Inconel 718 vs Inconel 625
Inconel 718 and 625 are both nickel-chromium superalloys, but serve different purposes. 718 is a precipitation-hardened workhorse for strength to 700 C. 625 is solid-solution-strengthened for corrosion resistance and weldability to higher temperatures.
Inconel 718
Composition (wt %)
| Element | Min | Max |
|---|---|---|
| Ni | 50.0 | 55.0 |
| Cr | 17.0 | 21.0 |
| Fe | 14.0 | 21.0 |
| Mo | 2.8 | 3.3 |
| Nb | 4.75 | 5.5 |
| Ti | 0.65 | 1.15 |
| Al | 0.2 | 0.8 |
| Co | 0.0 | 1.0 |
Inconel 625
Composition (wt %)
| Element | Min | Max |
|---|---|---|
| Ni | 58.0 | 100.0 |
| Cr | 20.0 | 23.0 |
| Mo | 8.0 | 10.0 |
| Nb | 3.15 | 4.15 |
| Fe | 0.0 | 5.0 |
| Ti | 0.0 | 0.4 |
| Al | 0.0 | 0.4 |
| Co | 0.0 | 1.0 |
Key differences
| Aspect | Inconel 718 | Inconel 625 | Engineering impact |
|---|---|---|---|
| Strengthening mechanism | Precipitation hardening (gamma-double-prime Ni3Nb) | Solid-solution strengthening (Mo, Nb) | 718 needs age hardening to reach full strength. 625 reaches its properties directly from annealed condition. |
| Maximum service temperature | ~700 C (Ni3Nb phase dissolves above) | ~980 C | For hot-section parts above 700 C, 625 or a gamma-prime alloy (e.g. Waspaloy) is the right choice. |
| Weldability | Strain-age cracking risk in HAZ | Excellent - welds without cracking | 625 is preferred for welded assemblies. 718 welds need careful thermal control and often post-weld solution + age. |
| Niobium content | 4.75-5.50% Nb (for strengthening) | 3.15-4.15% Nb | Nb is the strengthening element in 718 and a stabiliser against carbide precipitation in 625. |
When to choose Inconel 718
- Aerospace turbine discs and shafts
- High-strength fasteners to 650 C
- Oil and gas downhole tools
When to choose Inconel 625
- Bellows, expansion joints, exhaust systems
- Seawater and acidic chemical handling
- Welded aerospace combustor liners
Frequently asked questions
Which is more corrosion resistant - 718 or 625?
625 is the better corrosion alloy, especially in chloride and reducing acid service. 718 is acceptable in mild environments but prioritises high-temperature strength.
Can I substitute 625 for 718?
Not without re-analysis. 625 has much lower room-temperature yield strength. Substitution works only when corrosion or weldability, not strength, governs the design.
Do both alloys need heat treatment?
718 requires solution + precipitation age to develop the strengthening phase. 625 is typically supplied in the annealed condition and used as-received.