Inconel 718 vs 625: Which Nickel Alloy for Your Application?
RAS Materials Engineering Team
Applications
Bottom line up front: Inconel 718 gives you precipitation-hardenable strength to 650°C — it’s the turbine disc alloy. Inconel 625 gives you universal corrosion resistance to 980°C — it’s the seawater and chemical processing alloy. If your application needs both, 718 wins on strength; 625 wins on corrosion.
TL;DR — 718: Ni-Cr-Nb, age-hardenable, 650°C max, 1275 MPa UTS, AMS 5662. 625: Ni-Cr-Mo, solid-solution, 980°C max, outstanding pitting resistance, NACE MR0175.
Chemical Composition: What’s Actually Different?
Inconel 718 derives its strength from gamma-double-prime (Ni3Nb) precipitation — that 5% niobium is the key. Without proper solution treatment at 980-1010°C, the strengthening phase never fully dissolves and creep life drops by 60%.
Inconel 625 relies on molybdenum (8-10%) and niobium (3-4%) in solid solution for both strength and corrosion resistance. No heat treatment required for corrosion performance.
| Element | 718 | 625 | What It Does |
|---|---|---|---|
| Ni | 50-55% | 58% min | Base — corrosion resistance |
| Cr | 17-21% | 20-23% | Oxidation resistance |
| Mo | 2.8-3.3% | 8-10% | Pitting resistance (625 wins) |
| Nb | 4.75-5.5% | 3.15-4.15% | 718: precipitation hardening; 625: solid-solution |
| Ti | 0.65-1.15% | 0.4% max | 718: gamma-prime former |
| Al | 0.2-0.8% | 0.4% max | 718: gamma-prime former |
Temperature Limits: Where Each Alloy Excels
Inconel 718: Rated for -253°C to 650°C. Above 650°C, gamma-double-prime coarsens and strength drops rapidly. For 700°C+ service, consider Waspaloy or Inconel X-750.
Inconel 625: Rated from cryogenic to 980°C. Oxidation resistance holds to 980°C, but mechanical strength drops significantly above 650°C. The alloy was designed for corrosion resistance, not creep strength.
Corrosion Resistance: The Deciding Factor
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Seawater: 625 wins decisively. PREN (Pitting Resistance Equivalent Number) >50 vs 718’s ~30. For offshore and subsea, 625 is the standard. Only Monel K-500 competes in this environment.
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Sour Gas (H2S): Both are NACE MR0175 compliant. 625 has better resistance to chloride SCC. 718 is commonly used for high-strength wellhead components where 625 lacks the required mechanical properties.
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Oxidizing Acids: 625 outperforms 718 in nitric and phosphoric acid. 718 is acceptable in most organic acids but should be avoided in reducing acids like HCl.
Cost Comparison (Relative)
At time of writing, Inconel 718 typically costs 10-15% less than 625 per kilogram due to lower molybdenum content. However, 718 requires heat treatment (solution + two-step aging) which adds processing cost. For components requiring post-weld heat treatment, the total fabrication cost difference narrows considerably.
Selection Checklist
- Operating temperature above 650°C? → 625 (or consider 617/X-750 above 700°C)
- Seawater or high-chloride environment? → 625
- Need maximum tensile strength (>1200 MPa)? → 718 (heat treated)
- Sour gas with H2S? → Both NACE compliant; 625 for corrosion, 718 for strength
- Welding without post-weld heat treat? → 625 (easier to weld)
- Fatigue-critical rotating component? → 718 (superior fatigue strength)
Bottom line: If you’re building a turbine disc — 718. If you’re building a seawater piping system — 625. If you need both and can’t decide — send us your operating conditions. Our metallurgy team will recommend the optimal grade.
Standards: AMS 5662/5663 (718), ASTM B443/B444 (625), NACE MR0175.
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