What is Inconel 718?

Related: Nickel Alloys

Inconel 718 was originally developed for steam power plants, but became more widely used in aerospace for its resistance to oxidation at elevated temperatures. Its ready availability, combined with high strength and excellent corrosion resistance in seawater, later made it a strong candidate for oil and gas applications. It is the most widely specified grade in its family, and we stock it as solid bar for demanding oil and gas and aerospace applications.

Nickel-chromium alloys are among the most widely used nickel-based alloys. They form a protective oxide film that prevents further oxidation, and they retain their mechanical properties across a wide temperature range. They are often known by the Special Metals Corporation tradename Inconel, or the Haynes International tradename Hastelloy^®. There are at least fifteen commercially available nickel-chromium grades, with scope for further development to support new applications.

How is Inconel 718's strength achieved?

Additions of niobium (Nb), titanium (Ti) and aluminium (Al) form a series of precipitates with nickel during controlled solution annealing and ageing, giving a significant increase in strength and hardness compared with alternative nickel alloys. This treatment is variously called ageing, age hardening or precipitation strengthening. The aim is to form very fine precipitates, widely dispersed through the bar. These "pin" microstructural features such as grain boundaries and restrict them from moving under load. The effect is achieved by combining a carefully selected composition with multi-stage heat treatments, and the alloy is available in a number of increasing strength levels depending on the extent of heat treatment.

Thermal properties

Inconel 718 retains much of its mechanical strength up to 650°C (1200°F), and resists oxidation at far higher temperatures. Its melting point is in the range 1260-1340°C (2300-2440°F). Nickel alloys such as Inconel 718 suit high-temperature service because most other metals become brittle or oxidise when exposed to heat. Nickel-based alloys quickly form a protective oxide layer when heated in an oxygen-containing atmosphere, preventing further oxidation, and they have a relatively high melting point - only a handful of more exotic alloys outperform them at high temperatures.

Fabrication

Nickel-based alloys such as Inconel 718 work harden during machining, drilling and fabrication. As it is already a high-strength alloy in the aged condition, careful selection of tooling, coolant and CNC machine settings is essential. It can be readily welded, usually with a matched filler metal when welded to itself. Alloy 718 machines readily in either the annealed or age-hardened condition: the age-hardened condition gives a better surface finish, while the annealed condition gives longer tool life.

Where is Alloy 718 used?

Alloy 718 is used across many industries where components need high strength, corrosion resistance, high-temperature capability and resistance to stress corrosion cracking. This combination lets it withstand the aggressive conditions in gas turbines, jet engines and nuclear reactors.

• Aerospace: introduced in the 1960s, Alloy 718 was first used in aerospace because it retains its high strength at elevated temperatures. This supported the development of jet engines, and it remains widely used in engine components such as gas turbine discs, blades, casings and fasteners, plus high-speed airframe parts such as wheels, spacers and buckets, and high-temperature bolts and fasteners.
• Oil and gas: because it was readily available, Alloy 718 was taken up by the oil and gas industry as wells moved offshore and drilling went deeper, into higher temperatures, pressures and corrosive contaminants. It is used in downhole drilling tools, wellhead components, valve trim, pump shafts, sheathing, gate valves, choke stems, packers and tubing hangers, for its resistance to chlorides, stress corrosion and sulphide stress cracking combined with high strength and impact toughness. Other nickel-based alloys such as Hastelloy C276 and Inconel 625 offered sufficient corrosion resistance but insufficient strength without extensive cold working, while martensitic stainless steels were limited by hydrogen embrittlement and stress corrosion cracking, so Inconel 718 quickly became a default choice for pressure-containing and load-bearing components in aggressive environments.
• Chemical processing: reactors, valves and heat exchangers, for its ability to withstand corrosive chemicals and acids as well as elevated temperatures.
• Automotive: mainly motor racing, where high-temperature strength is needed in turbocharger turbines and exhaust components.
• Energy and nuclear: reactor components and steam-based systems.
• Marine and general engineering: occasional use in seawater-exposed fasteners, valve trim and heat exchangers.

Which specifications apply to Alloy 718?

Alloy 718 is covered by several major specifications. In the oil and gas market the key standards are API 6A, API 6A CRA and NACE MR0175 (ISO 15156); for aerospace, AMS 5662 and AMS 5663 dominate. Its wide use reflects its high performance and ready availability in a variety of product forms.

API 6A

API 6A (also known as ISO 10423, originally issued in 1994 and last revised in 2018) is maintained by the American Petroleum Institute. Formed in 1919, API aims to promote safety across the natural gas and oil industry globally, and one of its main activities is bringing together subject matter experts to develop and maintain standards covering materials and operational practices. The standard's title - 'Petroleum and natural gas industries - Drilling and production equipment - Specification for valves, wellhead and Christmas tree equipment' - indicates the target applications for this high-performance alloy.

