Tungsten and Tungsten Alloys
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 (melting point 1660°C) and zirconium (1850°C), which are used chiefly at intermediate temperatures. Therefore, chromium (melting point 1875°C) is usually classed as a refractory metal.
Alloying of tungsten (W) has been relatively less studied than some of the other refractory metals. Most of the tungsten used far in aerospace applications has been in the unalloyed form, which is much easier and less expensive to produce and fabricate. Also, it has been found that, particularly at temperatures above 2200°C (4000°F), the strengthening effects of many alloying agents decrease disproportionately.
Tungsten is consumed in four forms:
- Tungsten carbide
- Alloying additions
- Pure tungsten
- Tungsten-based chemicals
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 wearing applications. Characteristically, most of these carbides have high hardness, good electricity,thermal conductivity and high stability. These properties account for the principal applications: structures resistant to chemical reaction, uses in which wear resistance is of major importance, and high-temperature radiant-energy sources. The brittleness of carbides, however, has prevented their use as single-phase materials in highly stressed structural applications and has led to the development of metal-bonded composites (cemented carbides or cermets).
Metallic tungsten and tungsten alloy mill products account for about 16% of consumption. Tungsten and tungsten alloys dominate the market in applications for which a high-density material (19.3 g/cm3) is required, such as kinetic energy penetrators, counterweights, flywheels and governors. Other applications include radiation shields and x-ray targets. In wire form, tungsten is used extensively for lighting, electronic devices, and thermocouples.
Tungsten and tungsten alloys can be pressed and sintered into bars and subsequently fabricated into wrought bar, sheet or wire. Many tungsten products are intricated and required machining or molding and sintering to near-net shape and cannot be fabricated from standard mill products.
Tungsten Alloys. Three tungsten alloys are produced commercially: tungsten-ThO2, tungsten-molybdenum and tungsten-rhenium. The W-ThO2 alloy contains a dispersed second phase of 1% to 2% thorium. The thorium dispersion enhances thermionic electron emission, which in turn improves the starting characteristics of gas tungsten arc welding electrodes. It also increases the efficiency of electron discharge tubes and imparts creep strength to wire at temperatures above one-half the absolute melting point of tungsten.
Tungsten Heavy-Metal Alloys (WHAs). These are a category of tungsten-base materials that typically contain 90 to 98 wt% W. Most commercial WHAs are two-phase structures. The principal phase being nearly pure tungsten in associated with a binder phase containing the transition metals plus dissolved tungsten. As a consequence, WHAs derive their fundamental properties from those of the principal tungsten phase, which provides for both high density and high elastic stiffness. It is these two properties that give rise to necessary applications for this family of materials.
The current uses of WHAs are spanning a wide range of consumer, industry and government applications that include:
- Damping weights for computer disk drive heads
- Balancing weights for ailerons in commercial aircraft, helicopter rotors, and for guided missiles
- Kinetic energy penetrators for defeating heavy armor
- Fragmentation warheads
- Radiation shielding, radio isotope containers and collimalion apertures for cancer therapy devices
- High performance lead-free shot for waterfowl hunting
- Gyroscope components
- Weight distribution adjustment in sailboats and race cars
WHAs typically consist of 90 to 98 wt% W in combination with some mix of nickel, iron, copper, and/or cobalt. The bulk of WHA production falls into the 90 to 95% W range.
The choice of alloy composition is driven by several considerations. The primary factor is the density required by the given application. Further considerations include corrosion resistance, magnetic character, mechanical properties and postsinter heat treatment options.
Mechanical and Physical Properties Tungsten has high tensile strength and good creep resistance. However, its high density, poor low temperature ductility, and strong reactivity in air limit its usefulness. Maximum service temperatures for tungsten range from 1925 to 2500°C (3500 to 4500°F), but surface protection is required for use in air at these temperatures.
Wrought tungsten (as-cold worked) has high strength, strongly directional mechanical properties and some room-temperature toughness. However, recrystallization occurs rapidly above 1370°C (2500°F) and produces a grain structure that is crack sensitive at all temperatures.
COLLETED BY KAREN FROM www.chinatungsten.com
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