About Cobalt Alloys
Aerodyne Alloys is an international supplier of Cobalt Alloys. We have sales staff and processing staff experienced with the sales, distribution and processing of Cobalt alloys. The following information provides an overview of cobalt alloys. To place a cobalt order, review our Cobalt Specifications and contact Aerodyne Alloys to order by phone.
Cobalt alloys are useful in applications that require:
- Magnetic properties
- Corrosion resistance
- Wear resistance
- Strength at elevated temperatures
Many of the properties of the cobalt alloys (wear resistant, corrosion resistant, and heat resistant--strong even at high temperatures) arise from the crystallographic nature of cobalt, in particular its response to stress, the solid-solution-strengthening effects of chromium, tungsten, and molybdenum, the formation of metal carbides, and the corrosion resistance imparted by chromium.
Generally the softer and tougher compositions are used for high-temperature applications such as gas-turbine vanes and buckets. The harder grades are used for resistance to wear.
Cobalt-Base Wear-Resistant Alloys
The cobalt-base wear alloys of today are little changed from the early alloys of Elwood Haynes. The most important differences relate to the control of carbon and silicon (which were imparities in the early alloys). The main differences in the current Stellite alloy grades are carbon and tungsten contents (hence the amount and type of carbide formation in the microstructure during solidification). Carbon content influences hardness, ductility, and resistance to abrasive wear. Tungsten also plays an important role in these properties.
Chemical composition of Stellite alloys is approximately:
- Cr ~ 25-30%
- Mo = 1% max
- W = 2-15%
- C ~ 0.25-3.3%
- Fe = 3% max
- Ni = 3% max
- Si = 2% max
- Mn = 1% max.
- Co = rest of balance
Types of Cobalt Wear
The type of wear encountered in a particular cobalt application is an important factor that influences the selection of a wear-resistant material. There are several distinct types of wear which generally fall into three main categories:Abrasive wear, Sliding wear, Erosive wear.
Abrasive wear is encountered when hard particles or hard projections are forced against, and moved relative to a surface. The terms high and low stress abrasion relate to the condition of the abrasive medium after interaction with the surface.
If the abrasive medium is crushed, then the high stress condition is said to prevail. If the abrasive medium remains intact, the process is described as low stress abrasion. Typically, high stress abrasion results from the entrapment of hard particles between metallic surfaces (in relative motion), while low stress abrasion is encountered when moving surfaces come into contact with packed abrasives, such as soil and sand. In alloys such as the cobalt-base wear alloys, which contain a hard phase, the abrasion resistance generally increases as the volume fraction of the hard phase increases.
Abrasion resistance is, however, strongly influenced by the size and shape of the hard phase precipitates within the microstructure and the size and shape of the abrading species.
Of the three major types of wear, sliding is perhaps the most complex, not in concept, but in the way different materials respond to sliding conditions. Sliding wear is a possibility whenever two surfaces are forced together and moved relative to one another. The chances of damage are increased markedly if the two surfaces are metallic in nature, and if there is little or no lubrication present.
Cobalt-Based High-Temperature Alloys
For many years, the predominant user of specialty high-temperature alloys was the gas turbine industry. In the case of aircraft gas turbines, the chief material requirements were elevated-temperature strength, resistance to thermal fatigue, and oxidation resistance. For land-base gas turbines, which typically burn lower grade fuels and operate at lower temperatures, sulfidation resistance was the major concern. Today, the use of high-temperature alloys is more diversified, as more efficiency is sought from the burning of fossil fuels and waste, and as new chemical processing techniques are developed.
In general, cobalt-base high-temperature alloys have the following chemical composition:
- Cr = 20-23%
- W = 7-15%
- Ni = 10-22%
- Fe = 3% max
- C = 0.1-0.6%
- Co = rest of balance.
Although cobalt-base alloys are not as widely used as nickel and nickel-iron alloys in high-temperature applications, cobalt-base high-temperature alloys nevertheless play an important role, by virtue of their excellent resistance to sulfidation and their strength at temperatures exceeding those at which the gamma-prime- and gamma-double-prime-precipitates in the nickel and nickel-iron alloys dissolve. Cobalt is also used as an alloying element in many nickel-base high-temperature alloys.
Cobalt-Base Corrosion-Resistant Alloys
Although the cobalt-base wear-resistant alloys possess some resistance to aqueous corrosion, they are limited by grain boundary carbide precipitation, the lack of vital alloying elements in the matrix (after formation of the carbides or Laves precipitates) and, in the case of the cast and weld overlay materials, by chemical segregation in the microstructure. By virtue of their homogeneous microstructures and lower carbon contents, the wrought cobalt-base high-temperature alloys (which typically contain tungsten rather than molybdenum) are even more resistant to aqueous corrosion, but still fall well short of the nickel-chromium-molybdenum alloys in corrosion performance.
To satisfy the industrial need for alloys which exhibit outstanding resistance to aqueous corrosion, yet share the attributes of cobalt as an alloy base (resistance to various forms of wear, and high strength over a wide range of temperatures), several low-carbon, wrought cobalt-nickel-chromium-molybdenum alloys are produced.
Chemical composition of these alloys is:
- Cr = 20-25%
- W = 2%
- Mo = 5-10%
- Ni = 9-35%
- Fe = 3% max
- C = 0.8% max
- N = 0.1% max
- Co = rest of balance.
Sources of Cobalt
Cobalt is not a rare metal and ranks 33rd in abundance. It exists in a widly scattered area around the world but is currently being produced from 17 countries.
Cobalt is also present in the deep-sea nodules which occur in the Mid-Pacific and are estimated to contain anywhere from 2.5-10 million tonnes of cobalt. At a world production level of 42,000 tonnes, this is 60 to 230 years of usage. Current land sources are estimated at over 100 years, so no long-term shortage is in sight.
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