​‌​​‌​

Incoloy 925 / UNS N09925

​‌​​‌​

Incoloy 925, also widely known as Alloy 925, is a high-nickel alloy. Containing around 45% nickel, it is made thoroughly corrosion resistant by the various other elements it contains. Also designated UNS N09925, the material can be hardened by ageing. Formed essentially from a combination of nickel, chromium and iron, it carries various additions such as molybdenum, copper, titanium and aluminium. The material has very good, high mechanical strength, and at the same time its resistance to corrosion is excellent. In Alloy 925, the combination of nickel with molybdenum and copper produces outstanding resistance to a wide range of chemicals, while the titanium and aluminium it contains give Incoloy 925 additional mechanical strength during heat treatment.

Like all nickel materials, Incoloy Alloy 925 is a costly material. It is chosen where high mechanical strength is required and where excellent corrosion resistance is required at the same time. It is used in valves, in hangers working in special chemicals, in special nipples and fasteners, at component joints, in screws and nuts, in various marine applications, in special shafts, in pumps and in many round bar components that are loaded and work under load. Another application area for Alloy 925 is the glass industry.​‌​​‌​

​‌​​‌​

What 925 Actually Is — and the First Correction

Incoloy 925 (UNS N09925) is an age-hardenable 825. The Ni-Cr-Fe-Mo-Cu backbone is the same; titanium is raised from 0.6–1.2 % to 1.9–2.4 % and 0.10–0.50 % aluminium is added, so that ageing precipitates γ′ Ni₃(Al,Ti). The corrosion chemistry is essentially unchanged; the strength triples.​‌​​‌​

The first correction is in the material number: many pages print 2.4858 for 925 — 2.4858 is alloy 825. Alloy 925 is 2.4852 (EN NiCr20FeMo3TiCuAl). Some sites put 2.4858 in the page title and copy 925 data underneath it.

Composition (B805): Ni 42.0–46.0 % · Cr 19.5–22.5 % · Fe 22.0 % min (balance) · Mo 2.50–3.50 % · Cu 1.50–3.00 % · Ti 1.90–2.40 % · Al 0.10–0.50 % · Nb(+Ta) ≤0.50 % · Mn ≤1.00 % · Si ≤0.50 % · C ≤0.03 % · S ≤0.03 % · P ≤0.03 %. Some sources also give a 0.08 % minimum for Nb; the originator and one European mill print a maximum only — know both. Oilfield (API 6ACRA) bar commonly carries restricted chemistry: C ≤0.025 % · P ≤0.020 % · S ≤0.003 % · Si ≤0.35 % — tighter than ASTM and it must be stated on the order. One distributor’s “Cu 0.23 % max” is impossible (copper is a deliberate 1.5–3.0 % addition) — a typographical error.

What the additions do: Titanium (1.9–2.4 %) is the primary strengthener; with aluminium it forms the coherent γ′ Ni₃(Al,Ti) precipitate on ageing — this is the whole difference between 825 and 925. Aluminium (0.10–0.50 %) raises the γ′ volume fraction and stability, and acts as a deoxidiser. Molybdenum (2.5–3.5 %) gives pitting and crevice resistance (PREN ≈26). Copper (1.5–3.0 %) is inherited from 825 and markedly improves resistance in reducing acids, especially sulphuric.​‌​​‌​

Heat Treatment and Ageing — the Centrepiece of This Page

​‌​​‌​

​‌​​‌​

Solution anneal

980–1040 °C (1800–1900 °F), minimum 30 minutes, maximum 4 hours. For sections 25 mm (1 in) and under, cool at a rate equivalent to air cooling or faster; water quench all sizes over 1 in. One European mill specifies 980–1040 °C for 2 hours followed by water quenching — the temperature band is verified in two sources, the hold time and cooling detail differ. A second, shorter anneal appears for pre-weld and re-solution work: 1040 °C / 1 h / air cool. An aggregator figure of “1010 °C for at least 2 h, air cooled” conflicts with the water-quench requirement above 1 in — do not follow it for heavy sections.​‌​​‌​

The standard two-step age

732–749 °C (1350–1380 °F), nominally 740 °C — hold 6 to 9 hours; furnace cool to 621 °C ± 8 °C; hold at 621 °C until the total precipitation heat-treatment time reaches 18 hours; then air cool or faster. The originator and the ASTM B805 cycle match on all of these points. Divergence (do not average): one European mill specifies 740 °C for 6 h, furnace cool to 621 °C, hold 6 h, air cool — roughly 12 hours total, not 18. A six-hour-shorter low-temperature step lands you lower in the strength band; if your order is to B805 or 6ACRA, the 18-hour total governs.​‌​​‌​

Why two steps: the 732–749 °C step nucleates and grows γ′ Ni₃(Al,Ti); the furnace cool and the long 621 °C hold precipitate a second, finer γ′ population. That is what lifts yield strength into the 110 ksi band without pushing hardness past the NACE cap.

