Super alloys are nickel, iron-nickel, and cobalt based alloys which are generally used at temperatures above around 1000°F(540°C). The iron-nickel-based superalloys such as Alloy 718 are an extension of stainless steel technology and generally are wrought. Cobalt-based and nickel-based superalloys may be wrought or cast, depending on the application or composition involved.

A large number of super alloys have been invented and studied, many have been patented. However, many of them have been winnowed down over the years; only a few are extensively used.

Appropriate compositions of superalloys can be forged, rolled to sheet, or otherwise produced in a variety of forms. The more highly alloyed compositions normally are processed as castings. Fabricated structures can be built up by welding or brazing, but many highly alloyed compositions containing a large amount of hardening phase are difficult to weld. Properties can be controlled by adjusting composition and by processing (including heat treatment), and excellent elevated-temperature strengths are available in finished products.

Characteristic of super alloys

• When temperatures go above about 1000°F (540°C), like said at the beginning, ordinary steels and titanium alloys are no longer strong enough for application. Steels also may suffer from enhanced corrosion attack.

• When the highest temperatures (below the melting temperatures, which are about 2200 to 2500°F (1204 to 1371°C) for most
alloys) must be achieved and strength is the consideration, then nickel-based superalloys are the materials of choice.

• Nickel-based superalloys can be used to a higher fraction of their melting points than just about any other commercially available
materials. Refractory metals have higher melting points than superalloys but do not have the same desirable characteristics as superalloys and are much less widely used.

• Cobalt-base superalloys may be used in stead of nickel-based superalloys, dependent on actual strength needs and the type of corrosive attack expected.

• At lower temperatures, and dependent on the type of strength needs for an application, iron-nickel-base superalloys find more use than cobalt or nickel based superalloys.

• Most wrought superalloys have fairly high levels of chromium content to provide corrosion resistance. In the cast alloys, chromium was high to start but was significantly reduced over the years in order to accommodate other alloy elements that increased the elevated temperature strength of superalloys. In the superalloys based on nickel, the aluminum content of the alloys increased as chromium decreased. Thus, the oxidation resistance of nickel superalloys remained similar to original levels or even increased. However, resistance to other types of corrosion attack decreased.

• Superalloy strength properties are directly related not only to the chemistry of the alloy but also to melting procedures, forging and working processes, casting techniques, and, above all, to heat treatment following forming, forging or casting.

• Superalloys have great oxidation resistance, in many instances, but not enough corrosion resistance. For many applications at the highest temperatures, above about 1400°F (760°C), as in aircraft turbines, superalloys must be coated. For very long-time applications at temperatures at or above about 1200°F (649°C), as in land-based gas turbines, superalloys may have to be coated.

Applications

The high-temperature applications of superalloys are extensive, including components for aircraft, chemical plant equipment, and petrochemical equipment.

Aircraft/industrial gas turbine components:
Disks, Bolts, Shafts, Cases, Blades, Vanes, Combustors, Afterburners, Thrust reversers

Steam turbine power plant components:
Bolts, Blades, Stack-gas reheaters

Selected automotive components:
Turbochargers, Exhaust valves

Metal processing:
Hot work tools and dies, Casting dies

Medical components:
Dentistry, Prosthetic devices

Space vehicle components:
Aerodynamically heated skins, Rocket-engine parts

Heat treating equipment:
Trays, Fixtures, Conveyor belts

Nuclear power systems:
Control-rod drive mechanisms, Valve stems, Springs, Ducting

Chemical and petrochemical industries:
Bolts, Valves, Reaction vessels, Piping, Pumps