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Incoloy 825 / (2.4858) / UNS N08825

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Incoloy 825 (2.4858) is one of the three most widely used nickel alloys. Similar in composition to 904L (1.4539) stainless steel, this material has far higher corrosion resistance than any stainless steel.

Alloy 825 is also designated UNS N08825 in the UNS system. Resembling stainless steel in composition, the material is formed essentially from nickel and chromium. In Incoloy 825 iron is added to the nickel and chromium, and various further elements such as molybdenum, copper and titanium are added to that trio in order to strengthen the material against corrosion and to raise its resistance.​‌​​‌​

With outstanding corrosion resistance, this material was designed and produced for use in extremely harsh and hostile environments. Carrying a very high content of quality alloying elements, it is expensive, and both the service environment and the necessity of the material should be assessed carefully before it is specified. Usable from time to time even in sulphuric and phosphoric acid environments, the material is highly resistant to corrosion thanks to the additional elements it contains such as copper and molybdenum. It has better corrosion resistance than stainless steels, and thanks to the combination of all these elements Alloy 825 withstands even nitric acid, nitrates and chloride environments.

Its applications and service environments are very wide and numerous. Incoloy 825 (Alloy 825) is frequently chosen and used in environments where chemical reactions take place, in environments where chemical processes are carried out, in filters, in pollution control systems, in oil and gas systems, in acid production plants, in nuclear power plant components, and in the collection and containment of radioactive waste.​‌​​‌​

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Standards by Product Form

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Incoloy 825 (UNS N08825 / W.Nr. 2.4858) is a Ni-Fe-Cr-Mo-Cu alloy built to work in both reducing and oxidising environments. Despite the “Incoloy” name it is not a high-temperature alloy — it is a wet-corrosion alloy, and above about 540 °C it loses ductility and toughness to phase formation.

Composition: Ni 38.0–46.0 % · Cr 19.5–23.5 % · Fe 22.0 % min · Mo 2.50–3.50 % · Cu 1.50–3.00 % · Ti 0.60–1.20 % · C ≤0.05 % · Mn ≤1.0 % · Si ≤0.5 % · Al ≤0.2 % · S ≤0.03 %. Mill limits are tighter than ASTM in places: one European mill runs C ≤0.025 %, S ≤0.015 %, P ≤0.02 % and Co ≤1.0 %; another gives S ≤0.010 %. A heat that passes B425 may fail a mill-grade enquiry. And note: one distributor publishes titanium as 0.06–1.2 % — that is a typo for 0.60–1.20 %; a genuine 0.06 % titanium would destroy the stabilisation the alloy depends on.

What the three signature additions do. Titanium (0.6–1.2 %): in the originator’s words it “serves, with an appropriate heat treatment, to stabilize the alloy against sensitization to intergranular corrosion” — it ties up carbon as TiC so chromium carbides do not precipitate on grain boundaries and deplete them of chromium. Note the conditional clause: titanium alone does not do it. Molybdenum (2.5–3.5 %): aids resistance to pitting and crevice corrosion. Copper (1.5–3.0 %): with nickel and molybdenum it gives “outstanding resistance to reducing environments such as those containing sulfuric and phosphoric acids” — this is where 825 beats plain Ni-Cr-Mo grades, and why the alloy exists as a distinct grade rather than being replaced by 904L or 625. Chromium gives resistance to oxidising substances (nitric acid, nitrates, oxidising salts); nickel gives resistance to chloride-ion stress-corrosion cracking.​‌​​‌​

Physical properties: density 8.14 g/cm³ · melting range 1370–1400 °C · specific heat (20 °C) 440 J/kg·K · thermal conductivity about 11.1 W/m·K (sources diverge between 10.8 and 11.1) · modulus of elasticity 196 GPa (static) or 193 GPa (dynamic) — both defensible. Magnetic behaviour: fully austenitic and effectively non-magnetic in the annealed condition (µ = 1.005 at 200 Oe); the Curie point is below −196 °C, so it stays non-magnetic to cryogenic temperatures. No source quantifies the permeability rise after cold work — so do not claim it “remains non-magnetic after cold working”. The mean coefficient of thermal expansion could not be verified and is not printed here.​‌​​‌​

Heat Treatment — It Says “Annealed”, But Which Anneal?

