Superior High-Temperature Strength • Precipitation Hardening • Aerospace Industry Standard
Inconel 718 is the most widely used nickel-based superalloy, combining exceptional strength, corrosion resistance, and ease of fabrication. Its precipitation-hardening capability delivers tensile strengths exceeding 1380 MPa while maintaining excellent high-temperature properties up to 650°C. The material of choice for gas turbine engines, rocket motors, and critical aerospace components.
Inconel 718 is a precipitation-hardenable nickel-chromium alloy containing significant amounts of iron, niobium, and molybdenum along with lesser amounts of aluminum and titanium. Unlike solid-solution strengthened alloys, Inconel 718 develops its exceptional strength through controlled heat treatment that forms hardening precipitates (primarily Ni₃Nb gamma double-prime phase). This gives it the highest strength of any common nickel superalloy while maintaining excellent fabricability.
The ability to precipitation harden Inconel 718 is its defining characteristic. Components can be machined, welded, and formed in the relatively soft solution-treated condition, then transformed into high-strength parts through a precisely controlled aging heat treatment. This processing flexibility makes it the most widely used superalloy for complex aerospace components.
| Element | Weight % | Purpose |
|---|---|---|
| Nickel (Ni) | 50.0 - 55.0 | Base metal, matrix for precipitation hardening |
| Chromium (Cr) | 17.0 - 21.0 | Oxidation and corrosion resistance |
| Iron (Fe) | Balance (≈18.5) | Reduces cost, contributes to strength |
| Niobium + Tantalum (Nb + Ta) | 4.75 - 5.50 | PRIMARY hardening element (forms γ" precipitates) |
| Molybdenum (Mo) | 2.80 - 3.30 | Solid solution strengthening |
| Titanium (Ti) | 0.65 - 1.15 | Forms γ' precipitates, age hardening |
| Aluminum (Al) | 0.20 - 0.80 | Forms γ' precipitates, age hardening |
| Cobalt (Co) | ≤ 1.0 | Minor alloying element |
| Manganese (Mn) | ≤ 0.35 | Deoxidizer |
| Carbon (C) | ≤ 0.08 | Carbide former, grain boundary strengthening |
Precipitation Hardening Chemistry: Niobium is the critical element—it forms Ni₃Nb (gamma double-prime) precipitates during aging that provide the exceptional strength. Titanium and aluminum form additional Ni₃(Ti,Al) (gamma-prime) precipitates for supplementary strengthening.
Important: Properties vary significantly with heat treatment condition. Values below are for fully age-hardened condition (solution treated + aged).
| Property | Metric | Imperial |
|---|---|---|
| Tensile Strength (Ultimate) | 1240-1380 MPa | 180,000-200,000 psi |
| Yield Strength (0.2% offset) | 1030 MPa (min) | 150,000 psi (min) |
| Elongation at Break | 12% (minimum) | |
| Reduction of Area | 15% (minimum) | |
| Modulus of Elasticity | 200 GPa | 29.0 × 10⁶ psi |
| Rockwell Hardness | 40-47 HRC (typical) | |
| Brinell Hardness | 388-444 HB (aged) | |
| Shear Strength | 760 MPa | 110,000 psi |
| Shear Modulus | 77.2 GPa | 11.2 × 10⁶ psi |
Inconel 718's high strength is achieved through a two-step heat treatment process: solution treatment followed by precipitation hardening (aging). This controlled process develops the strengthening precipitates that give the alloy its exceptional properties.
Alternative Single-Step Aging: 760°C (1400°F) for 10 hours → Air cool
Simpler but may result in slightly lower strength
A key advantage of Inconel 718 is that complex parts can be machined, welded, and formed in the solution-treated (soft) condition, then transformed to full strength through aging. This allows fabrication of intricate components that would be impossible to machine after hardening.
Turbine discs, compressor discs, turbine blades, combustor cans, afterburner parts, thrust reversers, engine mounts, shafts, casings, fasteners, and critical rotating components requiring high strength at elevated temperatures.
Rocket motor cases, liquid-fueled rocket engines, spacecraft components, airframe parts, landing gear components, fasteners, bellows, springs, and structural parts requiring high strength-to-weight ratio.
Nuclear reactor components, steam generator tubing, reactor vessel internals, control rod drive mechanisms, springs, and fasteners in high-radiation, high-temperature environments.
Downhole tools, wellhead equipment, valves, pumps, compressor components, subsea equipment, and high-pressure high-temperature (HPHT) applications in sour gas service.
Cryogenic storage tanks, liquid hydrogen and oxygen systems, LNG equipment, and cryogenic pump components where the alloy maintains excellent toughness at extremely low temperatures.
High-strength fasteners, springs, pressure vessels, heat treating fixtures, tooling for high-temperature forming, and components requiring exceptional creep resistance.
| Operation | Annealed Condition | Aged Condition |
|---|---|---|
| Turning | Speed: 60-100 SFM (18-30 m/min) | Speed: 20-40 SFM (6-12 m/min) |
| Milling | Speed: 70-120 SFM (21-37 m/min) | Speed: 30-50 SFM (9-15 m/min) |
| Drilling | Speed: 25-50 SFM (7.5-15 m/min) | Extremely difficult, avoid if possible |
| Grinding | Standard parameters | Primary method for aged material |
Processing Strategy: Machine all complex features, threads, and precision dimensions in the solution-treated condition. After aging, only perform light grinding or EDM operations for final dimensions. This approach maximizes productivity and tool life while achieving the required strength.
Inconel 718 has excellent weldability when in the solution-treated (annealed) condition. The alloy can be welded using most common processes, and the weld zone can be aged after welding to develop full strength properties.
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