Inconel 625 vs Inconel 825 Buttweld Fittings: A Buyer Guide

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

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

1. Ordering 625 because "Inconel" sounds premium. If your service is reducing acid, 825 may actually outperform 625 and cost less.
2. Mixing 625 fittings with 825 pipe. Galvanic and weld metallurgy issues arise; use matching grades or qualify the dissimilar weld.
3. Forgetting that 825 strength drops above 540 degC. It is not a high-temperature alloy.
4. Skipping the Nb+Ta range check on 625. A low Nb+Ta value reduces strength and weldability; verify on the MTC.
5. 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