Inconel 718
Inconel 718 (UNS N07718) is a precipitation-hardenable nickel-chromium-iron alloy strengthened by niobium and molybdenum. Unlike other age-hardened nickel alloys, its slow precipitation kinetics allow welding without spontaneous cracking. After double-aging heat treatment, it reaches tensile strengths above 1240 MPa and can operate at temperatures up to 700°C.
1. Chemical Composition
The table below lists the full elemental specification for Inconel 718. The niobium and aluminum-titanium ratios control the balance between gamma-prime and gamma-double-prime precipitation strengthening.
| GRADE | UNS Designation | Standard (Pipe) | C | Mn | P | S | Si | Ni | Cr | Mo | Cu | Fe | Ti | Al | Co | Nb | B |
| Inconel 718 | N07718 | B983 | ≤0.08 | ≤0.35 | ≤ 0.015 |
≤ 0.015 |
≤0.35 | 50.0-55.0 | 17.0-21.0 | 2.8- 3.3 |
≤0.30 | BAL | 0.65-1.15 | 0.2- 0.8 |
≤1.0 | 4.75-5.50 | ≤0.006 |
2. Mechanical Properties
The table below shows Inconel 718 in the precipitation-hardened (aged) condition. This is the standard condition for structural applications that need high tensile and fatigue strength.
| Grade | Standard | Condition | Tensile Strength , MPa (ksi) |
Yield Strength MPa (ksi) | Elongation , % |
Heat Treatment Procedure |
| Inconel 718 | B983 | Solution Ann + prec hard | ≥1034 (150) | ≥862 (125) | ≥20 | Solution Annealing Treatment：1875±25°F (1024 ± 14°C) for 1 to 2 h, water quench Precipitation Hardening Treatment：1425-1475°F (774-802°C) hold 6 to 8 h, air cool |
3. Equivalent Grade
The table below shows the corresponding international designations for Inconel 718 (UNS N07718) in EN/DIN, JIS, GB/T, and other national standards.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| Inconel 718 | N07718 | - | GH4169 | NCF718 | - | 2.4668 | - |
4. Key Technical Advantages
- Weldability Among Precipitation-Hardened Alloys: The high niobium content in Inconel 718 causes gamma-double-prime to precipitate slowly compared to the gamma-prime phase in alloys like Waspaloy or Rene 41. This significantly lowers strain-age cracking susceptibility during postweld heat treatment, which matters for fabricated pressure components.
- High Tensile and Fatigue Strength to 700°C: After solution anneal and double-aging (718°C + 621°C), Inconel 718 reaches tensile strengths above 1240 MPa and retains over 80% of its room-temperature strength at 650°C. It is the most widely specified alloy for gas turbine discs and shafts.
- Delta Phase Stability Control: Orthorhombic delta phase (Ni3Nb) forms at grain boundaries in the 850–980°C range. Through heat treatment, this can be controlled to limit grain growth and improve notch ductility. This microstructural feature is not available in other nickel superalloys.
Technical Note: For service above 700°C, Inconel 718 loses strength rapidly due to dissolution of gamma-double-prime. Above this temperature, consider gamma-prime-strengthened alloys such as Waspaloy or Inconel 725 for sustained load-bearing applications.
5. Common Manufacturing Standards
ASTM B637: Standard Specification for Precipitation-Hardening and Cold Worked Nickel Alloy Bars, Forgings, and Forging Stock — covers N07718 bar and rod for precipitation-hardened applications.
AMS Standards (Non-linked — no XTD page):
- AMS 5596: Sheet, strip, and plate — Inconel 718 solution treated
- AMS 5663: Bars, forgings, rings — solution treated and aged
- AMS 5589: Seamless tubing — solution treated
DIN/EN Standards:
- EN 10216-5: Seamless steel tubes for pressure purposes (Material No. 2.4668)
- DIN 17742: Nickel alloys — specification
JIS Standards:
- JIS H 4552: Nickel and nickel alloy seamless pipes and tubes
GB/T Standards:
- GB/T 15007: Corrosion-resistant alloy grades (GH4169)
Summary Table — Standard Equivalents:
| ASTM | AMS | EN/DIN | JIS | GB/T |
| ASTM B637 | AMS 5589 | EN 10216-5 / DIN 17742 | JIS H 4552 | GB/T 15007 |
6. Primary Applications
- Gas Turbine Discs and Shafts: Rotating components in aircraft and industrial gas turbines where the combination of high tensile strength, fatigue resistance, and creep strength to 700°C is needed. Inconel 718 is the standard choice for this service.
- Aerospace Structural Components: Engine casings, thrust reversers, and nacelle structure tubing that require certified high-strength nickel alloy tubular material per AMS specifications.
- Oil and Gas Downhole Tools: High-strength tubing and casing connectors, completion tools, and subsurface safety valve bodies in HPHT (high pressure, high temperature) well environments.
- Cryogenic Vessels and Piping: Liquid oxygen and liquid hydrogen storage systems. Inconel 718's austenitic structure keeps its toughness and strength at temperatures down to -253°C without a ductile-to-brittle transition.
- Nuclear Reactor Components: Fuel assembly hardware, control rod guide tubes, and reactor internal fasteners in light-water reactor environments that need high strength and radiation resistance.