What Is Inconel 718?
Inconel 718 (UNS N07718) is a precipitation-hardened nickel-chromium alloy combining exceptional strength with good corrosion resistance. Unlike solution-annealed alloys such as Inconel 625, Inconel 718 achieves its extreme mechanical properties through age-hardening, producing yield strengths above 1000 MPa. It is the most widely used superalloy in the world, serving both aerospace and oil and gas applications.
| Property | Value |
|---|---|
| UNS designation | N07718 |
| Common names | Inconel 718, Alloy 718 |
| Nickel | 50.0-55.0% |
| Chromium | 17.0-21.0% |
| Molybdenum | 2.80-3.30% |
| Niobium + Tantalum | 4.75-5.50% |
| Titanium | 0.65-1.15% |
| Aluminum | 0.20-0.80% |
| Iron | Balance (~18%) |
| Carbon | 0.08% max |
| Tensile strength (min) | 1241 MPa (180 ksi); aged |
| Yield strength (min) | 1034 MPa (150 ksi); aged |
| Elongation (min) | 12% |
| Hardness | 331-401 HBW (35-44 HRC); aged |
| Density | 8.19 g/cm3 |
| Max service temp | 650°C (1200°F); strength-limited |
Precipitation Hardening
Inconel 718 is strengthened by gamma double-prime (Ni3Nb) and gamma-prime (Ni3(Al,Ti)) precipitates formed during aging heat treatment. The standard heat treatment cycle:
- Solution anneal: 955-1010°C, air cool or faster
- First aging step: 720°C for 8 hours, furnace cool to 620°C
- Second aging step: 620°C for 8 hours, air cool
This produces the characteristic ultra-high strength while maintaining adequate ductility. The niobium-driven gamma double-prime precipitation is sluggish, which gives 718 excellent weldability for a precipitation-hardened alloy.
718 vs 625: Key Differences
| Property | Inconel 625 | Inconel 718 |
|---|---|---|
| Strengthening | Solid solution | Precipitation hardened |
| Yield (min) | 414 MPa (60 ksi) | 1034 MPa (150 ksi) |
| Corrosion resistance | Superior | Good |
| PREN | >50 | ~32 |
| Max service temp | 980°C (oxidation) | 650°C (strength) |
| Primary advantage | Corrosion | Strength |
| Typical application | Flowlines, chemical | Fasteners, wellheads, downhole |
Oil and Gas Applications
- Subsea wellhead: compact flange connectors, hubs, and clamps for HP/HT service (per API 17D)
- Downhole tools: completion equipment, packers, and safety valve components
- Fasteners: high-strength bolting for critical flanged connections per ASTM A453 Grade 660
- Christmas tree components: valve stems, actuator parts, and sealing elements
- Drilling: MWD/LWD tool housings, drill collar connections
ASTM Specifications
| Product Form | ASTM Specification |
|---|---|
| Forgings | B564 |
| Bars, forgings, rings | B637 |
| Plate, sheet, strip | B670 |
| Seamless pipe/tube | B829 |
NACE Compliance
For sour service per NACE MR0175/ISO 15156, Inconel 718 is acceptable in the age-hardened condition provided hardness does not exceed 40 HRC. API 6A and API 17D reference 718 extensively for wellhead and tree components in sour environments.
Mill test certificates must document full chemistry, age-hardening heat treatment parameters, mechanical properties, and hardness. NDT includes ultrasonic and fluorescent penetrant inspection. Hydrostatic testing follows the applicable equipment standard.
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