Molybdenum and tungsten manufacturer-Zhengzhou Sanhui Co., Ltd has been dealing with refractory metals for 25 years.

In all high-temperature applications, the behaviour of the refractory metals in a gas atmosphere and in contact with other furnace materials must be taken into consideration.

Furnaces with molybdenum and/or tungsten parts are usually operated with hydrogen as a protective atmosphere. Hydrogen offers the advantage of reducing any existing oxides of molybdenum or tungsten. The humidity of the hydrogen should be lower than 5g H2O/m3. Above this level, molybdenum and tungsten begin to oxidize by “water-cycling”, whereby oxides form, evaporate and condense on cooler furnace areas where they are reduced to metal by hydrogen.

Condensed water, i.e. in warm and humid weather, may also cause water-cycling and, in consequence, unwanted dendritic growth on furnace parts. Therefore, water should always be removed by vacuum or other means before the next heating cycle.

Sometimes oxidation and reduction may also be caused by impurities of the material to be heat-treated.

High-temperature furnaces containing molybdenum and tungsten parts may also be operated in high vacuum or with argon.

Whenever tantalum is used for high-temperature applications its behaviour to the gas atmosphere has to be considered. Even in high vacuum, undesirable reactions between tantalum and traces of evolved gases may occur. These can result from the outgassing of the material to be heat-treated, leakages, improper operation of the vacuum equipment, etc..