NACE / ISO 15156 — What the Certificate Proves and What It Does Not

N09925 is listed in Annex A of ANSI/NACE MR0175/ISO 15156-3, among the precipitation-hardened nickel-based alloys, sharing a table with N07718. But the standard does not approve alloys: it approves an alloy in a stated metallurgical condition, below a stated hardness, inside a stated environmental envelope.​‌​​‌​

Welding, Machining and Forming​‌​​‌​

Preferred processes are GTAW and GMAW. For GMAW in spray-arc transfer the current must not exceed 180 A. SAW and SMAW are NOT recommended by the mill — a widely copied aggregator page lists both as applicable methods; that is wrong. Filler: INCO-WELD 725NDUR = AWS A5.14 ERNiCrMo-15 (UNS N07725); filler chemistry Ni+Co 55.0–59.0 %, Cr 19.0–22.5 %, Mo 7.0–9.5 %, Nb+Ta 2.75–4.00 %, Ti 1.0–1.7 %. The originator states this gives better properties than a matching 925-composition wire — deliberate over-alloying, because a weld deposit cannot be given the base metal’s wrought-plus-aged microstructure.

Post-weld ageing — read this carefully. The cycle published for 725NDUR is 1038 °C/1 h + 732 °C/8 h, furnace cool to 621 °C, 8 h, air cool. That cycle begins with a full solution anneal: so for a welded 925 assembly intended to reach full aged strength, the correct route is to solution anneal the whole weldment, then age — not “age the weld only”. Directly ageing an as-welded joint leaves the HAZ in a mixed condition: the region that saw 800–1000 °C during welding is partially solutioned, and the region that saw 600–750 °C is already partly aged and will over-age, so hardness across the HAZ becomes non-uniform. That matters because NACE compliance is a hardness requirement on every part of the component, weld and HAZ included.​‌​​‌​

No preheat, interpass temperature or heat-input limits are printed on this page — neither the originator nor the European mill reviewed publishes figures for this alloy; they must be fixed by a qualified WPS/PQR. Other pitfalls: (1) γ′-strengthened Ni-Fe-Cr alloys are susceptible to strain-age cracking in the HAZ if welded in the fully aged condition — weld solution-annealed, then age. (2) The 180 A GMAW ceiling exists to hold heat input down. (3) Sulphur and phosphorus picked up from contaminated surfaces promote hot cracking. (4) A weld procedure qualified on 825 does not qualify 925 — different filler, different PWHT, different acceptance hardness.

Corrosion — Where It Is Strong and Where It Is Not

Physical properties: density 8.08 g/cm³ · melting range 1311–1366 °C · modulus of elasticity 199 GPa · shear modulus 77 GPa · Poisson’s ratio 0.293 (one source 0.281) · electrical resistivity 1.17 µΩ·m · thermal conductivity 12.0 W/m·K · specific heat 435 J/kg·°C · mean CTE (20–100 °C) 13.2 × 10⁻⁶/K. Magnetic permeability 1.001 at 200 oersteds — effectively non-magnetic, and it stays that way even cold-worked and aged (there is no martensitic transformation), which makes it usable for MWD/LWD and downhole instrument housings. Unit trap: several sites republish the mill’s 7.8 µin/in·°F as “µm/m·°C”; the correct conversion is ≈14.0 µm/m·°C.

Frequently Asked Questions​‌​​‌​

Incoloy 925, 825, Monel K-500 or Inconel 718 — when is each actually required?