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Not hardenable by heat treatment. It is solid-solution strengthened; strength above the annealed minimums is obtained only by cold work (1000 MPa tensile in cold-drawn tubing).

Sensitisation, in the originator’s own words: “Heavy sections may become sensitized during cooling from the hot-working temperature, and therefore be subject to intergranular corrosion in certain media. A stabilizing anneal… restores resistance to corrosion.” Titanium stabilisation is only realised if the material gets a stabilising heat treatment; it is not automatic.

Embrittlement / phase window: “Exposure to temperatures above about 540 °C (1000 °F) can result in microstructural changes (phase formation) that significantly lower ductility and impact strength.” The continuous-service ceiling is given as about 550 °C. Which phase forms (σ, Ti-rich or other) could not be verified and is not guessed here.​‌​​‌​

Welding, Machining and Forming

Welding — two legitimate but different philosophies​‌​​‌​

There is a real distinction that the trade literature mixes up. (A) Matching-composition consumables (Ni-Fe-Cr-Mo-Cu, i.e. 825-like): AWS A5.14 ERNiFeCr-1 (ISO 18274 S Ni 8065) for GTAW/GMAW; the originator’s matching pair is INCOLOY Filler Metal 65 (gas-shielded) and INCOLOY Welding Electrode 135 (SMAW); another mill offers AWS A5.4 E383-16 as an SMAW alternative — note that this is a stainless (27Cr-31Ni-4Mo-Cu) electrode, not a nickel-base one. (B) Over-alloyed Ni-Cr-Mo (625-type) consumables — what the originator now recommends: “For most applications, INCONEL Welding Electrode 112 for shielded metal-arc welding and INCONEL Filler Metal 625 for gas-shielded processes are used.” Filler Metal 625 = AWS A5.14 ERNiCrMo-3. (C) Highest corrosion resistance: INCO-WELD 686CPT electrode and filler metal.

Buyer-facing summary: 625-type (ERNiCrMo-3) is the workhorse and gives an over-matched, over-alloyed weld; ERNiFeCr-1 / FM 65 is the chemistry-matching choice and is what you want when weld-metal chemistry must match the base metal for a corrosion or NACE case. No source consulted recommends ERNiCr-3 (alloy 82), ENiCrFe-2/-3 (the 600/800 family) or any 316L-type filler for 825 — if a fabricator proposes one, ask for the source. Parameters: heat input <1.0 kJ/mm, interpass temperature <100 °C (single-sourced). No source specifies a preheat requirement — that is the absence of a requirement, not a statement that none is needed; do not claim either way. “No post-weld heat treatment is required” is stated, but qualify it: heavy sections, or weldments that will see intergranular-attack-prone media, may still warrant a stabilising anneal even though PWHT is not a general requirement.​‌​​‌​

Machining and forming

“All standard machining operations are readily performed… The alloy normally has optimum machining characteristics in the annealed temper.” Cutting speeds are not printed on this page — the separate machining publication the originator defers to could not be reached. Cold forming: “properties and practices are essentially the same as for alloy 600. Although the work-hardening rate is somewhat less than for the common grades of austenitic stainless steels, it is still relatively high.” That directly contradicts the very common trade claim that 825 work-hardens faster than 304/316. For the hot-working range sources diverge: 870–1180 °C and 900–1150 °C — quote both; the second source also instructs rapid cooling after hot working, which ties directly to the sensitisation warning.​‌​​‌​

