selection of a wear-resistant material.
Abrasive wear is encountered when hard particles, or hard projections (on a counter-face) are forced against, and moved relative to a surface. The terms high and low stress abrasion relate to the condition of the abrasive medium (be it hard particles or projections) 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. Sliding Wear. 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-Base High-Temperature Alloys For many years, the predominant user of 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. 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.
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