Inconel 625 and Inconel 718 are the two nickel alloys that appear most often in aerospace specifications. Both contain niobium and both work at high temperature — but they take their strength from completely different mechanisms.
Solid solution, or precipitation?
Inconel 625 is a solid solution alloy. Molybdenum (8.00–10.0%) and niobium remain dissolved within the nickel matrix and strengthen the material by making the atomic lattice harder to shear. No heat treatment is needed; the material is used in the annealed condition.
Inconel 718 is precipitation hardening. Its composition contains 4.75–5.50% niobium + tantalum, 0.65–1.15% titanium and 0.20–0.80% aluminium. During the ageing treatment these elements combine with nickel to form finely dispersed precipitates and multiply the strength. Molybdenum, meanwhile, falls to 2.80–3.30%.
Another important difference is in iron: in 625 iron is max 5.00% while in 718 it is 12.0–24.0%. Iron makes 718 markedly more economical — one of the reasons 718 is so widespread in aerospace.
The price of that is a temperature limit. The precipitates in 718 begin to lose their stability above roughly 650 °C. 625, by contrast, can be used at far higher temperatures because its strength does not depend on precipitates.
Comparison in numbers
| PROPERTY | Inconel 625 | Inconel 718 |
|---|---|---|
| Hardening mechanism | Solid solution | Precipitation (ageing) |
| Nickel (Ni) | min 58.0% | 50.0 – 55.0% |
| Chromium (Cr) | 20.0 – 23.0% | 17.0 – 21.0% |
| Iron (Fe) | max 5.00% | 12.0 – 24.0% |
| Molybdenum (Mo) | 8.00 – 10.0% | 2.80 – 3.30% |
| Niobium + Tantalum | 3.15 – 4.15% | 4.75 – 5.50% |
| Titanium (Ti) | max 0.40% | 0.65 – 1.15% |
| Aluminium (Al) | max 0.40% | 0.20 – 0.80% |
| Boron (B) | — | max 0.006% |
| Carbon (C) | max 0.10% | max 0.08% |
| UNS | N06625 | N07718 |
| W.Nr (DIN/EN) | 2.4856 | 2.4668 |
| EN chemical symbol | NiCr22Mo9Nb | NiCr19Fe19Nb5Mo3 |
| AMS | 5599 · 5666 · 5837 | 5596 · 5663 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
Inconel 625 is preferred — seawater and chemical environment equipment, exhaust and combustion chamber components, welded structures, parts needing oxidation resistance at very high temperature. Its weldability is excellent and it requires no heat treatment.
Inconel 718 is required — turbine discs and blades, rocket engine parts, highly stressed fasteners, cryogenic applications, any part needing high strength below 650 °C. It is the nickel alloy most frequently called out in aerospace specifications, through AMS 5596 and AMS 5663.
Neighbouring alloys
If strength is needed at temperatures above those 718 can handle, Waspaloy comes into play; we explained the difference in this article. For the unalloyed equivalent of 625, see our Inconel 600 – 625 comparison.
Technical datasheets
Inconel 625 — chemical composition, mechanical values, standard equivalents
Inconel 718 — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.