THE prominent REFRACTORY
METALS are:
Niobium (also known as columbium)
Tantalum
Molybdenum and
Tungsten.
They have the highest melting
temperatures of all metals
Melting temperature, °C
Niobium 2468
Tantalum 2996
Molybdenum 2610
Tungsten 3410
Refractory metals have the lowest vapor
pressures of all metals.
Vapor pressure at 2500 K, MPa
Niobium 5.3
Tantalum 0.11
Molybdenum 80
Tungsten 0.0093
The refractory metals are readily degraded by
oxidizing environments at moderately low
temperatures, a property that has 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 have found widespread
application in:
the aerospace,
electronics,
nuclear and high-energy physics, and
chemical process industries.
Molybdenum
Metallurgy & Applications
•Molybdenum is alloyed with steel, making it
stronger and resistant to heat.
•The iron and steel industries account for more
than 75% of molybdenum consumption.
•One large area of usage for molybdenum is for
production of alloy steels.
•Some of these alloy steel grades are used for
manufacture of automotive parts.
Molybdenum finds application in the
form of glass melting furnace
electrodes and components that are
resistant to molten glass.
Molybdenum electrodes for glass
melting
Molybdenum is used in the form
sprayed coatings on automotive
piston rings and machine
components to reduce friction
and improve wear.
Piston rings coated with Molybdenum
Molybdenum
resistance heating
elements are used
in electric furnaces
that operate at
temperatures up
to 2205 °C
All metal hot zone employed in a modern
vacuum furnace: Mo & its alloys find
prominent application
Strength of pure molybdenum and
carbide strengthened molybdenum alloys
Tungsten
Metallurgy & Applications
Tungsten carbide accounts for about 65%
of tungsten consumption. It is combined
with cobalt as a binder to form the so-called
cemented carbides, which are used in
cutting and wear applications.
Metallic tungsten and tungsten alloy mill
products account for about 16% of
consumption.
•WC-Co is the most common hard metal
composition.
•Hard metal is used both as bulk material
and as coatings for wear resistant
applications.
•To save on the cost and weight, hard
metal coatings got developed.
•Coatings are mostly produced by High
velocity Oxy-Fuel (HVOF) spraying, one
of the most important thermal spray
processes.
Tungsten & its alloys and
their applications
Grade
Applications
Unalloyed tungsten
Heating elements for high temperature vacuum
furnaces to 3000C, heat shields in high
temperature furnaces, radiation shields in
medical & X-ray equipment
Doped tungsten containing minute
quantities of Al (15ppm),
Si(50ppm), K(90ppm) and oxygen
(35ppm)
Non-sagging filament elements for incandescent
lamps
Solid solution alloys containing
High temperature thermocouple elements,
various amounts of Mo (2-20%) or
propulsion system components, rotating
rhenium (1-25%)
anodes for X-ray tubes
Dispersion strengthened alloys
containing 1-2% ThO2
Filament alloys for the lighting industry, high
temperature structural material
Tungsten heavy alloys (W-Ni-Cu or
W-Ni-Fe alloys)
Counterweights and balances, radiation
shielding, heat sinks and electrical contacts
and anti-armor kinetic energy penetrators
Niobium
Metallurgy & Applications
Most niobium is consumed as a ferroalloy
used in the production of high-strength lowalloy, stainless steels and superalloys.
The consumption of niobium-base metals
and alloys accounts for about 6% of the
total.
Alloying with Nb leads to increase in strength and
toughness of high strength low alloy steels due
to grain refining.
Major weight reduction can be achieved by using
Nb-alloyed steels in wide ranging applications.
Stainless steels also alloyed with Nb to improve
corrosion resistance
Important niobium alloys
for high temperature service
Strength category
Alloy
Compositio Application
n
Low strength, high Nb-1Zr Nb-1Zr
ductility,
excellent
fabricability
C103
Nb-10Hf1Ti
Sodium vapor lamps,
nuclear
applications
Moderate strength PWCand ductility
11
Space nuclear power
systems
FS-85
Nb-1Zr0.1C
Rocket components
operating in the
range 1095-1370
oC
Nb-28TaAerospace
10W-1Zr
applications
Coatings
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Niobium and its alloys are coated to
prevent oxidation in high temperature
service(>425C)
Early coatings centered on pack
cementation and chemical vapor deposition.
Problems of increase in DBTT of metal and
part distortion encountered.
Later slurry coatings of complex aluminides
and subsequently silicides were devised to
overcome these problems
Si-20Cr-20Fe composition became the
mainstay coating
Tantalum
Metallurgy & Applications
Tantalum for electronic
applications
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The largest use for tantalum is in
electrolytic capacitors. Porous sintered
powder metallurgy anodes are used in
both solid and wet electrolytic
capacitors, and, to a lesser extent,
precision tantalum foil is used in foil
capacitors. Tantalum wire is used
extensively in capacitor manufacture.
The dielectric film of tantalum oxide is
electrolytically formed on the tantalum
surface in the manufacturing process
Tantalum has excellent resistance to corrosion by
most acids, by most aqueous salt solutions, and
by organic chemicals.
It also has good resistance to many corrosive
gases and liquid metals.
Tantalum and, to a lesser extent, molybdenum
have been used for many years in the chemical
process industries.
The severe corrosion problems accompanying
many chemical processes have given impetus to
greater use of refractory alloys.
Tantalum and tantalum-clad steel processing
equipment have performed well in hightemperature sulfuric acid service.