Rhenium is classified as a scarce refractory metal. It has the second-highest melting point of any of the metals and has a relatively high density.
Rhenium is produced as a powder from molybdenite, which also contains rhenium sulfide. The rhenium powder is consolidated by pressing and resistance sintering in a vacuum or hydrogen atmosphere. Rhenium bars produced in this way have excellent ductility at room temperature and can be rolled or cold-worked. However, the metalwork hardens rapidly and must be annealed after relatively small reductions.
Rhenium has a higher tensile strength at high temperatures than any of the other metals. For example, it is reported to have a strength of 49, 000 psi at 3000 F. The metal is rapidly oxidized in air at elevated temperatures, so when it is exposed to elevated temperatures it must be contained under vacuum or a protective atmosphere.
Because of the high price and limited supply of rhenium, it is employed in only a few very special applications that make use of its unusual properties. When used in electrical contacts, rhenium has even greater arcing and sparking resistance than tungsten. Under certain conditions, rhenium filaments have less tendency than tungsten filaments to plate out on the cool glass walls of electronic tubes.
This is a complex reaction involving traces of water vapor in the evacuated tube and is called the “water cycle” reaction. The addition of 40 to 50 percent rhenium to molybdenum alloys renders them ductile after heating to temperatures above the recrystallization temperature. Rhenium has a similar effect in several other alloys of refractory metals.
Rhenium is useful as an alloying element in thermocouple alloys of platinum or platinum-group metals. It is also useful as a thermocouple element with molybdenum or tungsten to measure temperatures above the range of the platinum thermocouples.
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