A wellhead is the collection of components at the surface of an oil and gas well that acts as the interface for the drilling and production equipment. It must contain the pressure of the oil and gas within the well and be strong enough to support the various items joined to it. Surface pressure control is provided by a Christmas tree - the collection of valves, spools and fittings used to control flow to or from the well, which may also house additional functions such as chemical injection and the monitoring of conditions, flow rates and temperatures.

The specific components described in detail within this standard are:

• wellhead equipment - casing-head housings, casing-head spools, tubing-head spools, cross-over spools, multi-stage head housings and spools
• connectors and fittings - cross-over connectors, tubing-head adapters, top connectors, tees and crosses, fluid-sampling devices, adapter and spacer spools
• casing and tubing hangers - mandrel hangers, slip hangers
• valves and chokes - single valves, multiple valves, actuated valves, valves prepared for actuators, check valves, chokes, surface and underwater safety valves and actuators, back-pressure valves
• loose connectors - weld neck connectors, blind connectors, threaded connectors, adapter and spacer connectors, bull plugs, valve-removal plugs
• other equipment - actuators, clamp hubs, pressure boundary penetrations, ring gaskets, running and testing tools, wear bushings

The standard covers equipment operating at pressures from 2,000 psi to 20,000 psi (13.8-138MPa) and temperatures from -60°C (-75°F) to 121°C (250°F). It focuses more on the design criteria of the components than on the material properties of the metals specified, so it is used in combination with API 6A CRA and NACE MR-0175.

API 6A CRA

API 6A CRA (first issued in 2015, building on standards such as API 6A718 from 2004) is titled 'Age-hardened Nickel-based Alloys for Oil and Gas Drilling and Production Equipment'. As a supplement to API 6A, it covers a group of age-hardened nickel alloys, including Alloy 718, Alloy 725 (Inconel 725, UNS N07725) and Alloy 925 (Inconel 925, UNS N09925), as well as Alloys 716, 935 and 945. From a material supplier's perspective, it covers the most relevant aspects: production route, chemical composition, mechanical properties, microstructure and test conditions.

NACE MR0175 (ISO 15156-3)

NACE MR0175, in full 'Metals for Sulphide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments', specifies the types of corrosion-resistant materials that can be used in specific oilfield environments. One notable feature is its use of limits for material hardness, as hardness is the only practical material measurement that can be conducted in the field to validate a specification. Despite this restriction, hardness correlates reasonably with overall mechanical properties, and the limits apply to both the parent metal and any weld features.

Because the standard focuses purely on sulphide stress corrosion cracking, it does not address general or pitting corrosion, nor does it stipulate minimum mechanical properties, the required manufacturing route or general testing standards - so it is not a complete selection tool, but a useful complement to supplier standards such as API. The hydrogen sulphide threshold limits are based on a combination of real-world experience and laboratory testing, and the standard has evolved over time so that both operating limits and material requirements vary considerably. It also provides guidance for material selection where the hydrogen sulphide thresholds are exceeded, allowing fit-for-purpose testing to qualify grades for a particular application. NACE MR0175 is now published and maintained as ISO 15156-3.

AMS 5662 and AMS 5663

AMS stands for Aerospace Materials Specification, a range of standards maintained by SAE (Society of Automotive Engineers) in the United States which, despite the name, cover aerospace as well as automotive and commercial vehicles.

AMS 5662 is the base specification for Alloy 718 typically used in the aerospace market ('Nickel Alloy, Corrosion and Heat Resistant, Bars, Forgings, and Rings'). It covers the product in the solution annealed condition only, so it is a relatively low strength and low hardness (20-25 HRC) condition compared with the more common solution annealed and aged condition. It is therefore much easier to machine than Alloy 718 in the aged condition, where strength and hardness are increased significantly. For some components it is desirable to machine, or at least proof machine, in this condition before heat treatment, then complete finish machining afterwards. AMS 5662 is commonly supplied with a capability test to demonstrate that, with the appropriate heat treatment temperatures and times, it can meet the requirements of AMS 5663.

AMS 5663 takes the same product as AMS 5662, but controlled heat treatments known as ageing achieve significantly increased mechanical properties, not dissimilar to those of API 6A CRA.

Inconel 718 compared with Inconel 625

Inconel 625 has a higher alloy content and therefore better corrosion resistance in most environments, albeit at greater cost. Inconel 718 has higher strength, thanks to a combination of alloying additions and a heat treatment that allows it to age harden.