Tungsten, tantalum, molybdenum, columbium, vanadium, and chromium may be classed as relatively abundant refractory metals; that is, free world reserves of contained metal are over 100,000 tons for each metal.
The first four show promise in a considerably higher temperature range than the last two, and sometimes the term “refractory metals” is used narrowly to mean only these four elements and their alloys. At the present time, these four metals are the only metals that are reasonably abundant whose alloys show promise of high strength above 2000 F.
It is these elements that are being used or studied for such ultrahigh temperature applications as rocket nozzles, leading edges and “hot structures” of certain reentry vehicles, special electronic components, advanced gas turbines, and ramjet engines.
Tungsten, molybdenum, vanadium, chromium, and to a lesser extent columbium have been used for years as alloying elements in other metals, especially iron and steel.
In addition, tungsten and molybdenum have been used in the lamp and electron-tube industry for more than 50 years. Commercially pure tantalum has been used for many years, in modest quantities, as a structural metal in the chemical industry and elsewhere for its corrosion resistance.
It is only since World War II, however, that these metals have been seriously considered for use as elevated-temperature structural metals in their own right. At the present time, a sizable fraction of their total production is for structural uses.
Within the past decade, it has become evident that structural materials having higher service temperatures than nickel- and cobalt-base alloys are desirable to achieve higher power and efficiency for turbojet, ramjet, and rocket engines. Consequently, a number of extensive research and development programs have been initiated to evaluate the abundant refractory metals for critical high-temperature applications.
Development work has been difficult with these metals because of their low-temperature brittleness (except for tantalum and columbium), their tendency to oxidize at elevated temperatures, problems in achieving increased purity, problems in forging and welding, etc.
Nevertheless, new techniques for melting, purifying, consolidating, coating, fabricating, and welding have been developed which have overcome many of the original problems. At the present time, several of the abundant refractory metals and alloys are being used as structural materials in a number of high-temperature applications.
Characteristics of the abundant refractory metals and some of their current applications are discussed in the following sections. However, these characteristics are, to a large extent, dependent on the purity, processing variables, section size, etc. As research studies continue on the abundant refractory metals, it will be possible to more accurately define their potential characteristics and properties for high-temperature structural applications.
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