Hastelloy X and Inconel 625 often appear on the same drawing in the hot sections of gas turbines and combustion systems. Both are solid solution alloys and neither requires heat treatment. The difference is in which condition they were optimised for.
High iron, low molybdenum
According to our datasheets, Hastelloy X has nickel at 47.5–61%, chromium at 20.5–23.0%, iron at 17–20% and molybdenum at 8.0–10.0%. Inconel 625 has nickel at min 58.0%, chromium 20.0–23.0%, iron max 5.00% and molybdenum 8.00–10.0%.
Chromium and molybdenum are almost identical. The decisive difference is in iron: 17–20% in Hastelloy X against below 5% in 625. Iron lowers the cost and contributes to structural stability at high temperature; in exchange it weakens resistance to aqueous corrosion.
The second difference is niobium. Inconel 625 contains 3.15–4.15% niobium + tantalum; Hastelloy X contains none. Niobium gives 625 solid solution strength and stability after welding.
The result: Hastelloy X is a furnace and combustion chamber alloy designed for oxidation resistance and creep strength at temperatures reaching 1200 °C. Inconel 625 is a far more broadly used alloy that withstands both high temperature and aqueous corrosion.
Comparison in numbers
| PROPERTY | Hastelloy X | Inconel 625 |
|---|---|---|
| Nickel (Ni) | 47.5 – 61% | min 58.0% |
| Chromium (Cr) | 20.5 – 23.0% | 20.0 – 23.0% |
| Iron (Fe) | 17 – 20% | max 5.00% |
| Molybdenum (Mo) | 8.0 – 10.0% | 8.00 – 10.0% |
| Niobium + Tantalum | — | 3.15 – 4.15% |
| Carbon (C) | — | max 0.10% |
| Cobalt (Co) | Present | max 1.00% |
| Aluminium (Al) | — | max 0.40% |
| Titanium (Ti) | — | max 0.40% |
| UNS | N06002 | N06625 |
| W.Nr (DIN/EN) | 2.4665 | 2.4856 |
| EN chemical symbol | NiCr22Fe18Mo | NiCr22Mo9Nb |
| AMS | 5536 · 5754 | 5599 · 5666 · 5837 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
Hastelloy X is required — gas turbine combustion chamber liners, transition ducts, flame holders, industrial furnace muffles and retorts, heat treatment fixtures. Where oxidation and creep resistance at very high temperature come first. If the specification calls out AMS 5536 or AMS 5754, this is the grade being asked for.
Inconel 625 is required — marine and chemical equipment, exhaust and fuel systems, flue gas desulphurisation, offshore pipelines, parts where temperature and aqueous corrosion occur together. If there is an aqueous environment, the iron in Hastelloy X becomes a problem.
Neighbouring alloys
For precipitation hardening alternatives at high temperature, see our Inconel 625 – 718 and Inconel 718 – Waspaloy comparisons.
Technical datasheets
Hastelloy X — chemical composition, mechanical values, standard equivalents
Inconel 625 — 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.