Corrosion Behaviour — the Alloy’s Whole Reason to Exist

Frequently Asked Questions

825, 625, C-276, 904L or super duplex — when is each actually required?​‌​​‌​

These five are not a quality ladder; they solve different problems. 904L is a stainless steel — the cheapest route to copper-bearing sulphuric-acid resistance, but its nickel content is far below 825’s 38–46 %, so it offers much less chloride-SCC margin. Incoloy 825 is the value grade where you need both reducing-acid resistance (from its 1.5–3.0 % Cu plus 2.5–3.5 % Mo, which the originator credits with “outstanding resistance to reducing environments such as those containing sulfuric and phosphoric acids”) and high chloride-SCC immunity from nickel. Super duplex (S32750) beats 825 badly on strength and on pitting (PREN ≥40; no published PREN exists for 825) and is usually cheaper — but in sour service it is limited to 232 °C and 0.20 bar (3 psi) H₂S, whereas 825 is listed as acceptable under ISO 15156 “with no environmental limits”. That H₂S ceiling is the true cross-over point: below about 3 psi H₂S, super duplex usually wins on cost and strength; above it, 825 is the entry ticket. Inconel 625 (Ni ≥58 %, Mo 8–10 %, Nb 3.15–4.15 %) roughly triples the molybdenum and takes you to 982 °C service — you pay for it, and you need it only when pitting or crevice severity, or temperature, exceeds 825. C-276 is the strong-reducing-acid (HCl/HF) alloy; no source consulted recommends 825 for those duties, so specifying 825 there is a false economy.

Sour service: what does the mill certificate actually prove?​‌​​‌​

Four independent sources confirm 825 is listed in NACE MR0175 / ISO 15156, and one mill states it is acceptable “in the cold worked and annealed condition with no environmental limits in respect of partial pressures of H₂S or elemental sulfur”. But that listing is about the alloy, not about your heat. A standard EN 10204 3.1 certificate for ASTM B425 or B423 proves chemistry and room-temperature tensile properties. It does not by itself prove: that the delivered hardness is within the ISO 15156 cap; that the heat-treatment condition is the one the standard’s table covers; that any cold work is within the permitted band; or that welds and weld consumables are qualified. Be aware of two honest gaps: this research could not verify the ISO 15156-3 Annex A table number or the maximum hardness figure for N08825 in any source — treat any web page quoting one without citing the standard as unreliable. One mill’s own datasheet also contradicts itself, saying “no environmental limits… elemental sulfur” in one place and, quoting institute tests, “in the absence of elementary sulfur” in another. Therefore specify on the purchase order: (1) “manufactured, tested and certified in full compliance with NACE MR0175/ISO 15156-3, latest edition, for material type N08825”; (2) the delivery condition (annealed / cold worked) and maximum hardness with test method and location; (3) EN 10204 3.2 where a third party is warranted; (4) that weld procedures and consumables are separately MR0175-qualified.

“It says annealed on the certificate” — but which anneal?​‌​​‌​

This is the most under-reported trap on 825, and it costs real money. Incoloy 825 relies on its 0.6–1.2 % titanium to prevent sensitisation — but the originator’s own wording is conditional: titanium “serves, with an appropriate heat treatment, to stabilize the alloy against sensitization to intergranular corrosion”. The same bulletin warns that “heavy sections may become sensitized during cooling from the hot-working temperature, and therefore be subject to intergranular corrosion in certain media. A stabilizing anneal… restores resistance to corrosion.” Now look at what three mills publish as “the” anneal: the originator says 930–980 °C with the lower end explicitly preferred for stabilisation; a European mill says 920–980 °C, preferably 940 ± 10 °C; a third specifies a solution anneal at 1000–1100 °C for tube. These are two different heat treatments, not a tolerance band. A high-temperature solution anneal dissolves titanium carbides; if cooling through the carbide range is not fast enough — likely in thick plate, forgings and heavy fittings — the part can leave the mill “annealed” and still be sensitised. What to do: for heavy sections, or any duty where intergranular attack matters, specify the stabilising anneal at 930–950 °C with a rapid quench, name it explicitly on the order, and require an intergranular-corrosion test (an ASTM A262 or G28-type practice agreed with the mill) on the delivered condition. Note too the instruction to cool rapidly after hot working.

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