Molybdenum, tantalum, tungsten and zirconium are all classified as refractory metals due to their high temperature melting points and wear resistance. Though all can handle high temperatures, refractory metals should be ONLY used in a vacuum or inert atmosphere when exceeding 500°C as they begin to react, sometimes quickly, with oxygen. By wider definition, Titanium is also considered a refractory.

Molybdenum
Molybdenum bolts offer high temperature stability combined with low coefficient of thermal expansion – so they stay strong without expanding or loosing shape even in extreme heat over repeated cycles. Molybdenum fasteners can handle temperatures up to 2900°F/1600°C in a vacuum environment.

Tantalum
Tantalum fasteners are best known for their corrosion resistance. Tantalum bolts out-perform Hastelloy and zirconium fasteners in the harshest environments like with elevated concentrations and high temperatures of hydrochloric and sulfuric acid. Tantalum is also used for medical devices as it is inert and very compatible with the human body. In addition, it is 50% denser than lead and is radio-opaque. For medical devices this can be important as the tantalum fasteners easily show up on x-rays. Along with these unique characteristics, tantalum fasteners are excellent in high temperature environments as well. In semi-conductor processing tantalum is used in high temperature furnaces as it will not contaminate sensitive electronics.

Titanium
Titanium screws are best known for their unparalleled strength to weight ratio as well as their saltwater and chloride corrosion resistance. Titanium grade 2 is primarily used for marine projects and applications that require resistance to chlorides such as pulp and paper bleaching and water treatment. Titanium grade 5 is 4 times stronger than 316 stainless steel at nearly half the weight. It is used often for high strength, salt water/chloride resistance, and medical applications.

Tungsten
Tungsten’s extreme high density of 19.3 gm/cc (70% denser than lead), as well as its radiopaque attributes provide unique capabilities for weight distribution and balancing, medical devices, as well as vacuum furnace applications.

Zirconium
Though not as resistant as tantalum, Zirconium offers excellent corrosion protection exceeding even Hastelloy in hydrochloric acid. Zirconium fasteners provide excellent corrosion resistance in many acids but will rapidly corrode when exposed to oxidizing media.