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Incoloy 925 (UNS N09925) and Inconel 625 (UNS N06625) are both high-performance nickel-base alloys used in oil & gas and marine environments, but they were developed for different primary design drivers. Incoloy 925 is an age-hardenable Fe–Ni–Cr–Mo–Cu alloy derived from Alloy 825 chemistry, engineered to deliver 110–120 ksi yield strength with NACE MR0175 SSC resistance for downhole completions in sour (H₂S/CO₂) wells. Inconel 625 is a solid-solution strengthened Ni–Cr–Mo–Nb alloy with ~21–23% Cr, ~8–10% Mo, and ~3.15–4.15% Nb—renowned for exceptional pitting/crevice corrosion resistance (PREN ≥ 45), seawater immunity, and usable high-temperature strength to ~650–870°C. Choosing incorrectly can mean overspending or underspecifying safety. This guide breaks it down technically.
1. Chemical Composition & Metallurgical Difference
|
Element |
Incoloy 925 (N09925) |
Inconel 625 (N06625) |
|---|---|---|
|
Ni |
38.0 – 46.0% |
≥ 58.0% (58–63%) |
|
Cr |
19.5 – 22.5% |
20.0 – 23.0% |
|
Fe |
≥ 22.0% (Balance) |
≤ 5.0% (typ. 2–4%) |
|
Mo |
2.5 – 3.5% |
8.0 – 10.0% |
|
Cu |
1.5 – 3.0% |
≤ 0.50% (not added) |
|
Nb (+Ta) |
1.8 – 2.4% (Ti+Al form γ′) |
3.15 – 4.15% (forms γ″ in aged condition) |
|
Ti |
1.8 – 2.4% (part of γ′) |
≤ 0.40% |
|
Al |
0.10 – 0.50% (γ′ co-former) |
≤ 0.40% |
|
C |
≤ 0.03% |
≤ 0.10% (L-grade ≤ 0.03%) |
|
Strengthening Mechanism |
Age hardening → γ′(Ni₃(Ti,Al)) precipitation after 650–680°C aging |
Solid-solution + minor γ″ (if aged/ welded & aged); primarily solution-strengthened |
Key takeaway: 625 has roughly double the Mo (→ far better pitting/crevice), far less Fe (→ better seawater & broader acid resistance), and much higher Ni (→ better high-temp & chloride SCC). 925 trades some corrosion margin for lower material cost + built-in high-yield-age capability.
2. Mechanical Properties & Strength Comparison
|
Property (RT) |
Incoloy 925 (Aged per ASTM B805) |
Inconel 625 (Annealed per ASTM B443) |
|---|---|---|
|
Tensile Strength Rm |
≥ 965 MPa (140 ksi); typ. 1000–1150 MPa |
≥ 827 MPa (120 ksi); typ. 860–1000 MPa |
|
Yield Strength Rp0.2 |
≥ 758 MPa (110 ksi); typ. 790–930 MPa |
≥ 414 MPa (60 ksi); typ. 450–550 MPa |
|
Elongation A₅₀ |
≥ 12%; typ. 15–20% |
≥ 30%; typ. 35–45% |
|
Hardness |
28–38 HRC (aged) |
~85–95 HRB (annealed); can be work-hardened |
|
High-Temp Strength |
γ′ overages > ~590°C → strength drops; not for high-T creep |
Retains useful strength to ~650–760°C; some aged variants (625 Plus®) to ~870°C |
|
Low-Temp Toughness |
Charpy V-notch ≥ 34 J @ −46°C |
Excellent to −196°C (no DBTT) |
Rule: Need ≥110 ksi yield for sour-well load-bearing → Incoloy 925. Need high general corrosion + pitting + seawater + moderate-high temp strength but no ultra-high yield → Inconel 625 (or 625 aged/Plus for higher YS).
