Tungsten - Tungsten alloys

Tungsten is produced by means of the powder metallurgical process with a purity of at least 99,95%. Wolfram, chemically pure, possesses outstanding properties, such as:

  • A extremely high melting point (3420°C) - the highest of all metals – with a low vapor pressure (at 2000°C < 4.10-s [Pa])
  • A high elastic modulus
  • Good thermal stability with a low thermal expansion coefficient.
  • High density of 19.3 g/cm³ (at 20°C)
  • Good chemical resistance against inorganic acids, alkaline solutions, organic acids, non-metals, glass without oxidization agents, molten glass and gases
  • Its corrosion behavior with respect to molten metals is consistently good

In the aforementioned physical and chemical values, tungsten displays properties typical of refractory metals. These are all metals with a melting point higher than that of platinum (1772°C). They are deployed up to around 2900°C (3173K).
These excellent properties found in tungsten are also shared by molybdenum. However, some properties are even more pronounced in tungsten than in molybdenum.

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Tungsten is mainly used as a basic material in numerous tungsten alloys, e.g. lanthanum oxide-doped tungsten, tungsten heavy alloy, tungsten–copper (EROMET). It has fewer uses in its pure form, e.g. crucibles in the coating industry, heat shielding and heating elements.

Processing
Tungsten is difficult to process, displaying great resistance to any kind of forming and machining (high level of wear on tools). Tungsten and its alloys are easier to process when the brittle-ductile transition temperature is taken into account. At room temperature its ductility is very low.

Just small amounts of alloy additives lower the brittle-ductile transition temperature. All tungsten alloys are therefore easier to process than pure tungsten.

The better processing temperatures are well over the brittle-ductile transition temperature, which is 200°C for a sheet 1 mm thick and 380°C for a sheet 7 mm thick. Turning, milling with tungsten-carbide, drilling with HSS and grinding with silicon carbide are all possible. When punching, cutting and bending, attention must be paid to the sheet thickness and the processing temperature. The material should be bent at right angles to the direction of rolling (microstructure lengthways). If this is not structurally possible, the processing temperature must be increased considerably. Bending radius ≥ sheet thickness should be adhered to.
Guideline values for bending temperature
  • thickness 1 mm 300 - 400°C
  • thickness 4 mm 600 - 700°C
  • thickness 7 mm 650 - 800°C
Guideline values for punching and cutting temperatures
  • thickness 1 mm 400 - 500°C
  • thickness 4 mm 850 - 950°C
  • thickness 7 mm 950 - 1000°C
The material can also be riveted, bolted, soldered and welded if certain rules are adhered to. Riveting and bolting should be preferred to soldering and especially to welding. The recrystallization temperature has a major effect on the mechanical properties of all refractory metals and alloys in the high temperature range. Above the recrystallization temperature there is a change in the microstructure that leads to a fall in major strength values and a higher susceptibility to breaking. It is therefore always desirable to have the highest possible recrystallization temperature. A high level of deformation together with targeted alloying and the inclusion of additives significantly increase the recrystallization temperature. At the same time, the mechanical properties are optimized with respect to the intended application. Machinability is considerably improved.