Learn About Refractory Metals

The term 'refractory metal' is used to describe a group of metal elements that have exceptionally high melting points and are resistant to wear, corrosion, and deformation.

Industrial uses of the term refractory metal most often refer to five commonly used elements:

• Molybdenum (Mo)
• Niobium (Nb)
• Rhenium (Re)
• Tantalum (Ta)
• Tungsten (W)

However, broader definitions have also included the less commonly used metals:

• Chromium (Cr)
• Hafnium (Hf)
• Iridium (Ir)
• Osmium (Os)
• Rhodium (Rh)
• Ruthenium (Ru)
• Titanium (Ti)
• Vanadium (V)
• Zirconium (Zr)

The Characteristics

The identifying feature of refractory metals is their resistance to heat. The five industrial refractory metals all have melting points in excess of 3632°F (2000°C).

The strength of refractory metals at high temperatures, in combination with their hardness, makes them ideal for cutting and drilling tools.

Refractory metals are also very resistant to thermal shock, meaning that repeated heating and cooling will not easily cause expansion, stress, and cracking.

The metals all have high densities (they're heavy) as well as good electrical and heat conducting properties.

Another important property is their resistance to creep, the tendency of metals to slowly deform under the influence of stress.

Owing to their ability to form a protective layer, the refractory metals are also resistant to corrosion, although they do readily oxidize at high temperatures.

Refractory Metals & Powder Metallurgy

Due to their high melting points and hardness, the refractory metals are most often processed in powder form and never fabricated by casting.

Metal powders are manufactured to specific sizes and forms, then blended to create the right mixture of properties, before being compacted and sintered.

Sintering involves heating the metal powder (within a mold) for a long period of time. Under heat, the powder particles begin to bond, forming a solid piece.

Sintering can bond metals at temperatures lower than their melting point, a significant advantage when working with the refractory metals.

Carbide Powders

One of the earliest uses for many refractory metals arose in the early 20th century with the development of cemented carbides.

Widia, the first commercially available tungsten carbide, was developed by Osram Company (Germany) and marketed in 1926. This led to further testing with similarly hard and wear resistant metals, ultimately leading to the development of modern sintered carbides.

The products of carbide materials often benefit from mixtures of different powders. This process of blending allows for the introduction of beneficial properties from different metals, thereby, producing materials superior to what could be created by an individual metal. For example, the original Widia powder was comprised of 5-15% cobalt.

Note: See more on refractory metal properties in the table at the bottom of the page

Applications

Refractory metal-based alloys and carbides are used in virtually all major industries, including electronics, aerospace, automotive, chemicals, mining, nuclear technology, metal processing, and prosthetics.

The following list of end-uses for refractory metals was compiled by the Refractory Metals Association:

Tungsten Metal

• Incandescent, fluorescent, and automotive lamp filaments
• Anodes and targets for x-ray tubes
• Semiconductor supports
• Electrodes for inert gas arc welding
• High capacity cathodes
• Electrodes for xenon are lamps
• Automotive ignition systems
• Rocket nozzles
• Electronic tube emitters
• Uranium processing crucibles
• Heating elements and radiation shields
• Alloying elements in steels and superalloys
• Reinforcement in metal-matrix composites
• Catalysts in chemical and petrochemical processes
• Lubricants

Molybdenum

• Alloying additions in irons, steels, stainless steels, tool steels and nickel-base superalloys
• High-precision grinding wheel spindles
• Spray metallizing
• Die-casting dies
• Missile and rocket engine components
• Electrodes and stirring rods in glass manufacture
• Electric furnace heating elements, boats, heat shields, and muffler liner
• Zinc refining pumps, launders, valves, stirrers and thermocouple wells
• Nuclear reactor control rod production
• Switch electrodes
• Supports and backing for transistors & rectifiers
• Filaments & support wires for automobile headlight
• Vacuum tube getters
• Rocket skirts, cones, and heat shields
• Missile Components
• Superconductors
• Chemical process equipment
• Heat shields in high-temperature vacuum furnaces
• Alloying additives in ferrous alloys & superconductors

Cemented Tungsten Carbide

• Cemented Tungsten Carbide
• Cutting tools for metal machining
• Nuclear engineering equipment
• Mining and oil drilling tools
• Forming dies
• Metal forming rolls
• Thread guides

Tungsten Heavy Metal

• Bushings
• Valve seats
• Blades for cutting hard and abrasive materials
• Ball point pen points
• Masonry saws and drills
• Heavy Metal
• Radiation shields
• Aircraft counterweights
• Self-winding watch counterweights
• Aerial camera balancing mechanisms
• Helicopter rotor blade balance weights
• Gold club weight inserts
• Dart bodies
• Armament fuses
• Vibration damping
• Military Ordnance
• Shotgun pellets

Tantalum

• Electrolytic capacitors
• Heat exchangers
• Bayonet heaters
• Thermometer wells
• Vacuum tube filaments
• Chemical process equipment
• High-temperature furnaces components
• Crucibles for handling molten metal and alloys
• Cutting tools
• Aerospace engine components
• Surgical implants
• Alloy additive in superalloys

Physical Properties of Refractory Metals

Type	Unit	Mo	Ta	Nb	W	Rh	Zr
Typical Commercial Purity		99.95%	99.9%	99.9%	99.95%	99.0%	99.0%
Density	cm/cc	10.22	16.6	8.57	19.3	21.03	6.53
	lbs/in2	0.369	0.60	0.310	0.697	0.760	0.236
Melting Point	Celcius	2623	3017	2477	3422	3180	1852
	°F	4753.4	5463	5463	6191.6	5756	3370
Boiling Point	Celcius	4612	5425	4744	5644	5627	4377
	°F	8355	9797	8571	10,211	10,160.6	7911
Typical Hardness	DPH (vickers)	230	200	130	310	--	150
Thermal Conductivity (@ 20 °C)	cal/cm2/cm°C/sec	--	0.13	0.126	0.397	0.17	--
Coefficient of Thermal Expansion	°C x 10 -6	4.9	6.5	7.1	4.3	6.6	--
Electrical Resistivity	Micro-ohm-cm	5.7	13.5	14.1	5.5	19.1	40
Electrical Conductivity	%IACS	34	13.9	13.2	31	9.3	--
Tensile Strength (KSI)	Ambient	120-200	35-70	30-50	100-500	200	--
	500°C	35-85	25-45	20-40	100-300	134	--
	1000°C	20-30	13-17	5-15	50-75	68	--
Minimum Elongation (1 inch gauge)	Ambient	45	27	15	59	67	--
Modulus of Elasticity	500°C	41	25	13	55	55
	1000°C	39	22	11.5	50	--	--

Source: http://www.edfagan.com