How refractory metals’ chemical properties have impacts on the materials processing operations

Refractory metals, tungsten and molybdenum, are easily influenced by their chemical environments, such as oxidation, corrosion, and volatilization. Refractory metals and their alloys are structural materials exhibiting high-temperature resistance. These metals exhibit good high-temperature performance in a protective environment (vacuum, hydrogen, and inert gases). Nevertheless, realizing this performance in an oxidizing environment is quite challenging. Refractory metals applied in the aerospace, aviation, and atomic energy industries must withstand temperatures higher than 1,000°C under a given atmosphere. However, tungsten and molybdenum exhibit poor oxidation resistance. Molybdenum will experience “disastrous” oxidation at 720°C–800°C (caused by MoO3 volatilization). Tungsten will also undergo “disastrous” oxidation due to the volatilization of WO3 above 1,000°C. Therefore, methods for overcoming the poor oxidation resistance of refractory metals exposed to oxidative environments are desired.

Coating process for refractory metals

Coating process is the best solution at present. For example, iridium can maintain low oxygen permeability and strength at 2,100°C. Consequently, this material is commonly used in ultrahigh-temperature oxidation-resistant coatings employed for high-temperature structural materials (such as tungsten and molybdenum) exposed to an oxidizing environment.

Welding process for refractory metals

In welding process, the chemical composition of the base material directly affects the metallurgical properties and consequently the weldability of the material. Therefore, selecting appropriate base material is essential for identifying a suitable welding method. Refractory metals and their alloys are active materials that inert gases should protect during welding.

Surface treatments for refractory metals

In the surface treatment process, surface cleaning can be achieved via chemical treatment such as alkali washing and acid pickling. Carbide and oxide particles are produced in refractory metals during deposition and dispersion strengthening through chemical reactions. These particles will affect tungsten and molybdenum’s plasticity, toughness, strength, and hardness. When various deep processing technologies are combined, the influence of chemical composition on the machinability of materials increases and becomes more complex than that associated with individual technologies. The solid solution strengthening of tungsten can be realized by adding Ta, Nb, Cr, V. Similarly, its softening can be achieved by adding Re. Impurity elements (such as O and C) have a significant influence on the plasticity of tungsten. Thus, high-content impurities will rapidly increase the strength and reduce the plasticity of tungsten.