The refractory metals, which include tungsten, molybdenum, tantalum, and niobium, have exceptionally high melting temperatures (above 1925 °C) and, consequently, have the potential for high temperature service. Applications include filaments for lightbulbs, rocket nozzles, nuclear power generators, tantalum- and niobium-based electronic capacitors, and chemical processing equipment. The metals, however, have a high density, limiting their specific strengths.
Definition according to DIN EN 10052: „Annealing carried out in a medium that allows the original metallic surface finish to be maintained by preventing oxidation of the metal.“
Annealing is a „heat treatment consisting of heating and soaking at a suitable temperature followed by cooling under conditions such that, after return to ambient temperature, the metal will be in a structural state closer to that of equilibrium.“
A medium is an „environment in which the product is placed during a heat treatment operation. The medium can be solid, liquid or gaseous. “
The protective gas consists of nitrogen (argon) and hydrogen. The maximum concentration of the hydrogen is approx. 5 %. We use the brand name Neutrotherm.
The hydrogen content in nitrogen (argon)-hydrogen mixture exceeds 5 %. We use the brand name Hydrotherm.
Classical refractory metals are molybdenum, tungsten, niobium, and tantalum. Mo and W form carbides and volatile oxides. Both hydrogen and nitrogen may be considered inert to pure Mo and W, but internal nitriding can occur in alloys containing Ti, Zr, and Hf.
Because Nb and Ta absorb oxygen, nitrogen, carbon and hydrogen from the atmosphere, annealing takes place under vacuum or argon with very low dew points.
Definition according to ISO 3252: Sintering is a ”thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.”
Sintering of molybdenum and tungsten takes place under pure hydrogen.
Tantalum and niobium are processed in vacuum to remove oxygen, in argon or helium.
BRAZING AND HIGH-TEMPERATURE BRAZING
Brazing is a joining process wherein metals are bonded together using a filler metal with a melting temperature greater than 450 °C, but lower than the melting temperature of the base metal.
High-temperature brazing is flux-free brazing under exclusion of air (vacuum, protective gas) with filler metals whose melting temperature is above 900 °C.
Purified dry hydrogen and inert gas (helium, argon) atmospheres are suitable for brazing molybdenum. For brazing pure molybdenum, the purity of the hydrogen atmosphere is not critical. A low dew point is required for brazing of titanium-bearing molybdenum alloys, like TZM.
Tungsten can be brazed in an inert gas atmosphere (helium, argon), in hydrogen, or vacuum.
Vacuum brazing is preferred for brazing niobium, and tantalum. High-purity helium or argon can also be used.