The refractory metals are conveniently described as those which, first of all, melt
at temperatures well above the melting points of the common alloying bases, iron,
cobalt, and nickel. Second, it seems appropriate to consider the refractory metals
as those which have higher melting points than do titanium and zirconium, which are
used chiefly at intermediate temperatures. On the other hand, chromium is usually
classed as a refractory metal.
The refractory metals include niobium (also known as columbium), tantalum, molybdenum,
tungsten, and rhenium. With the exception of two of the platinum-group metals, osmium
and iridium, they have the highest melting temperatures and lowest vapor pressures of
all metals. The refractory metals are readily degraded by oxidizing environments at
moderately low temperatures, a property that restricted the applicability of the
metals in low-temperature or nonoxidizing high-temperature environments. Protective
coating systems have been developed, mostly for niobium alloys, to permit their use
in high-temperature oxidizing aerospace applications.
Refractory metals at one time were limited to use in tamp filaments, electron tube
grids, heating elements, and electrical contacts; however, they have since found
widespread application in the aerospace, electronics, nuclear and high-energy
physics, and chemical process industries.
The refractory metals are extracted from ore concentrates, processed into intermediate
chemicals and then reduced to metal. The refractory metals, except for niobium, are
produced exclusively as metal powders, which are consolidated by sintering and/or
melting. They can be formed, machined, and joined by conventional methods. They
are ductile in the pure state and have high interstitial solubilities for carbon,
nitrogen, oxygen and hydrogen. Because of the high solubilities in niobium and
tantalum, these embrittling contaminants normally do not present problems in
fabrication. The process for niobium differs only in that the metal is most commonly
reduced by aluminothermic reduction of oxide.
Niobium
Niobium and its alloys exhibit properties that provide technological capabilities
unique among refractory metals, although it is the least refractory of these metals.
Its cryogenic ductility and ease of fabrication are excellent, being superior to
those of molybdenum and tungsten but not as good as those of tantalum. Niobium
oxidizes noncatastrophically; it is superior to molybdenum and tungsten and about
equal to tantalum in this characteristic. It is in abundant supply; the estimated
free-world reserves of niobium are much greater than those of tantalum and tungsten
and probably are even superior to those of molybdenum.
Niobium is a ductile and soft metal at elevated temperatures. Strength can be improved
by alloying to make it competitive with molybdenum (and molybdenum alloys), its
closest rival for use at temperatures through at least 2500°F. The advantages of
niobium alloys may dictate their ultimate use in preference to other refractory metals
to temperatures as high as 3300°F. Lack of oxidation resistance is a major barrier to
the use of niobium alloys in structural applications at elevated temperatures.
Niobium Alloys. Commercial application of niobium is dominated by
its use as an alloying element in steels. Almost 75% of all niobium metal is used as
minor alloying additions in low-alloy steel. Another 20 to 25% is used as alloy
additions in nickel-base superalloys and heat-resisting steels. Only 1 to 2% of all
niobium is used in the form of niobium-base alloys and pure niobium metal including
superconducting niobium-titanium alloy, which accounts for over one-half of all
niobium alloys produced. Originally, niobium metal was produced by powder metallurgy
methods which involved high temperature vacuum sintering and carbon reduction.
Powder Production. Powders are produced from ingot by hydriding,
crushing, and dechydriding; in addition, some recent efforts have been directed
toward producing complex metals-table alloy powders, such as niobium-aluminum and
niobium-silicon alloys, by liquid metal atomization and rapid quenching. The particle
structure of degassed hydride niobium powder is completely analogous to that of a
tantalum powder produced in a similar process for capacitors.
Molybdenum
More molybdenum is consumed annually than any other refractory metal. Molybdenum
ingots, produced by melting of P/M electrodes, are extruded, rolled into sheet and
rod, and subsequently drawn to other mill product shapes, such as wire and tubing.
These materials can then be stamped into simple shapes. Molybdenum is also machined
with ordinary tools and can be gas tungsten arc and electron beam welded, or brazed.