Start from the precise relationship: 925 is an age-hardenable 825. The two share the Ni-Cr-Fe-Mo-Cu backbone — 825 is Ni 38–46 / Cr 19.5–23.5 / Mo 2.5–3.5 / Cu 1.5–3.0 / Ti 0.6–1.2; 925 tightens Ni to 42–46 and Cr to 19.5–22.5, raises Ti to 1.9–2.4 % and adds Al 0.10–0.50 % so that ageing precipitates γ′ Ni₃(Al,Ti). The corrosion chemistry is essentially unchanged; the strength triples. Annealed 825 bar runs about 324 MPa (47 ksi) yield typical; 925 solution-annealed-and-aged is guaranteed to 758 MPa (110 ksi) and runs 120 ksi typical. So: choose 825 when the part carries little load — heat-exchanger tube, vessel lining, piping — and you want an ASME-listed, weldable, cheaper material. Choose 925 when the same corrosion environment now has to carry stress — hangers, mandrels, landing nipples, packers, valve stems, high-strength fasteners. Monel K-500 is also γ′-hardened, but it is Ni-Cu with no chromium: excellent in flowing seawater and non-oxidising acids, but with no passivating chromium it is not a sour-service CRA in the way 925 is, and it is documented as susceptible to hydrogen embrittlement under cathodic protection. Inconel 718 is γ″ Ni₃Nb-strengthened, reaches about 150 ksi yield and shares the NACE table with 925 — but with less chromium and no copper it is weaker in reducing acids, and it costs more. Use 718 when you need strength 925 cannot reach; use 925 when 718’s extra strength is not required, which is the common oilfield case.​‌​​‌​

Sour service: what does the mill certificate actually prove?

A mill certificate proves material identity and condition. It proves nothing about whether the material is legal in your well. MR0175/ISO 15156-3 does not approve alloys; it approves an alloy in a stated metallurgical condition, below a stated hardness, inside a stated environmental envelope. For N09925 the caps are 35 HRC cold-worked, 38 HRC solution-annealed-and-aged, 40 HRC cold-worked-and-aged, and the envelope runs from 232 °C / 30 psia H₂S down to 149 °C / 400 psia, with elemental sulfur permitted only to 135 °C. A certificate that says “N09925, NACE MR0175” and nothing else is not compliance evidence — it is a marketing line. The certificate must state the condition, the measured hardness (not “≤38 HRC” but the value and where it was taken), and ideally the heat-treatment cycle actually run. Put on the order, explicitly: the product-form standard and edition (ASTM B805-24 for bar, B872-26 for plate, B983-21 for seamless tube); API 6ACRA if it is API 6A equipment, with the Annex A microstructure acceptance; the condition (hot-worked SA+aged and cold-worked SA+aged carry different yield minima, 110 against 105 ksi); the 110–140 ksi yield band, which is a maximum as well as a minimum; hardness 26–38 HRC; Charpy 47 J average / 43 J minimum at −60 °C; ANSI/NACE MR0175/ISO 15156-3 with the edition named; and restricted chemistry (S ≤0.003 %, P ≤0.020 %) if your specification demands it. Finally: hardness must be met in welds and HAZ too, so the WPS is part of the compliance chain, not separate from it.​‌​​‌​

The most expensive paperwork error: “ASTM B805 pipe”, and the forgings gap

Distributors routinely advertise “Incoloy 925 seamless pipe to ASTM B805” and “925 forgings to ASTM B805 / B564“. Both are wrong, and both will fail a document review. ASTM B805-24 is titled “Precipitation Hardening Nickel Alloys Bar and Wire.” Its scope is hot- or cold-finished rounds, squares, hexagons and rectangles, plus cold-finished wire — and it explicitly excludes forgings, pipe, tube, plate and sheet. The correct citations are ASTM B983-21 for seamless pipe and tube, ASTM B872-26 for plate, sheet and strip, and ASTM B1007-21 for welded tube (tube only, ⅛–6 in; it does not cover pipe). The forgings gap is the expensive one: ASTM B564-25 “Nickel Alloy Forgings” does not list N09925 — its list includes the parent N08825 but not 925 — and B805 excludes forgings. Several fastener distributors cite ASTM B637 for N09925 forgings; B637’s inclusion of this grade could not be verified against ASTM. So there is no confirmed ASTM forging specification for this alloy. What to do: buy 925 forgings to API 6ACRA plus a named customer specification — the pattern real oilfield suppliers use. If a quotation offers 925 forgings “to ASTM B805” or “to ASTM B564”, the supplier has not read either standard.​‌​​‌​

​‌​​‌​