Alloy 718 (UNS N07718, 2.4668) is used across aerospace and oil and gas for its combination of high strength, temperature stability and corrosion resistance. It was originally developed for potential application in supercritical (very high temperature) steam power plants.
The alloy is predominantly a nickel-chrome base, so it resists oxidation at elevated temperatures. Additions of Nb, Ti, Al and Ni form a series of precipitates during the controlled solution annealing and ageing steps, giving a significant increase in strength and hardness over alternative nickel alloys. These precipitates also provide excellent temperature stability and creep resistance, which is why the alloy became more widely used in aerospace.
Aerospace
The dramatic rise in air travel drove demand for more powerful, more efficient turbines running at higher temperatures. In turbines, Alloy 718 is used for the more critical rotating parts: shafts, sheets, blades and discs. It is also used elsewhere in modern aircraft, in airfoils, supporting structures and pressure vessels.
A less common application is in tanks, containers, rings and pressure vessels in rocket engines, which exploit the same temperature stability, albeit at cryogenic temperatures down to 250degC.
Oil and gas
Outside aerospace, the largest single market for Alloy 718 is assorted oil and gas applications. Very high temperature performance matters less here, but the alloy provides high strength and excellent corrosion resistance in seawater. Its popularity in 1960s and 1970s aerospace meant it was already widely available in a range of product forms and sizes, so early target applications included fasteners, valve gates, seats, stems and trim.
As oil and gas well environments became more severe, stress corrosion and hydrogen embrittlement failures appeared in production equipment. Alloy 718 already outperformed most of the alloys previously used, such as Alloy K-500, and its composition could be refined to improve performance in specific applications. It is now available against several specifications, including AMS5662/AMS5663 for common aerospace use and API 6A CRA for oil and gas, and it can be supplied in a number of conditions (the extent of ageing) to reach several different strength levels.
Our stock programme is based on API 6A CRA '120ksi', which translates to a minimum yield strength of 125ksi. The most common sizes we supply are from ½" to 9" diameter.
Oil and gas applications in detail
The range of oil and gas applications is extensive. Down-hole uses include packers (controlling the flow from the well), hangers (carrying the weight of the tubing string) and sub-sea valves. Drill tools use Alloy 718 for its elevated strength and non-magnetic nature, and wellhead Christmas trees make extensive use of it for blocks and valves.
The shift towards Alloy 925
Although Alloy 718 remains extremely popular, there has been a very gradual shift towards Alloy 925 for some completion-equipment applications. Its lower nickel content offers a valuable cost saving, and NACE MR0175 allows Alloy 925 to be used within similar temperature and pressure limits as Alloy 718, even though its yield strength is slightly lower. We carry an increasing stock of Alloy 925 (UNS N09925) in the most popular sizes up to 9" diameter.