products are 3% Rhenium Tungsten, 5% Rhenium
Tungsten and 25% Rhenium Tungsten. The addition of rhenium to the tungsten
increases the ductility and shock resistance at higher temperatures encountered
in thermocouple and electronic applications among others.
Surface Finishes
A colloidal graphite lubricant, which imparts a black coating to "as drawn"
tungsten wire, is used to protect the wire from oxidation and to lubricate the
wire as it passes through the die. Many applications require tungsten wire with
characteristics other than those obtainable in "as drawn" wire. A variety of
processes produce wire with a cleaned surface or with a special finish.
After drawing operations have been completed, the wire may be cleaned by
exposure to elevated temperatures in a reducing atmosphere. Treating the surface
chemically or electrolytically can also clean tungsten wire. (The diameter of
the wire may be further reduced after drawing by electrolytic etching.)
The cleaned wire can be electroplated with such elements as gold, silver, nickel
and copper, for use primarily in electronic tubes.
Packaging
All H. Cross Company products are delivered in packaging specifically designed
for the product's application. As an example, wire is furnished on returnable
plastic reels containing a single length of wire.
If required, H. Cross Co. can adopt standard packaging methods or develop new
ones for your special needs. |
