Inconel 625 — Ni-Cr-Mo Superalloy vs Inconel 718 — Nickel Superalloy Properties, Heat Treatment & Aerospace Applications
Side-by-side engineering comparison of Inconel 625 — Ni-Cr-Mo Superalloy (AMS 5666) and Inconel 718 — Nickel Superalloy Properties, Heat Treatment & Aerospace Applications (AMS 5662). Compare mechanical properties, chemical composition, density, yield strength, international equivalents, and typical applications to select the right material for your project.
Quick Verdict
Choose Inconel 625 for applications requiring exceptional corrosion resistance in the solution-annealed condition — chemical processing, marine exhaust, and seawater systems where its inherent strength (414 MPa yield) is sufficient and welding is frequent. Choose Inconel 718 when maximum strength is required — gas turbine disks, aerospace fasteners, and cryogenic components where its precipitation-hardened 1034 MPa yield (2.5× 625's) justifies the added heat treatment complexity....
Corrosion resistance and weldability in solution-annealed condition
Maximum precipitation-hardened strength for aerospace turbine applications
Quick Comparison
| Property | Inconel 625 — Ni-Cr-Mo Su | Inconel 718 — Nickel Supe |
|---|---|---|
| Standard | AMS 5666 | AMS 5662 |
| Category | ||
| Density | 8440 | 8190 |
| Yield Strength | 517 MPa | 1100 MPa |
| Tensile Strength | 930 MPa | 1375 MPa |
| Key Applications | <p>Ni-Cr-Mo superalloy for marine, aerospace, and chemical processing. Outstanding fatigue strength.</p>... | <p>Inconel 718 is a precipitation-hardening nickel-chromium superalloy used extensively in jet engine turbine components... |
International Equivalents
| Inconel 625 — Ni-Cr- Equivalents | Inconel 718 — Nickel Equivalents |
|---|---|
| UNS N06625 | UNS N07718 |
| 2.4856 | 2.4668 |
| — | NC19FeNb |
How to Choose
| Choose Inconel 625 — Ni-Cr-Mo Su when... | Inconel 625 provides 414 MPa yield in the solution-annealed condition with unmatched corrosion resistance (PREN >45) and excellent weldability — ideal for chemical processing, marine, and nuclear applications. |
| Choose Inconel 718 — Nickel Supe when... | Inconel 718 achieves 1034 MPa yield after two-step aging — 2.5× 625's strength — making it the dominant aerospace superalloy for turbine discs, blades, and fasteners operating up to 700°C. |
Decision Checklist: Which Is Right for You?
| Criterion | Inconel 625 — Ni-Cr- | Inconel 718 — Nickel |
|---|---|---|
| Corrosion Resistance And Weldability In Solution-Annealed Condition | ✓ | |
| Maximum Precipitation-Hardened Strength For Aerospace Turbine Applications | ✓ | |
| Higher yield strength | ✓ | |
| Strength — 718's aged yield strength (1034 MPa) is 2.5× 625's annealed... | ✓ | |
| Heat treatment — 625 is solid-solution strengthened (use as-annealed);... | ✓ | |
| Corrosion resistance — 625 is superior in aggressive wet corrosion due... | ✓ |
Selection Guide
Choose Inconel 625 for applications requiring exceptional corrosion resistance in the solution-annealed condition — chemical processing, marine exhaust, and seawater systems where its inherent strength (414 MPa yield) is sufficient and welding is frequent. Choose Inconel 718 when maximum strength is required — gas turbine disks, aerospace fasteners, and cryogenic components where its precipitation-hardened 1034 MPa yield (2.5× 625's) justifies the added heat treatment complexity. 718 for strength; 625 for corrosion and weldability.
Key Decision Factors
- Strength — 718's aged yield strength (1034 MPa) is 2.5× 625's annealed strength (414 MPa); if 625's strength is adequate, 718's precipitation-hardening complexity is unnecessary
- Heat treatment — 625 is solid-solution strengthened (use as-annealed); 718 requires complex two-step aging (720°C/8h + 620°C/8h) to develop full strength
- Corrosion resistance — 625 is superior in aggressive wet corrosion due to higher Mo (8-10% vs 718's 3%) and Nb content; 718 is optimized for strength, not corrosion
- Weldability — 625 is one of the most weldable superalloys (no PWHT needed); 718 is weldable but requires full solution+aging after welding to restore properties in the HAZ
When to Use Each
Use Inconel 625 — Ni-Cr- for:
625's Ni-Cr-Mo-Nb composition (PREN >45) provides unmatched resistance to pitting, crevice corrosion, and chloride SCC — standard for scrubbers, flue gas desulfurization, and reactor vessels.
