AISI 316L and Inconel 625 are not two members of the same family — one is an iron-based stainless steel, the other a nickel-based alloy. There is not one step between them, but several.
From an iron matrix to a nickel matrix
In 316L, nickel is 10.0–14.0%; most of the balance is iron. In Inconel 625, nickel is the base metal at min 58.0% and iron is reduced to max 5.00%.
A nickel matrix brings two things. The first is immunity to stress corrosion cracking: this is 316L’s greatest weakness in hot chloride environments, and the risk disappears as the nickel content rises. The second is high-temperature strength: while 316L’s structural service limit is relatively low, 625 retains its strength at far higher temperatures.
The difference in alloying elements is large as well. Molybdenum rises from 2.0–3.0% to 8.00–10.0% — four times as much. Chromium rises from 16.0–18.0% to 20.0–23.0%. On top of that, 3.15–4.15% niobium + tantalum, which 316L does not contain at all, is added; this gives 625 high strength without heat treatment and provides stability after welding.
The price of this is cost: Inconel 625 costs several times as much as 316L. That is why 625 is chosen only where 316L genuinely is not enough.
Comparison in numbers
| PROPERTY | AISI 316L | Inconel 625 |
|---|---|---|
| Base metal | Iron | Nickel |
| Nickel (Ni) | 10.0 – 14.0% | min 58.0% |
| Chromium (Cr) | 16.0 – 18.0% | 20.0 – 23.0% |
| Molybdenum (Mo) | 2.0 – 3.0% | 8.00 – 10.0% |
| Iron (Fe) | Balance | max 5.00% |
| Niobium + Tantalum | — | 3.15 – 4.15% |
| Carbon (C) | max 0.03% | max 0.10% |
| Cobalt (Co) | — | max 1.00% |
| Tensile strength Rm | 485 MPa | — |
| Yield strength Rp0.2 | 170 MPa | — |
| Elongation A50 | 40% | — |
| Density | 8.00 g/cm³ | 8440 kg/m³ |
| UNS | S31603 | N06625 |
| W.Nr (DIN/EN) | 1.4404 | 2.4856 |
| EN chemical symbol | — | NiCr22Mo9Nb |
| AMS | 5653 · 5507 | 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?
AISI 316L is sufficient — food, pharmaceutical and beverage pipework, general chemical process equipment, welded tanks and pipes, moderate chloride loads, room and mild temperatures. It is economical and readily available in every form.
Inconel 625 is required — seawater and offshore equipment, flue gas desulphurisation systems, hot acidic processes, aircraft exhaust and fuel systems, nuclear and chemical reactor internals. Anywhere pitting, crevice corrosion or stress cracking is seen on 316L.
If the difference strains the budget, it makes sense to look at the intermediate steps before jumping straight to 625.
Intermediate steps
AISI 904L, duplex AISI 318 and super duplex AISI F53 gradually fill the gap between 316L and Inconel 625. For a step up on the nickel alloy side, see our Hastelloy C-276 – C-22 comparison.
Technical datasheets
AISI 316L — 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.