Refractory Metals

Nov. 15, 2002
Refractory metals are characterized by their extremely high melting points, which range well above those of iron, cobalt, and nickel.

Refractory metals are characterized by their extremely high melting points, which range well above those of iron, cobalt, and nickel. They are used in demanding applications requiring high-temperature strength and corrosion resistance. The most extensively used of these metals are tungsten, tantalum, molybdenum, and columbium (niobium). They are mutually soluble and form solid-solution alloys with each other in any proportion. These four refractory metals and their alloys are available in mill forms as well as products such as screws, bolts, studs, and tubing.

Although the melting points of these metals are all well above 4,000°F, they oxidize at much lower temperatures. Accelerated oxidation in air occurs at 190°C for tungsten, 395°C for molybdenum, and 425°C for tantalum and columbium. Therefore, protective coatings must be applied to these metals if they are to be used at higher temperatures. Tensile and yield strengths of the refractory metals are substantially retained at high temperature.

Columbium and tantalum: These metals are usually considered together because most of their working characteristics are similar. They can be fabricated by most conventional methods at room temperature. Heavy sections for forging can be heated, without protection, to approximately 425°C.

Out of several commercial-grade tantalum alloys, those containing tungsten, columbium, and molybdenum generally retain the corrosion resistance of tantalum and provide higher mechanical properties. Columbium is also available in alloys containing tantalum, tungsten, molybdenum, vanadium, hafnium, zirconium, or carbon. Alloys provide improved tensile, yield, and creep properties, particularly in the 1,100 to 1,650°C range.

Most sheet-metal fabrication of columbium and tantalum is done in the thickness range of 0.004 to 0.060 in. Columbium, like tantalum, can be welded to itself and to certain other metals by resistance welding, tungsten inert-gas (TIG) welding, and to itself by inert-gas arc welding. Electron-beam welding can also be used, particularly for joining to other metals. However, surfaces that are heated above 315°C during welding must be protected with an inert gas to prevent embrittlement.

Principal applications for tantalum are in capacitor anodes, filaments, gettering devices, chemical-process equipment, and high-temperature aerospace engine components. Columbium is used in superconducting materials, thin-film substrates, electrical contacts, heat sinks, and as an alloying addition in steels and superalloys.

Molybdenum: Probably the most versatile of the refractory metals, molybdenum is also a natural resource of the United States. It is an excellent structural material for applications requiring high strength and rigidity at temperatures to 3,000°F where it can operate in vacuum or under inert or reducing atmospheres.

Unalloyed molybdenum and its principal alloy, TZM, are produced by powder-metallurgy methods and by vacuum-arc melting. Both are commercially available in ordinary mill product forms: forging billets, bars, rods, wire, seamless tubing, plate, strip, and thin foil. Compared to unalloyed molybdenum, the TZM alloy (Mo-0.5%Ti-0.1%Zr) develops higher strength at room temperature and much higher stress-rupture and creep properties at all elevated temperatures. At 1,800 to 2,000°F, TZM can sustain a 30,000-psi stress for over 100 hr, three times that for unalloyed molybdenum.

Molybdenum and TZM are readily machined with conventional tools. Sheet can be processed by punching, stamping, spinning, and deep drawing. Some parts can be forged to shape. Molybdenum wire and powder can be flame sprayed onto steel substrates to salvage worn parts or to produce long-wearing, low-friction surfaces for tools.

In nonoxidizing environments, the metal resists attack by hydrochloric, hydrofluoric, sulfuric, and phosphoric acids. Molybdenum oxidizes at high temperatures to produce volatile, nontoxic, molybdenum trioxide; however, parts such as gimbled nozzles have been used successfully in rocket and missile-guidance systems when exposure time to the very-high temperatures of ballistic gases was brief.

Molybdenum parts can be welded by inertia, resistance, and spot methods in air; by TIG and MIG welding under inert atmospheres; and by electron-beam welding in vacuum. The best welds are produced by inertia (friction) welding and electron-beam welding; welds produced by the other techniques are less ductile. Generally, arc-cast metal develops better welds than do powder-metallurgy products. Heavy sections of molybdenum should be preheated and postheated when they are welded to reduce thermal stresses.