625 is standard for seawater heat exchangers, subsea piping, and offshore flare tips where its combination of corrosion resistance, weldability, and moderate strength (414 MPa) outperforms all stainless steels.
625 is the preferred filler (ERNiCrMo-3) for overlay cladding carbon steel with corrosion-resistant layers — its solid-solution strengthening means no post-weld heat treatment is required to achieve full properties.
Use Inconel 718 — Nickel for:
718's precipitation-hardened 1034 MPa yield (aged condition) and 650°C service capability make it the dominant superalloy for jet engine rotating components — 35% of all superalloy tonnage.
718 retains 1000+ MPa yield to 650°C and excellent fatigue resistance — standard for engine mounting bolts, airframe fasteners, and rocket motor components.
718 maintains useful toughness to -253°C (liquid helium temperature), making it standard for LNG and rocket propellant handling where high strength at cryogenic temperatures is required.
Frequently Asked Questions
What is the main difference between Inconel 625 — Ni-Cr-Mo Superalloy and Inconel 718 — Nickel Superallo?
Inconel 625 — Ni-Cr-Mo Superalloy (AMS 5666) provides 517 MPa yield strength at 8440 density, while Inconel 718 — Nickel Superalloy Properties, Heat Treatment & Aerospace Applications (AMS 5662) delivers 1100 MPa at 8190. The choice depends on whether your application prioritizes corrosion resistance and weldability in solution-annealed condition or maximum precipitation-hardened strength for aerospace turbine applications.
Can Inconel 625 — Ni-Cr-Mo Superalloy be substituted for Inconel 718 — Nickel Superallo?
In many applications, these materials can be cross-referenced, but direct substitution should always be verified against specific project specifications, especially for corrosion resistance and weldability in solution-annealed condition, maximum precipitation-hardened strength for aerospace turbine applications, and operating environment. Consult your engineer of record.
Can Inconel 718 be used without aging (solution-annealed only)?
Yes, but its yield strength in the solution-annealed condition is only ~414 MPa (similar to 625), negating the primary reason to choose 718. The 1034+ MPa strength that makes 718 valuable comes from the gamma-double-prime (Ni3Nb) precipitates formed during aging. If you don't need the aged strength, use 625 instead — it provides better corrosion resistance and weldability at the same strength level.
Why is 718 the most widely used superalloy?
Three reasons: (1) its gamma-double-prime (Ni3Nb) precipitation kinetics are slow, meaning thick sections can be heat-treated without quench cracking — unlike gamma-prime alloys (Waspaloy, Udimet 720) which crack in heavy sections; (2) it is weldable (with proper PWHT) where most gamma-prime superalloys suffer strain-age cracking; (3) its 650°C service temperature covers the majority of jet engine and industrial gas turbine hot-section components. These factors combine to make 718 the workhorse superalloy — 35% of all superalloy tonnage.
Is Inconel 625 or 718 better for high-temperature service?
For service above 650°C, neither is ideal — 718's gamma-double-prime coarsens and transforms to stable delta phase above 650°C, losing strength; 625 retains strength to ~815°C but is much weaker than 718 below 650°C. For sustained service above 700°C, consider Waspaloy (750°C), Udimet 720 (760°C), or single-crystal nickel alloys like CMSX-4 (1100°C+) for turbine blade airfoils. 625 is preferred for static high-temperature components (ducting, exhausts) where strength is moderate but corrosion resistance matters.
How do 625 and 718 compare for welding?
625 is significantly easier to weld. As a solid-solution alloy, it needs no post-weld heat treatment — the weld and HAZ have essentially the same properties as the base metal. 718 is weldable in the solution-annealed condition but must be re-solution-annealed and aged after welding to restore precipitation-hardened strength in the HAZ. Without PWHT, 718 welds have annealed-strength (~414 MPa) zones that can be failure points under load. For weld overlays and repair cladding, 625 is strongly preferred.
What is the cost difference between Inconel 625 and 718?
718 typically costs 10-25% more than 625 per kilogram due to the more complex two-step heat treatment required and tighter quality controls for aerospace applications. However, 718's 2.5× strength advantage means less material is often required, making the cost-per-unit-strength similar. For purely corrosion-driven applications, 625 is more economical; for strength-driven aerospace applications, 718's cost is justified by weight savings. Both are 10-20× the cost of 316L stainless.