Nickel Alloy Inconel 718 - Properties And Applications

Overview

Inconel 718 is a high-strength, corrosion-resistant nickel-based superalloy. It possesses excellent mechanical properties, high-temperature strength, and resistance to oxidation and corrosion. Inconel 718 is commonly used in various industries, including aerospace, oil and gas, automotive, and power generation. Inconel exhibits high reflectivity to laser radiation, especially at shorter wavelengths. Reflectivity affects energy absorption and heat generation during laser processing. Higher reflectivity may require adjustments to laser parameters or the use of wavelength-specific lasers. Inconel has relatively high thermal conductivity, influencing heat dissipation and the overall energy distribution during laser treatment. It is essential to consider the heat transfer characteristics to achieve desired texture depth and minimize heat-affected zones.

Alloy 718, often referred to as Inconel 718, is precipitation hardened to give maximum strength and high creep-stress rupture strength. The alloy can be supplied in the annealed condition to enhance machinability followed by subsequent appropriate heat treatments. Capability tests are completed on test samples which have had the precipitation hardening process completed.

Alloy 718 is exceptional properties and technical specifications mean that it’s used for lots of highly technical and essential industrial applications. It is mostly chosen by design engineers thanks to its high strength in extreme temperature conditions at both ends of the spectrum. It can be used in environments from cryogenic all the way up to 1300°F/704°C. Throughout this entire range it exhibits exceptionally high yield, tensile and creep-rupture properties. It also shows excellent tensile and impact strength.

What are the Chemical Composition of Inconel 718?

Inconel 718 is primarily composed of nickel, with significant amounts of chromium, iron, niobium, molybdenum, and more minor additions of other elements, such as aluminum and titanium. These alloying elements contribute to unique properties. The alloy 718 is made up of 50-55% nickel+cobalt (with cobalt limited to 1% max) and 17-21% chromium. This combination gives the material its corrosion-resistance properties. This includes good resistance to oxidation, enabling it to withstand corrosive media present in many of its applications. The remainder of the composition includes niobium+tantalum (4.75-5.5%), molybdenum (2.8-3.3%) and titanium (0.65-1.15%), plus other balancing elements. It’s a potent and well-balanced combination of elements, responsible for the superalloy’s desirable technical properties.The table shows the composition of INCONEL 718:

Inconel 718 exhibits excellent mechanical properties, making it a highly desirable material in various industries.

Forging : Alloy 718 offers considerable resistance to deformation during forging. It should be forged in the range from 900 to 1120°C, heavy forging requiring higher temperature. In heating for forging, the material should be brought up to the temperature, allowed to soak a short time to ensure uniformity and withdrawn. In the last operation, the metal should be worked uniformly with a gradually decrasing temperature, finishing with light reduction below 955°C. Uniform reduction should be given to avoid duplex grain structure. Final reductions of 20% minimum should be used for open-die work, and 10% minimum for closed-die work. Parts should generally be air cooled after forging.

Tensile strength: Inconel 718 has a high tensile strength, typically ranging from 1034 MPa to 1380 MPa. This high tensile strength enables it to withstand heavy loads and resist deformation under mechanical stress.

High-temperature strength: Inconel 718 retains its strength and mechanical properties at elevated temperatures, making it suitable for applications that involve high temperatures and thermal stress, such as gas turbines, jet engines, and rocket motors. The γ’ precipitates, which form during the heat treatment, act as barriers to dislocation movement, effectively impeding creep deformation even at elevated temperatures. Under extreme operating conditions, this property ensures the structural integrity of critical aeroengine components, such as turbine blades and combustor liners.

Corrosion resistance: This alloy offers excellent resistance to corrosion and oxidation in various environments, including seawater, acids, and high-temperature gasses. Aero-engines encounter aggressive environments where oxidation and corrosion can severely compromise performance and durability. Nickel-based superalloys resist oxidation and corrosion thanks to the protective oxide layer that forms on their surface. The alloying elements, particularly chromium, and Aluminum, create a stable oxide scale that acts as a barrier, preventing further degradation and ensuring prolonged service life.

Fatigue Resistance: Inconel 718 has good fatigue resistance, which means it can withstand repeated stress cycles without failure. This property makes it suitable for cyclic loading applications, such as turbine disks and rotor blades. The alloying elements work in synergy to create a complex microstructure consisting of a solid solution matrix, dispersed γ’ precipitates, and carbides. The unique microstructure contributes to the superalloys’ exceptional high-temperature strength and resistance to creep deformation.

Weldability: Inconel 718 can be welded using various techniques, including gas tungsten arc welding (GTAW), plasma arc welding (PAW), and electron beam welding (EBW). However, proper welding procedures and post-weld heat treatment are required to ensure optimal properties.

Heat treatment: Inconel 718 undergoes a precipitation-hardening heat treatment to enhance its mechanical properties. This heat treatment involves solution annealing at a specific temperature followed by aging to develop the desired strength and hardness.

Applications of Inconel 718

3D printing

Inconel 718 has become associated with 3D printing technology, as this alloy works effectively in printing parts and components for a wide range of industries. Several studies have looked into the use of the superalloy in the additive manufacturing (AM) industry and further enhancements to maximise its features.

Gas turbine engine parts

Because of its efficient weldability and heat resistance, the superalloy has become an integral component of gas turbine engines.

Aerospace and aircraft manufacturing

Due to the strength and resistance of the material, Inconel 718 has become a popular component for turbojet engine parts such as compressor casings, discs and fan blades. The superalloy has also found its way into the production of rocket engines, especially with the alloy’s high heat resistance. This is the primary reason why Inconel 718 comprises about half of the components in conventional aircraft engine parts.

Die casting

This Inconel variant works well in hot environments, and so it’s a natural fit for extreme heat applications such as metal die casting. Other non-ferrous metals are more popular in this industry but Inconel 718 has more than enough toughness to protect itself from damage. As a result, dies or die holders made of Inconel 718 last longer and resist deformation better.

Oil And Gas

High temperatures, extremes of pressure, and corrosive gases can be damaging in the oil and gas industry. Therefore, materials with a high structural load require exceptional strength and corrosion resistance to be effective. With increased chromium and molybdenum content, Alloy 718 offers superior resistance to pitting and crevice corrosion, while the addition of niobium and titanium enables precipitation hardening for increased strength and durability.

Inconel 718 is widely used in a range of contexts in the oil and gas industry, including for gate valves, choke stems, high-strength fasteners, and in oil tools. Alloy 718 also plays a vital role in important downhole components, including safety valves, side-pocket inhibitors, seal assemblies, and hangers, providing reliable, corrosion-resistant, and robust solutions.