Because molybdenum has a modulus of elasticity of 47 10^6 psi at room temperature, it is used for boring bars and the quills for high-speed internal grinders to avoid vibration and chatter. Its relatively high electrical conductivity makes unalloyed molybdenum useful for electrical and electronic applications. It is used in the manufacture of incandescent lamps, as substrates in solid-state electronic devices, as electrodes for EDM equipment and for melting glass, and as heating elements and reflectors or radiation shields for high-temperature vacuum furnaces.

Because it retains usable strength at elevated temperatures, has a low coefficient of thermal expansion, and resists erosion by molten metals, the TZM alloy is used for cores in die casting of aluminum, and for die cavities in casting of brass, bronze, and even stainless steel. Dies of the TZM alloy weighing several thousand pounds are used for isothermal forging of superalloy components for aircraft gas turbines, and die inserts made of TZM have been used for extruding steel shapes. Piercer points of TZM are used to produce stainless-steel seamless tubing.

Tungsten: In many respects, tungsten is similar to molybdenum. The two metals have about the same electrical conductivity and resistivity, coefficient of thermal expansion, and about the same resistance to corrosion by mineral acids. Both have high strength at temperatures above 2,000°F, but because the melting point of tungsten is higher, it retains significant strength at higher temperatures than molybdenum does. The elastic modulus for tungsten is about 25% higher than that of molybdenum, and its density is almost twice that of molybdenum. All commercial unalloyed tungsten is produced by powder-metallurgy methods; it is available as rod, wire, plate, sheet, and some forged shapes. For some special applications, vacuum-arc-melted tungsten can be produced, but it is expensive and limited to relatively small sections.

Several tungsten alloys are produced by liquid-phase sintering of compacts of tungsten powder with binders of nickel-copper, iron-nickel, iron-copper, or nickel-cobalt-molybdenum combinations; tungsten usually comprises 85 to 95% of the alloy by weight. These alloys are often identified as heavy metals or machinable tungsten alloys. In compact forms, the alloys can be machined by turning, drilling, boring, milling, and shaping; they are not available in mill product forms because they are unable to be wrought at any temperature.

The heavy-metal alloys are especially useful for aircraft counterbalances and as weights in gyratory compasses. Heavy-metal inserts are used as the cores of high-mass military projectiles. Tungsten alloys are widely used for counterbalances in sports equipment such as golf clubs and tennis racquets. X-ray shielding is another important application of the tungsten alloys.

Filaments for incandescent lamps are usually coils of very fine unalloyed tungsten wire. Electronic tubes are often constructed with tungsten as the heaters; some advanced tubes use heaters made from a tungsten alloy containing 3% rhenium. A thermocouple rated to 4,350°F consists of one tungsten wire alloyed with 25% rhenium and another wire alloyed with 5% rhenium.

Nozzle throats of forged and machined unalloyed tungsten have been used in solid-fuel rocket engines; at one time, throats were machined from porous consolidations of tungsten powder that were infiltrated with silver for exposure to gases at temperatures near 3,500°C. Unalloyed tungsten is used for X-ray targets, for filaments in vacuum-metallizing furnaces, and for electrical contacts such as the distributor points in automotive ignition systems. Tungsten electrodes form the basis for TIG welding. Water-cooled tungsten tips are used for nonconsumable electrode vacuum-arc melting of alloys.

Cutting tools and parts that must resist severe abrasion are often made of tungsten carbide. Tungsten-carbide chips or inserts, with the cutting edges ground, are attached to the bodies of steel tools by brazing or by screws. The higher cutting speeds and longer tool life made feasible by the use of tungsten-carbide tools are such that the inserts are discarded after one use. Tungsten-carbide dies have been used for many years for drawing wire. Inserts of tungsten carbide are used in rotary bits for drilling oil and gas wells and in mining operations. Fused tungsten carbide is applied to the surfaces of mining machinery that is subjected to severe wear.

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