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Inconel 718 / (2.4668) / UNS N07718 / AMS 5596 / AMS 5663

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Inconel 718 (2.4668), also widely known as Alloy 718, is one of the most widely used of all nickel alloys. As with Inconel 625, Inconel 718 is formed from a nickel-chromium-molybdenum combination. The grade is also called Alloy 718. Corresponding to 2.4668 in the DIN standard, it also corresponds to UNS N07718. With the chemical designation NiCr19Fe19Nb5Mo3, this nickel alloy has extremely high resistance to corrosion.

Hardenable by ageing, the grade retains its mechanical properties across a very wide range of temperatures. This nickel alloy has very high tensile strength, and its rupture and fracture resistance are both very high. Able to withstand high temperatures, the material takes on an even harder structure after age hardening.​‌​​‌​

Alongside many materials that are difficult to weld, the weldability of Alloy 718 is very good. Suitable for use where welding is involved, Alloy 718 is, thanks to all these properties, among the most widely used and most readily available nickel alloys.

Produced for highly specialised applications and holding very specific specifications, Alloy 718 is frequently used in the aerospace industry. It is often chosen for parts in contact with chemicals, in marine vessels and components, nuclear reactor parts, rocket engine parts, storage tanks built for very specific purposes, valves, fasteners and turbine blades. Formed essentially from nickel, chromium and molybdenum, this nickel alloy is made still more durable by the niobium, aluminium and titanium it contains.​‌​​‌​

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Standards by Product Form

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The most common source of confusion when ordering Inconel 718 is that the same alloy is sold under different specification numbers depending on the solution-annealing temperature. AMS 5662, AMS 5663, AMS 5596 and AMS 5589 are based on solution annealing at 968 °C (1775 °F); AMS 5664, AMS 5597 and AMS 5590 on 1066 °C (1950 °F). Always state the product form, the specification and the delivery condition (annealed or aged) together — this prevents certificate mismatches before they happen.

AMS 5662 defines the solution-annealed, precipitation-hardenable condition (the material arrives soft and you age it after machining), while AMS 5663 defines the solution-annealed and aged condition — delivered at full strength. Order 5662 or 5664 if extensive machining is planned, 5663 if you want the part ready at strength.

Welding, Heat Treatment and Machining​‌​​‌​

Welding​‌​​‌​

Because Inconel 718 hardens through the γ″ (Ni₃Nb) phase rather than through aluminium and titanium, it is markedly more resistant to post-weld strain-age cracking than most other superalloys — in practice one of the easiest superalloys to weld. GTAW (TIG) is the primary process; GMAW, SMAW, electron beam and resistance welding are also used. The matching filler is AWS A5.14 ERNiFeCr-2 (AMS 5832). It can be welded in both the annealed and the aged condition, and it can be aged directly without an intermediate stress relief. Keep heat input low and interpass temperature below 90–100 °C. Avoid silver- and cadmium-bearing brazing alloys, which cause stress cracking in hardened material.

Heat treatment​‌​​‌​

The alloy strengthens by precipitation (age) hardening; the principal strengthening phase is γ″ (Ni₃Nb), with γ′ as a secondary contributor. Common route: solution anneal at 927–1010 °C (typically 954–982 °C for 1 hour, air cool), then a two-step age — 718 °C for 8 hours → furnace cool at 56 °C/hour to 621 °C → 621 °C for 8 hours (18 hours total) → air cool. The high-temperature route solution anneals at 1038–1066 °C and gives better transverse ductility and stress-rupture behaviour at slightly lower room-temperature strength. Typical minimums after solution + age: yield ≥ 1035 MPa, tensile ≥ 1240 MPa, elongation ≥ 12%.

Machining​‌​​‌​

The work-hardening rate is higher than that of austenitic stainless steels — this is the critical point. Set up the machine, fixture and tool holder as rigidly as possible; vibration translates directly into work hardening and tool failure. Use positive-rake carbide tooling and treat a tool with a 0.38 mm wear land as blunt and replace it. Use a low cutting speed with a steady, adequate feed; letting the tool dwell and rub leaves a work-hardened layer that makes the next pass worse. A generous flow of soluble-oil coolant is essential. The most efficient route is to rough in the annealed condition and finish after ageing; ageing causes roughly 0.0008 mm/mm of shrinkage.

Which Alloy for Which Duty?​‌​​‌​

Nickel-base alloys are not interchangeable — each is optimised around a different priority. A short guide:

Frequently Asked Questions

What is the difference between AMS 5662, AMS 5663 and AMS 5664?​‌​​‌​

All three cover the same alloy (N07718) as bar, forgings and rings; the difference is the delivery condition and the solution-annealing temperature. AMS 5662 = solution annealed at 968 °C, precipitation hardenable. AMS 5663 = solution annealed at 968 °C and precipitation heat treated, i.e. at full strength. AMS 5664 = solution annealed at 1066 °C, precipitation hardenable; coarser grain, better stress-rupture life and transverse ductility.

Is the temperature limit for 718 650 °C or 704 °C?​‌​​‌​

Both figures are correct but describe different things. 704 °C is the limit up to which the alloy retains high yield and tensile strength. 650 °C is the practical limit for long-term service under creep loading, above which γ″ coarsens and transforms to the δ phase and strength falls. Oxidation resistance extends to roughly 980 °C, but that is surface behaviour only, not load-bearing capability. Rule of thumb: up to 700 °C for static or short-term loading, do not exceed 650 °C for sustained load over a long service life.

Can aerospace-certified material (AMS 5663) be used for a sour-service oil and gas part?​‌​​‌​

Not automatically. Pressure-containing and pressure-controlling oil and gas equipment is ordered to API Std 6ACRA and NACE MR0175 / ISO 15156-3. Unlike AMS, these impose a hardness ceiling and minimum reduction of area, elongation, Charpy toughness and production-route requirements; the high-strength target of AMS 5663 can fall outside that hardness window. Order it as “UNS N07718, API 6ACRA, NACE MR0175 / ISO 15156-3 compliant”.

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