Comparison Guide
Inconel 718 vs Inconel 625 — Application Selection Guide
Inconel 718 and 625 are the two most-used nickel-based superalloys in oil & gas — but they're not interchangeable. 718 is precipitation-hardenable (higher strength when aged); 625 is solid-solution strengthened (better corrosion resistance, easier to weld). This guide covers the tradeoffs.
Property comparison
| Property | Inconel 718 (aged H900) | Inconel 625 |
|---|---|---|
| Yield strength (ksi) | ~150 ksi | ~60 ksi |
| Tensile strength (ksi) | ~180 ksi | ~120 ksi |
| Hardness | ~40–45 HRC | ~22–25 HRC |
| Max service temperature | ~1300°F (700°C) | ~1800°F (980°C) |
| Chloride SCC resistance | Good | Excellent |
| Weldability | Moderate (precipitation-hardened complexity) | Excellent |
| Machinability rating | Difficult (especially aged) | Moderate |
| Cost (approximate) | Similar | Similar (slightly higher) |
When to spec 718
High-strength requirements: aged 718 is nearly 3x the yield strength of 625. Downhole tool bodies under high torque, high-pressure valve stems, structural components in critical service.
Cost-sensitive applications where the higher strength allows thinner sections and less material.
When to spec 625
Corrosion resistance is the primary driver: chloride-heavy service, high-temperature aqueous corrosion, seawater. 625's higher chromium and molybdenum give it better chloride SCC and pitting resistance than 718.
Welded fabrications: 625's excellent weldability makes it the choice for pressure vessels, complex assemblies, and applications where the part will be welded post-machining.
Common mistakes
- Speccing 718 aged for weld-fabricated parts — Aging complicates post-weld heat treatment. 625 or annealed 718 is better for weld fabrication.
- Speccing 625 for high-strength structural — You'll need more material and thicker sections. 718 aged is cheaper per unit of strength.
- Assuming similar machining behavior — Aged 718 is significantly harder to machine than 625. Cost and lead time differ.
Frequently asked questions
Is Inconel 718 stronger than 625?
Yes — aged 718 has ~150 ksi yield vs ~60 ksi for 625. About 2.5x the yield strength for high-strength critical applications.
Is Inconel 625 more corrosion-resistant than 718?
For chloride SCC and pitting: yes, 625 is better. Both are excellent in most oilfield service; 625 has the edge in the harshest chloride environments.
Which is easier to machine?
Inconel 625 is moderately easier — lower cutting forces and less work-hardening tendency than aged 718. Both require sharp tooling and disciplined feeds.
Can I substitute 625 for 718?
Only if the strength difference is acceptable. Substitution is a design engineering decision — the alloys aren't interchangeable in most critical applications.