Alloy 718 is a precipitation-hardenable nickel-chromium alloy originally developed for supercritical steam power plants, where high-temperature strength and oxidation resistance were essential. Its composition is predominantly a nickel-chromium base, and additions of niobium, titanium and aluminium form a series of precipitates during controlled solution annealing and ageing. Those precipitates deliver a significant increase in strength and hardness compared with other nickel alloys, together with excellent temperature stability and creep resistance. That combination is why the grade moved quickly into aerospace service and now supports a much wider range of equipment.
Alloy 718 is designated UNS N07718 and Werkstoff 2.4688. The balanced chemistry keeps the alloy both strong and corrosion resistant, so a single specification can serve turbine hardware, sour-service wellhead equipment and cryogenic pressure vessels.
Nickel: 50.0-55.0 %
Chromium: 17.0-21.0 %
Niobium: 4.75-5.50 %
Molybdenum: 2.80-3.30 %
Titanium: 0.65-1.15 %
Aluminium: 0.20-0.80 %
Cobalt: 1.00 % max
Carbon: 0.08 % max
Iron: balance
Aerospace Applications of Alloy 718
Dramatic growth in air travel created demand for more powerful and more efficient turbines operating at higher temperatures. In turbine engines, Alloy 718 is used for the more critical rotating parts, including shafts, discs, blades and sheet fabrications, where yield strength, fatigue resistance and creep resistance must be retained at temperature. The same alloy appears elsewhere in modern aircraft for airfoils, supporting structures and pressure vessels.
Turbine discs, shafts, blades, seals and casings
Airframe airfoils and structural supports
Engine fasteners and high-temperature bolting
Rocket engine tanks, containers, rings and pressure vessels
In rocket hardware the grade is exploited for temperature stability at the opposite end of the scale, at cryogenic temperatures down to about -250 degC.
Oil and Gas Applications
Outside aerospace, oil and gas is the largest single market for the grade. Very high temperature performance matters less here; instead the alloy is selected for high strength combined with excellent corrosion resistance in seawater and in sour production environments. Because Alloy 718 had been produced in a wide variety of product forms and sizes for aerospace, early oilfield applications focused on fasteners, valve gates, seats, stems and trim.
As well environments became more severe, stress corrosion and hydrogen embrittlement failures appeared in production equipment. Alloy 718 already performed better than the alloys previously used in these duties, such as Alloy K-500, and further composition refinement followed for specific applications. Today the grade is supplied against several specifications, and the extent of the ageing heat treatment can be adjusted to create different strength levels.
| Component | Function in service |
|---|---|
| Packers | Controlling the flow from the well |
| Hangers | Carrying the weight of the tubing string |
| Subsea valves | Pressure control in subsea completions |
| Drill tools | Exploiting high strength and non-magnetic behaviour |
| Wellhead blocks and valves | Used extensively in wellhead assemblies |
Common stock diameters run from 1/2 in to 9 in (12.7 mm to 229 mm), and the widely distributed stock programme is based on the API 6A CRA 120 ksi minimum yield strength class. Where cost reduction is the priority, lower-nickel alternatives with slightly lower yield strength are sometimes considered for completion equipment, provided the applicable sour service specification permits that substitution at the required temperature and pressure limits.
Cryogenic Service and Other Markets
A less common but important group of applications covers tanks, containers, rings and pressure vessels in rocket engines, which rely on the same temperature stability at cryogenic temperatures. Beyond aerospace and oil and gas, the alloy is used for high-strength fasteners, springs, instrument components and marine hardware where strength and corrosion resistance must be combined in one material.
Specifications, Conditions and Product Forms
| Specification | Typical scope |
|---|---|
| AMS 5662 / AMS 5663 | Common aerospace bar, forgings and rings |
| API 6A CRA | Oil and gas wellhead and downhole equipment |
| NACE MR0175 / ISO 15156 | Sour service qualification |
| ASTM B637 / ASME SB637 | Bar, forgings and rings |
| DIN 2.4688 / EN designation | European material identification |
Product forms include round bar, flat bar, plate, sheet, strip, seamless and welded tube, wire, forgings, hexagon bar and finished fasteners. Solution annealed and aged conditions are the most common supply condition, and the ageing treatment is selected to match the required strength and ductility balance for the application.
Frequently Asked Questions
Q: What is Alloy 718 used for?
It is used for turbine discs, shafts and blades, airframe structures, oil and gas wellhead and downhole components, high-strength fasteners and cryogenic pressure vessels.
Q: Is Alloy 718 the same as UNS N07718?
Yes. Alloy 718 is designated UNS N07718 and Werkstoff 2.4688, and these identifiers appear on the material certificates issued for the grade.
Q: Why is Alloy 718 used in downhole equipment?
Its combination of high strength, seawater corrosion resistance and non-magnetic behaviour suits packers, hangers, subsea valves, drill tools and wellhead hardware.
Q: Can Alloy 718 be used at cryogenic temperatures?
Yes. The alloy retains useful toughness and phase stability at cryogenic temperatures down to about -250 degC, which is why rocket engine vessels use it.
Q: What strength condition is supplied?
The grade is normally supplied solution annealed and aged, with the ageing treatment adjusted to deliver several strength levels; the API 6A CRA 120 ksi class is common in oil and gas.
Q: Does Alloy 718 meet sour service requirements?
It is qualified under NACE MR0175 / ISO 15156 for defined temperature, pressure and hardness limits, so the permissible envelope depends on the specified condition.
