The Use of Tungsten in Vacuum Furnace Applications

time2018/08/22

The Use of Tungsten in Vacuum Furnace Applications

Tungsten is used in vacuum furnaces when there is a need for structural integrity at elevated temperature and/or in situations where other materials may degrade, such as when lower melting point eutectics are a concern. One example of its use in is roller rail assemblies in which graphite wheels are positioned between molybdenum rails using tungsten axles

Tungsten (chemical symbol W) is a member of the family of refractory metal (Mo, Nb, Re, Ta, W) and has the highest melting point and vapor pressure of this group. Due to this unique property, it is commonly used as a material of construction in specific areas of vacuum furnace hot zones operating above 1315ºC

Tungsten can also be used for heating elements given that it has the highest duty temperature, typically 2800°C . In practice, this rating is often downgraded as it is for all heating element material choices. Tungsten will become brittle, however, if exposed to oxygen or water vapor and is sensitive to changes in emissivity. In general, tungsten is resistant to corrosion below 60% relative humidity.

Depending on the design of the hot zone (cylindrical or rectangular), heating elements can be placed circumferentially in a 360° pattern, on just the two (2) sidewalls, on the top and bottom as well as the sidewalls or with the use of end elements, on all six (6) sides

The use of tungsten in hot zone construction for vacuum furnaces is due to the wide range of properties that it exhibits, namely:

· High melting point, 3420ºC (6187ºF)

· Low vapor pressure

· High hot strength

· Low thermal expansion

· High thermal conductivity

· High Young’s modulus

· High corrosion resistance against acids and molten metals

· Recrystallization temperature, between 1100º – 1400ºC (2012º – 2550ºF)

· Good electrical conductivity

· High modulus of elasticity