What is Inconel 718 used for?
Alloy 718 (Inconel 718, UNS N07718, 2.4688) was originally developed for potential use in supercritical (ultra-high temperature) steam power plants. The alloy composition is primarily nickel-chromium based, which means it is resistant to oxidation at high temperatures. However, during the controlled solution annealing and aging process steps, the addition of Nb, Ti, Al and Ni form a series of precipitates that significantly increase the strength and hardness compared to alternative nickel alloys. In addition, these precipitates have excellent temperature stability and creep resistance, which is why it is more widely used in aerospace applications.
The dramatic increase in air travel has led to an increase in demand for more powerful and efficient turbines, requiring higher temperature operation. Applications for Alloy 718 in turbines include the more critical rotating components (shafts, plates, blades and disks). However, it is also used elsewhere in modern aircraft for wings, support structures and pressure vessels.
Less common applications are tanks, vessels, rings, and pressure vessels in rocket engines, which utilize the same temperature stability, albeit with cryogenic temperatures as low as 250 degrees Celsius.
Outside of aerospace, the largest single market for Alloy 718 is in various oil and gas applications. While high temperature performance is not as concerning, this alloy has high strength and excellent corrosion resistance in seawater. Due to its popularity in aerospace applications in the 1960s and 1970s, it is relatively widely available in a variety of product forms and sizes. Immediate target applications therefore include fasteners, valves, valve seats, valve stems, and valve trim.
As oil and gas well environments become increasingly harsh, stress corrosion and hydrogen embrittlement failures have occurred in production equipment. Alloy 718 already outperforms most alloys previously used, such as Alloy K-500. However, the composition can be improved to improve performance in specific applications. As a result, Alloy 718 is now available in a number of different specifications, including AMS5662/AMS5663 for common aerospace applications, while API 6A CRA is most commonly used in oil and gas related applications. In addition, it can be supplied in a number of different conditions, i.e. the extent of the heat treatment process (aging) can produce several different strength levels.
The GNEE group stock program is based on API 6A CRA "120ksi", which means a minimum yield strength of 125ksi. The most common size distribution is from ½" to 9" in diameter.
The range of applications in the oil and gas sector is wide. Downhole applications include packers (control well flow), hangers (carry the weight of the tubing string), and subsea valves. Drilling tools will also use Alloy 718, taking advantage of the high strength and non-magnetic properties of this nickel alloy. Wellhead "Christmas trees" will make extensive use of Alloy 718 for blocks and valves.
Despite the popularity of Alloy 718, some common completion equipment applications have gradually moved to Alloy 925. Developed with a lower nickel content, this alloy offers valuable cost savings opportunities. NACE MR0175 allows Alloy 925 to be used within similar temperature and pressure limits as Alloy 718, albeit with a slightly lower yield strength. GNEE group has a growing inventory of Alloy 925 (UNS N09925), with the most common sizes up to 9" diameter
