Jet engine hot section
Compressor disks, turbine cases, shafts, fasteners. The 540 °C+ operating environment kills everything else.
Nickel superalloy · UNS N07718
Inconel 718
Nickel-chromium-iron precipitation-hardening superalloy that holds its strength from cryogenic temperatures up to 700 °C (1300 °F). The standard for jet-engine hot-section parts, gas turbines, and high-pressure energy applications. Hard-won performance — but worth it where it counts.
UTS
1380 MPa
Density
8.19 g/cc
Modulus
200 GPa
Mach.
12 /100
When to use
Pick Inconel when no other alloy survives the temperature.
Inconel 718 is what you spec when steel would creep, titanium would lose its property profile, and stainless would soften. It’s the dominant alloy for compressor and turbine disks, fasteners, and casings in jet engines — roughly 30% of an F-class gas turbine’s hot-section weight is Inconel 718.
The combination is unique: 1380 MPa UTS at room temperature, ~1275 MPa retained at 540 °C, excellent fatigue and creep resistance, oxidation resistance, and weldability. The trade-off is cost (4–8× stainless) and machining difficulty (12/100 — among the toughest production alloys to cut).
Strengths
What you get for the price.
- High-temp strengthHolds ~1275 MPa UTS at 540 °C. No other commonly-machined alloy comes close.
- Cryogenic capableDuctile down to liquid-hydrogen temperatures (−253 °C). Used in rocket turbopumps.
- Oxidation resistanceForms protective Cr₂O₃ scale up to 980 °C. Long service life in hot oxidizing environments.
- WeldableSluggish age-hardening response means weldable without strain-age cracking — unusual for a superalloy.
- Fatigue & creepExcellent endurance for cyclically loaded turbine components.
Trade-offs
What you pay for it.
- Stock cost$35–50/lb for bar. Material can easily be 70%+ of finished part cost.
- Machining costCycle times often 4–8× steel. Tool wear is the dominant variable.
- Work hardens fastSingle pass and the surface is harder than your tool. Constant feed, no dwell.
- HeatLike Ti, low thermal conductivity. Heat goes into the tool — high-pressure coolant non-negotiable.
- Density penaltySame density as steel (8.19 g/cc). Used because nothing lighter performs at temp.
Specs
Mechanical, physical & chemical data.
Typical values for solution-treated and aged (precipitation-hardened) Inconel 718 bar per AMS 5663. Solution-treated only (without aging), UTS is typically ~930 MPa (135 ksi) with far lower yield — almost always aged before use.
Mechanical
Properties (aged)
| Ultimate tensile strength | 1380 MPa (200 ksi) |
|---|---|
| Yield strength (0.2% offset) | 1035 MPa (150 ksi) |
| Elongation at break | 12% |
| UTS at 540 °C | 1275 MPa (185 ksi) |
| UTS at 650 °C | 1160 MPa (168 ksi) |
| Modulus of elasticity | 200 GPa |
| Hardness | 36 – 44 HRC |
| Stress rupture≥4% elongation, per AMS 5662/5663 | ≥23 h at 649 °C (1200 °F) / 689 MPa (100 ksi) |
Source: Special Metals datasheet; AMS 5663; elevated-temperature UTS from Rolled Alloys typical aged-bar data. Values shown are typical for STA bar; the AMS 5663 acceptance minimum is 1275 MPa (185 ksi).
Physical & thermal
Properties at room temp
| Density | 8.19 g/cc (0.296 lb/in³) |
|---|---|
| Melting range | 1260 – 1336 °C |
| Specific heat | 0.435 J/g·°C |
| Thermal conductivity | 11.4 W/m·K |
| CTE (20–100 °C) | 13.0 µm/m·°C |
| Electrical resistivity | 1.25 µΩ·m |
| Magnetic | Non-magnetic above ~10 K |
Thermal conductivity is roughly 2× titanium and 1/15 aluminum — heat retention drives the machining challenge.
Composition
Chemistry, weight %
| Nickel (Ni) + cobalt | 50.0 – 55.0 |
|---|---|
| Chromium (Cr) | 17.0 – 21.0 |
| Iron (Fe) | balance |
| Niobium (Nb) + tantalum | 4.75 – 5.50 |
| Molybdenum (Mo) | 2.80 – 3.30 |
| Titanium (Ti) | 0.65 – 1.15 |
| Aluminum (Al) | 0.20 – 0.80 |
| Carbon (C) | 0.08 max |
| Cobalt (Co) | 1.00 max |
Niobium drives gamma double-prime precipitation — the slower-aging mechanism that gives 718 its weldability.
Specifications
Common purchase specs.
AMS 5662
Bar, forgings — solution treated
AMS 5663
Bar, forgings — solution & aged
AMS 5664
Bar, forgings, rings — 1950 °F solution treated (unaged)
AMS 5596
Sheet, strip, plate
ASTM B637
Bar, forgings — precipitation-hardened nickel
ASTM B670
Plate, sheet, strip
MIL-N-24114
Plate, sheet, strip
DIN 17744
European nickel-base equivalents
Machining
Machining considerations.
Inconel 718 punishes mistakes. Use slow speeds, heavy feeds, sharp edges, and rigid setups. A worn tool work-hardens the surface, which then ruins the next tool — every operator knows the cascade.
Cutting speed
30–50 SFM with carbide. 80–150 SFM with ceramic inserts at higher feeds.
Feed rate
Heavy. Light feed = rubbing = work hardening = scrapped tool and surface.
Depth of cut
Deeper is better — keeps the cut below any work-hardened layer from the previous pass.
Tooling
Carbide grades for general work. SiAlON or whisker-reinforced ceramic for production turning.
Coolant
High-pressure (1000+ psi) through-spindle. Heat removal is the whole game.
Climb mill always
Conventional milling rubs and immediately work-hardens.
No dwell
Stop the feed and the surface hardens past your tool's capability.
Rigid setup
Vibration accelerates wear exponentially. Short overhangs, heavy fixturing, sharp tools.
Applications
Where Inconel 718 earns its cost.
Gas turbines & energy
Land-based gas turbine wheels, exhaust components, oil & gas downhole tools, rocket engine turbopumps.
Cryogenic & space
LH₂ / LOX rocket components — Inconel keeps ductility at −253 °C where steel goes brittle.
Compare
Related materials.
Lighter alternative
Ti-6Al-4V →
Half the density. Picks up where it can — up to ~316 °C (600 °F) long-term service.
Cheaper alternative
316L Stainless →
When operating temp stays below 425 °C and corrosion drives the spec.
Cost / weight
Aluminum 6061 →
For ambient-temp structures where 718's strength is overkill.
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