Material comparison
The stronger alloy has the lower temperature limit — and the reason is the strengthening mechanism.
Both are Inconel, both are nickel-based, and they are strengthened by completely different mechanisms. That mechanism is why 718 — the stronger of the two at room temperature — has the lower temperature ceiling.
Inconel 625 is a solid-solution strengthened alloy. Molybdenum and niobium dissolve into the nickel matrix and distort the lattice, which resists dislocation movement. This strengthening is thermodynamically stable: nothing has to be precipitated and nothing can dissolve back out at temperature.
Inconel 718 is precipitation hardened. Niobium, titanium and aluminium form fine gamma-double-prime and gamma-prime particles during ageing, and those particles block dislocations very effectively. The result is roughly twice the yield strength of 625 at room temperature. But the particles are only stable up to a point: above roughly 650 °C the gamma-double-prime coarsens and transforms, and the strength it provided is lost.
| Inconel 625 (N06625) | Inconel 718 (N07718) | |
|---|---|---|
| Strengthening | Solid solution | Precipitation |
| Nickel | min. 58% | 50 – 55% |
| Chromium | 20 – 23% | 17 – 21% |
| Molybdenum | 8 – 10% | 2.8 – 3.3% |
| Niobium | 3.15 – 4.15% | 4.75 – 5.5% |
| Iron | max. 5% | balance, approx. 18% |
| Yield strength, typical | min. 415 MPa (annealed) | min. 1030 MPa (aged) |
| Practical temperature ceiling | approx. 980 °C | approx. 650 °C |
| Density | 8.44 g/cm³ | 8.19 g/cm³ |
625 carries three times the molybdenum of 718 and less iron. In seawater, sour gas and chloride environments 625 is substantially better, and it is the grade that appears on NACE MR0175 / ISO 15156 material lists for sour service. 718 is specified in sour service too, but within tighter limits on hardness and environment.
The decision in one line. If the part has to carry load, 718. If the part has to survive the environment, 625. If both — and the temperature is below 650 °C — 718 in a qualified condition is usually the compromise, but the corrosion allowance needs reviewing.
718 is supplied and used in several conditions, and they are not interchangeable. AMS 5662 and AMS 5663 cover the common aerospace solution-treated and aged conditions; API 6A718 governs oil and gas service with tighter microstructural requirements. Specifying "Inconel 718" without the condition and governing specification leaves the most important variable undefined.
625 is supplied grade 1 (annealed) or grade 2 (solution annealed), the latter for service above roughly 600 °C where creep matters.
Aged 718 is one of the harder commercial alloys to machine. Where the geometry allows, rough machining in the solution-treated condition and ageing afterwards saves a great deal of tool life, provided the dimensional change on ageing is accounted for. 625 in the annealed condition is appreciably easier but still work-hardens readily.
Because its strength comes from precipitated particles that are only stable up to a point. Above roughly 650 °C the gamma-double-prime phase coarsens and transforms, and the strengthening disappears. 625's solid-solution strengthening has nothing to lose, so it degrades gradually rather than abruptly.
It is used, but 625 is the better choice. 718 carries roughly a third of the molybdenum and considerably more iron, so its pitting and crevice corrosion resistance is lower. Where both strength and seawater resistance are required, the trade-off should be engineered rather than assumed.
718 usually costs more per kilogram, mainly because of the heat treatment and the tighter process control its aerospace specifications demand. For aerospace-qualified material with full certification the gap widens further.
Yes, and it appears on NACE MR0175 / ISO 15156 material lists for sour environments. The applicable limits on temperature, partial pressure of hydrogen sulphide and chloride content must still be checked against the standard for the specific service.
Send us the operating temperature, the medium and the pressure. We will come back with a material recommendation and the cost difference between the two options.
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