Sometimes the standard grades of metal just aren’t enough to handle some of the harshest environments. When manufacturers are faced with the challenges posed by excessive temperatures, severe corrosion, or other difficult obstacles, they must turn to special high-performance alloys meant to handle these extremes.
One example of such a family of alloys is known under the trademark Inconel. These nickel-based alloys have a long history and track record of success in commercial applications. They offer manufacturers and designers an effective option when dealing with high temperature demands.
What is Inconel and how was it developed?
The Inconel family of alloys comprises a series of austenitic nickel-chromium-based superalloys. Inconel was originally developed by Wiggin Alloys in the 1940’s, but the company was later acquired by Special Metals Corporation, and this is usually the company we associate with the Inconel name.
This group of alloys is known for its oxidation corrosion resistance and for being able to withstand extreme heat and pressure. It has several very popular forms, all primarily nickel in their chemistry, including Inconel 600, 625, 718 and X-750. Each of the various alloys also can be found marketed under a variety of trade names, depending on the mill of manufacture. For instance, Inconel 718 can be purchased as Nicrofer 5219, Superimphy 718, Haynes 718, Pyromet 718, Supermet 718 or Udimet 718.
Inconel was originally developed in response to the difficulties more common metals have in extreme environments, including severe pressure, heat, and corrosion. The aerospace ambitions of the belligerent parties and other military applications surrounding World War II were a driving force in its creation. Jet and rocket propulsion systems were in their infancy and created new metallurgical challenges for the industry.
What gives nickel its ability to withstand high temperatures?
The primary characteristic associated with Inconel is its excellent resistance to high temperatures and corrosion. This is a result of the thick passivation oxide layer that forms on these metals when they are exposed to extremely high temperatures. As the metal responds, the layer that forms is stable in nature, helping to protect the surface from any further damage. This allows Inconel alloys to retain their strength at high temperatures in relation to other grades.
As such, Inconel is one of the default choices for high temp applications that would weaken other metals. Furthermore, the heat resistance of Inconel can be enhanced through solid solution strengthening or precipitation hardening (different alloys respond better to one or the other).
What other properties is Inconel known for?
In addition to high temperature resistance, Inconel offers several other beneficial properties thanks to its nickel and chromium chemistry. For example, Inconel alloys are typically austenitic in nature, meaning they are non-magnetic.
Another important trait of Inconel alloys is they offer excellent corrosion resistance, even in marine environments. Inconel 718 provides superior resistance to corrosion where lesser metals would start to corrode upon exposure to high temperatures, this alloy continues to offer protection. This is an important consideration for applications such as jet engines or gas turbines.
It should come as no surprise that Inconel alloys are difficult to machine using conventional methods. This is because these metals experience rapid work hardening.
The result is that these alloys must be machined in a very aggressive manner, using a hard but slow cut to reduce the number of passes required to the absolute minimum. It is also recommended that Inconel is machined in its solution annealed form. This means most of the machining will take place before any age hardening occurs.
There are other options for fabricating nickel based grades. 718, a popular fastener grade, can be roll-threaded by employing induction heat at 1300°F after full aging has taken place. The special properties of Inconel mean that even at such temperatures, there will be no increase in grain size.
Waterjet fabrication is an effective way to process Inconel plate. A more recent option is to use a whisker-reinforced ceramic cutter, which can cut away material up to eight times faster than traditional carbide cutters. Another possibility is selective laser melting. Grinding is also frequently used to process nickel based alloys.
What applications is Inconel well suited for?
Inconel is most often deployed for use in extreme environments. Common applications include gas turbines, turbochargers, electric submersibles, high temperature fasteners, chemical processing, high pressure vessels, heat exchangers, steam generators, nuclear reactors, natural gas processing, firearms and performance automotive.
As an example, Inconel studs were used in the Space Shuttle to hold the solid rocket boosters in place and support the whole weight of the Shuttle as it was prepared for takeoff.
The X-15 rocket-powered aircraft from North American Aviation was built in the 1960’s with an outer skin entirely built from Inconel. Space X also uses Inconel in many of its rocket components, including in the engine manifold used in the Falcon 9. In the automotive industry, both Tesla and Ford use Inconel alloys in their eco-friendly engines.