Molybdenum has outstanding electrical and heat-conducting capabilities and relatively
high tensile strength. Thermal conductivity is approximately 50% higher than that of
steel, iron or nickel alloys. It consequently finds wide usage as heatsinks. Its
electrical conductivity is the highest of all refractory metals, about one third
that of copper, but higher than nickel, platinum, or mercury. The coefficient of
thermal expansion of molybdenum plots almost linearly with temperature over a wide
range. This characteristic, in combination will raise heat-conducting capabilities,
accounts for its use in bimetal thermocouples. Methods of doping molybdenum powder
with potassium aluminosilicate to obtain a non-sag microstructure comparable to that
of tungsten also have been developed.
The major use for molybdenum is as an alloying agent for alloy and tool steels,
stainless steels, and nickel-base or cobalt-base super-alloys to increase hot strength,
toughness and corrosion resistance.
In the electrical and electronic industries, molybdenum is used in cathodes, cathode
supports for radar devices, current leads for thorium cathodes, magnetron end hats,
and mandrels for winding tungsten filaments.
Molybdenum is important in the missile industry, where it is used for high-temperature
structural parts, such as nozzles, leading edges of control surfaces, support vanes,
struts, reentry cones, heal-radiation shields, heat sinks, turbine wheels, and pumps.
Molybdenum has also been useful in the nuclear, chemical, glass, and metallizing
industries. Service temperatures, for molybdenum alloys in structural applications
arc, is limited to a maximum of about 1650°C (3000°F). Pure molybdenum has good
resistance to hydrochloric acid and is used for acid service in chemical process
industries.
Molybdenum Alloy TZM. The molybdenum alloy of greatest technological
importance is the high-strength, high-temperature alloy TZM. The material is
manufactured either by P/M or arc-cast processes.
TZM has a higher recrystallization temperature and higher strength and hardness at
room and at elevated temperatures than unalloyed molybdenum. It also exhibits adequate
ductility. Its superior mechanical properties arc due to the dispersion of complex
carbides in the molybdenum matrix. TZM is well suited to hot work applications
because of its combination of high hot hardness, high thermal conductivity, and
low thermal expansion to hot work steels.
Major uses include:
- Die inserts for casting aluminum, magnesium, zinc, and iron
- Rocket nozzles
- Die bodies and punches for hot stamping
- Tools for metalworking (due to the high abrasion and chatter resistance of TZM)
- Heat shields for furnaces, structural parts, and heating elements
Rhenium
Among the elements, rhenium has the highest melting point, except for tungsten and
carbon. Its density is exceeded only by osmium, iridium, and platinum. A
ductile-to-brittle transition temperature does not exist in pure rhenium. Rhenium
is the only refractory metal that does not form carbides.
Rhenium has a high electrical resistivity over a wide temperature range. This
characteristic, combined with a low vapor pressure, makes it ideally suited for
filament applications; additionally, it maintains ductility and is not affected by
the oxidation/reduction cycle experienced in these applications, as is tungsten.
One of the largest applications for rhenium is for mass spectrometer filaments,
where it is available in commercial (99.99%) and zone-refined (99.995%) purities.
Rhenium is not attacked by molten copper, silver, tin, or zinc. It dissolves readily
in molten iron and nickel, but is stable in the presence of aluminum. Rhenium has a
significant hardening effect on platinum. At elevated temperature, rhenium resists
attack in hydrogen and inert atmospheres. It is resistant to hydrochloric acid and
seawater corrosion and to the mechanical effects of electrical erosion.
Rhenium Alloys. Rhenium is a beneficial alloying addition with
other refractory metals. Rhenium greatly enhances the ductility and tensile strength
of refractory metals and their alloys Rhenium alloys arc used in nuclear reactors,
semiconductors, electronic tube components, thermocouples, gyroscopes, miniature
rockets, electrical contacts, thermionic converters, and other commercial and
aerospace applications. Tungsten-rhenium alloys, applied by vapor deposition,
are used to coat the surface of molybdenum targets in x-ray tube manufacture.
Other rhenium alloys (with tungsten or molybdenum) are used for filaments,
grid heaters, cathode cups, and igniter wires in photoflash bulbs.
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