Inconel 625 vs Inconel 718
Inconel 625 and 718 are both nickel superalloys for extreme environments, but they are optimized differently. 718 is age-hardenable, reaching very high strength (~1375 MPa tensile) for aerospace fasteners and turbine parts up to ~700 C. 625 is solid-solution strengthened, prioritizing the best corrosion resistance and weldability with strong service to ~980 C. Strength versus corrosion-and-weldability is the core tradeoff between these two premium alloys.
The verdict
Choose Inconel 718 for maximum strength via age hardening — high-load aerospace fasteners and turbine components to about 700 C. Choose Inconel 625 for the best corrosion resistance and weldability with solid high-temperature service to ~980 C. 718 is the strength alloy; 625 is the corrosion-and-weldability alloy.
Side-by-side data
| Property | Inconel 625 | Inconel 718 |
|---|---|---|
| Category | Nickel Alloy | Nickel Alloy |
| Density (g/cm³) | 8.44 | 8.19 |
| Tensile strength (MPa) | 860 | 1375 |
| Yield strength (MPa) | 480 | 1100 |
| Elongation (%) | 40 | 12 |
| Hardness | 190 HB | 42 HRC |
| Max service temp (°C) | 980 | 700 |
| Machinability | ●●●●● | ●●●●● |
| Corrosion resistance | ●●●●● | ●●●●● |
| Relative cost | ●●●●● | ●●●●● |
| Thermal cond. (W/m·K) | 9.8 | 11.4 |
| Typically used for | Seawater & high-temp corrosion | High-strength hot turbine parts |
Which should you choose?
Choose Inconel 625 when…
- Maximum corrosion resistance is the priority — 625 rates 5/5 vs 718's 4.5/5
- Weldability matters; 625 welds readily and retains corrosion resistance in the joint
- Continuous service runs very hot — 625 lists ~980 C vs 718's ~700 C
- You want high strength without age-hardening heat treatment (solid-solution strengthened)
- Seawater, acids, and aggressive chemistry are present alongside heat
- Sheet, piping, and welded fabrications are the target form
Choose Inconel 718 when…
- You need very high strength — tensile ~1375 MPa vs 625's ~860 MPa (age-hardened)
- Aerospace fasteners, turbine disks, and high-load hot components are the application
- Strength must hold at elevated temperature up to ~700 C
- Age-hardening (precipitation) heat treatment is acceptable in the process route
- Strength-to-weight in rotating and fastened parts is critical
- The part is highly stressed but not in the most aggressive corrosive media
Key differences that matter
- 718 is age-hardenable, reaching ~1375 MPa tensile versus 625's ~860 MPa
- 625 is solid-solution strengthened — strong without precipitation heat treatment
- 625 has the edge in corrosion resistance (5/5 vs 4.5/5) and weldability
- 625 serves hotter (~980 C) but 718 keeps high strength to ~700 C
- Both are nickel superalloys at the top cost tier (~5/5) and very hard to machine (1/5)
- 718 dominates structural hot parts: turbine disks, shafts, aerospace fasteners
- 625 dominates corrosion-and-weld-critical hot parts: ducting, bellows, chemical hardware
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What makes Inconel 718 so much stronger than 625?
718 is precipitation- (age-) hardenable: a heat treatment forms strengthening phases that push tensile strength to around 1375 MPa. 625 relies on solid-solution strengthening and reaches about 860 MPa. So 718 trades some weldability and a lower max temperature for dramatically higher strength, which is exactly what highly loaded aerospace and turbine parts need.
Which superalloy is easier to weld?
Inconel 625. It welds readily and retains its corrosion resistance and ductility in the weld zone, making it ideal for fabricated piping, ducting, and bellows. 718 can be welded but its age-hardening response complicates the process and post-weld heat treatment. For weld-intensive corrosion-critical fabrications, 625 is the more practical alloy.
Which handles higher temperatures?
For continuous service, 625 lists a higher ceiling (~980 C vs ~700 C). However, 718 retains very high mechanical strength up to about 700 C, which is what matters for loaded turbine and fastener applications. So 625 wins on sheer temperature and oxidation resistance, while 718 wins on strength at the temperatures it is rated for.
Are they similar in cost and machinability?
Yes — both sit at the top cost tier (~5/5) and both rate 1/5 for machinability, among the hardest materials in the dataset. They work-harden aggressively and demand slow speeds, rigid setups, and frequent tooling. Neither is cheap or easy to make, so material selection is driven by performance, not by any cost or machining advantage between them.
Which should I pick for turbine and aerospace parts?
It depends on the part. For highly stressed rotating or fastened components — turbine disks, shafts, bolts — 718's age-hardened strength to 700 C is the standard. For combustor liners, bellows, ducting, and weld-fabricated hot sections needing top corrosion resistance, 625 is preferred. Many engines use both, each where its strengths apply.
Property values are typical/nominal figures for early-stage guidance only and vary by temper, grade, supplier and heat treatment. Confirm critical specifications against a certified datasheet or with an mfgiq engineer before production.