The choice between Inconel 625 (UNS N06625) and Incoloy 825 (UNS N08825) welded pipes is rarely a matter of finding the "better" alloy; rather, it is an exercise in aligning metallurgical limits with regional compliance codes and localized project budgets. While both belong to the broad family of nickel-based alloys, their distinct core chemistry alters their weld pool dynamics, mechanical performance under stress, and resistance to specific chemical environments.
Our manufacturers can produce seamless and welded pipes to meet industry requirements. