Alloy 718 (Inconel 718, UNS N07718, 2.4688) was originally developed for potential application in supercritical (very high temperature) steam power plants. The alloy composition is predominantly a nickel-chrome base meaning it is resistant to oxidation at elevated temperatures. However, the addition of Nb, Ti, Al and Ni form a series of precipitates during the controlled solution annealing and ageing process steps, resulting in a significant increase in strength and hardness compared with alternative nickel alloys. In addition, these precipitates provide excellent temperature stability and creep resistance, which is why it became more widely used in aerospace applications.
The dramatic increase in air travel led to an increase in demand for more powerful and more efficient turbines, demanding higher temperature operation. Applications of Alloy 718 in turbines includes the more critical rotating parts (shafts, sheets, blades and discs). However, it is used elsewhere in modern planes for 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 of down to 250degC.
Outside of aerospace, the largest single market for Alloy 718 is in assorted Oil & Gas applications. Although very high temperature performance is less of a concern, this alloy provides high strength and excellent corrosion resistance in seawater. Due to its popularity in aerospace applications throughout the 1960’s and 1970’s, it was relatively widely available in a variety of product forms and sizes. Therefore, immediate 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 in production equipment were experienced. The performance of Alloy 718 was already better than most of the alloys previously used such as Alloy K-500. However, it was possible to refine the composition in order to improve the performance in specific applications. For this reason, Alloy 718 is now available against a number of different specifications including AMS5662/AMS5663 for common aerospace applications, whilst API 6A CRA is most common for Oil & Gas related applications. In addition, it can be supplied in a number of different conditions i.e. the extent of the heat treatment process (ageing) can create several different strength levels.
Langley Alloys stock programme is based on API 6A CRA ‘120ksi’, which translates to a minimum yield strength of 125ksi. The most common sizes distributed are from ½” to 9” diameter.
The range of applications in Oil & Gas is extensive. Down-hole applications includes packers (controlling the flow from the well), hangers (carrying the weight of the tubing string) and sub-sea valves. Drill tools will also make use of Alloy 718, exploiting the elevated strength and non-magnetic nature of this nickel alloy. Well-head ‘Christmas trees’ will make extensive use of Alloy 718 for the blocks and valves.
Although Alloy 718 is extremely popular, there has been a very gradual shift towards Alloy 925 for some of its common applications around completion equipment. Developed around a lower nickel content it provides a valuable cost-saving opportunity. NACE MR0175 allows Alloy 925 to be used under similar temperature and pressure limits as Alloy 718, even though its yield strength is slightly lower. Langley Alloys carries an increasing stock of Alloy 925 (UNS N09925) in the most popular sizes up to 9” diameter,
Inconel is a trademark belonging to Special Metal Inc and applied to a family of nickel – chrome alloys including Alloy 718.Such is the recognition of the trademark that it is generically used to describe such alloys even if produced by another manufacturer.