Incoloy 825 (UNS N08825 / 2.4858): Complete Alloy Composition & Mechanical Properties Guide

Incoloy 825 (UNS N08825 / W.Nr. 2.4858 / GB NS1402) is a titanium-stabilized, nickel-iron-chromium alloy with additions of molybdenum, copper, and a controlled low carbon content. It is not​ a precipitation-hardenable superalloy. Its corrosion resistance is derived from solid-solution strengthening combined with high nickel content (immunity to chloride-induced stress corrosion cracking), molybdenum (pitting/crevice resistance), and copper (resistance to reducing acids such as sulfuric and phosphoric acid). Titanium stabilization (Ti/C ≥ 8) prevents sensitization and intergranular corrosion after welding without requiring post-weld solution annealing. This guide provides strict, specification-based technical data for design reference and material evaluation.

1. Chemical Composition (wt%, per ASTM B424 / ASME SB424 / NACE MR0175)

Pitting Resistance Equivalent Number (PREN):​

PREN = %Cr + 3.3 × %Mo + 16 × %N ≈ 32–35​ (vs. 316L ≈ 24, 904L ≈ 33–35, 6Mo SS ≈ 40–43, Alloy C-276 ≈ 68+)

2. Physical Constants

3. Mechanical Properties — Solution-Annealed Condition (per ASTM B424 / B423 / B425)

Incoloy 825 is solution-annealed only​ (typically 980–1040°C, rapid quenched). It is not​ age-hardenable.

Room-Temperature Minimum Requirements (Plate / Tube / Bar):

Representative Elevated-Temperature Values (Solution-Annealed, Typical):

Incoloy 825 is not​ intended for high-load, long-term service > 450–480°C (creep strength is modest). For high-temperature load-bearing, consult Incoloy 800HT, Inconel 600/601, or A-286 / Inconel 718 depending on temperature.

4. Corrosion-Resistance Mechanism Summary

• Chloride Stress Corrosion Cracking (SCC):​ Ni ≥ 42% renders alloy essentially immune to transgranular Cl⁻-SCC that attacks 300-series SS > 60°C.

• Pitting / Crevice Corrosion:​ Mo (~3%) + high Cr → PREN 32–35; suitable for seawater splash/spray zones and desalination equipment (superior to 316L / 2205 in SCC, slightly less pitting-resistant than 6% Mo SS or C-276).

• Reducing Acids (H₂SO₄ / H₃PO₄):​ Cu (1.5–3%) enables service in sulfuric acid up to ~40 wt% at ≤ 60–80°C (dependent on aeration, impurities); also used in wet-process phosphoric acid service containing F⁻ and Cl⁻.

• Intergranular Corrosion after Welding:​ Ti-stabilized, low-carbon design prevents Cr-carbide precipitation at grain boundaries → weld HAZ remains resistant to intergranular attack per ASTM A262 Practice E (Strauss test) or ASTM G28.

• Sour Service (Oil & Gas):​ When supplied in solution-annealed condition with hardness ≤ 35 HRC (typically 70–90 HRB), meets NACE MR0175 / ISO 15156 for H₂S/CO₂/Cl⁻ environments (material must not be cold-worked to > HRC 35).

5. Applicable Standards by Product Form

6. Summary & Engineering Note

Incoloy 825 (UNS N08825 / W.Nr. 2.4858) is a titanium-stabilized Ni-Fe-Cr-Mo-Cu solid-solution alloy​ offering:

• Immunity to chloride-induced SCC (Ni ≥ 42%)

• Resistance to sulfuric / phosphoric acids (Cu addition)

• Resistance to seawater pitting/crevice corrosion (PREN 32–35, Mo addition)

• Intergranular corrosion resistance in welded condition (Ti-stabilized, low C)

• Good fabricability and weldability with conventional processes

It is the preferred cost-effective step-up from 316L / 904L​ in sour, acidic, or chloride-rich environments, and a more economically viable option than Hastelloy C-276 or Inconel 625​ where ultimate resistance to strong halide + strong reducing acids is not required.

For qualified material test reports (EN 10204 3.1), stock availability of plate / bar / seamless tube, or corrosion-appraisal consultation on Incoloy 825 (UNS N08825), please contact your technical representative.