3. Corrosion / Environmental Resistance Comparison
|
Environment |
Incoloy 925 (N09925) |
Inconel 625 (N06625) |
Comment |
|---|---|---|---|
|
Sour Gas H₂S/CO₂ (NACE MR0175 envelope) |
★★★★☆ (110 ksi YS, SSC resistant) |
★★★☆ (annealed OK in low H₂S; high strength aged may need review) |
925 purpose-built for this |
|
CO₂ Sweet Corrosion (≤150°C) |
★★★★☆ |
★★★★☆ |
Both far better than 13Cr/Cr-Mo |
|
Chloride SCC (aqueous, RT–150°C) |
★★★★★ (austenitic) |
★★★★★ (austenitic) |
Both immune vs. 300 SS |
|
Seawater Pitting / Crevice (PREN) |
PREN≈35–38 ★★★★ |
PREN≥45–48 ★★★★★ |
625 clearly superior for full-immersion seawater |
|
Sulfuric / Phosphoric Acid (dilute) |
★★★★ (Cu effect, like 825) |
★★★ (Mo helps but no Cu) |
925 slightly better in dilute H₂SO₄ |
|
Hydrochloric / HF (strong reducing) |
✗ Not recommended |
△ Limited (C-276 better) |
Neither is a true reducing-acid alloy |
|
High-Temp Oxidation (to ~870°C) |
Usable to ~750–800°C (strength fades >480°C) |
★★★★☆ to 870°C (no creep like 800HT but oxidation OK) |
625 better for warm exhaust/shroud |
|
Sulfidation (low O₂, high S, >650°C) |
Limited |
Slightly better (higher Ni/Cr) but not prime choice |
→ Inconel 601/671 if severe |
4. Typical Applications — When to Pick Which
✅ Choose Incoloy 925 (N09925) when:
Downhole sour (H₂S) + CO₂ well completions requiring API 110 ksi (or 120 ksi) yield: packer mandrels, polished bore receptacles, subsurface safety valve stems, hanger components.
Cost-sensitive vs. 625 but NACE MR0175 SSC compliance & Cl⁻ SCC immunity mandatory.
Service temperature typically −46°C to ~120–150°C (wellbore), occasionally to 175°C max for corrosion; nota high-temp alloy.
✅ Choose Inconel 625 (N06625) when:
Seawater full-immersion service: propeller shafts, fairleads, bellows, riser tensioning hardware, subsea bolting.
Marine / Offshore high pitting risk where PREN > 40 is required.
Chemical process with mixed acids + halides where broader acid + pitting resistance needed than 925 offers.
Aerospace / high-temp exhaust (to ~650–760°C useful strength, to 870°C oxidation): bellows, ducting, flame shields (though 718 or 600/601 may be preferred depending).
Clad plate / weld overlay for pressure vessels in sour service (625 clad on CS saves cost vs solid 925/625).
❌ Don't substitute blindly:
925 → for seawater full-immersion long term (pitting risk vs. 625).
625 → for high-yield sour-well load-bearing mandrels unlessyou specify aged/high-strength 625 variant (more expensive) and verify NACE compliance at that strength level.
5. Cost & Procurement Note
Relative Base Cost Index (Ni-price sensitive): Incoloy 925 ≈ 70–85% of Inconel 625 (fewer Mo, more Fe). Actual spread varies with LME Ni.
Availability: 925 usually in bar/forging (aged or ST); 625 widely available in sheet/plate/bar/pipe/tube/wire/clad.
Weld Filler:
925: typically ERNiCrMo-3 (Inconel 625 type) or ERNiCrMo-4 if matching higher Mo desired.
625: ERNiCrMo-3 (AWS A5.14) standard; post-weld heat treatment depends on service (often as-welded for corrosion; aged if strength recovery needed in aged version).
Shanghai COCESS Special Alloys Co., Ltd. stocks Incoloy 925 (UNS N09925) in round bar / forged rod (solution-annealed or solution + aged) and Inconel 625 (UNS N06625) in plate, sheet, bar, and seamless pipe. All shipments include EN 10204 3.1 MTC; UT, PMI, and third-party inspection (SGS / TÜV / BV / DNV) available. We assist with grade selection per your well conditions (p_H₂S, p_CO₂, Cl⁻, T, load) or marine environment (immersion vs. splash/spray).
Website: https://en.cocessalloys.com
Email: sales@cocessalloys.com
