Why this matters
Inconel 625 and Incoloy 825 are often shortlisted together for nickel-alloy piping packages, yet they are very different alloys with very different price points and service envelopes. Choosing the wrong one is one of the most expensive procurement mistakes in CRA piping.
This guide compares Inconel 625 (UNS N06625) and Incoloy 825 (UNS N08825) as buttweld fittings, on the criteria EPC buyers actually use.
Key technical facts
| Property | Inconel 625 (N06625) | Incoloy 825 (N08825) |
|---|---|---|
| Nickel base | Approx. 58% Ni minimum | Approx. 38-46% Ni |
| Chromium | Approx. 20-23% | Approx. 19.5-23.5% |
| Molybdenum | 8-10% | 2.5-3.5% |
| Niobium + Tantalum | 3.15-4.15% | None |
| Copper | None | 1.5-3.0% |
| Max service temperature | Up to about 982 degC (1800 degF) | About 540 degC (1004 degF) |
| Yield strength (annealed) | Approx. 414-528 MPa | Approx. 241-324 MPa |
| Tensile strength | Approx. 827-1276 MPa | Approx. 586-690 MPa |
625 is a nickel-chromium-molybdenum-niobium solid-solution-strengthened alloy; 825 is a nickel-iron-chromium alloy with copper and titanium additions, which is why it is technically an "Incoloy" rather than an "Inconel."
Decision matrix
| Service condition | Better choice | Why |
|---|---|---|
| Hot chloride / seawater / sour gas | Inconel 625 | High Mo + Cr drives PREN above 50 |
| Sulphuric or phosphoric acid | Incoloy 825 | Cu addition resists reducing acids |
| High temperature (above 540 degC) | Inconel 625 | 825 loses strength rapidly above this |
| Chloride stress corrosion cracking | Inconel 625 | Higher Ni reduces SCC susceptibility |
| General mixed acid or aqueous chemical | Incoloy 825 | Lower cost, adequate performance |
| Dynamic loading, fatigue | Inconel 625 | Higher fatigue strength |
For seamless butt-welding pipe fittings in oil and gas service, 625 is the more common choice for top-of-the-line CRA piping; 825 is often picked for upstream chemical injection and mixed-acid service to control cost.
Common procurement mistakes
- Ordering 625 because "Inconel" sounds premium. If your service is reducing acid, 825 may actually outperform 625 and cost less.
- Mixing 625 fittings with 825 pipe. Galvanic and weld metallurgy issues arise; use matching grades or qualify the dissimilar weld.
- Forgetting that 825 strength drops above 540 degC. It is not a high-temperature alloy.
- Skipping the Nb+Ta range check on 625. A low Nb+Ta value reduces strength and weldability; verify on the MTC.
- Using standard ASME B31.3 allowables without confirming the alloy temper. Annealed and solution-annealed conditions have different design stresses.
Buyer checklist
- Define the maximum and minimum design temperatures and the corrodent.
- For 625, confirm Nb+Ta within 3.15-4.15%, Mo within 8-10%, Cr within 20-23%.
- For 825, confirm Cu within 1.5-3.0% and Ti for stabilisation.
- Cross-check matching seamless steel pipes and forged flanges are the same UNS grade.
- Request mill MTCs to EN 10204 3.1 minimum and full PMI on receipt.
- Send your CRA buttweld fitting enquiry through our inquiry form with the corrodent and temperature spelt out.
Sources
- https://www.fushunspecialsteel.com/incoloy-825-vs-inconel-625/
- https://metalzenith.com/blogs/steel-compare/inconel-625-vs-incoloy-825
- https://www.aesteiron.com/blog/incoloy-825-vs-inconel-625.html
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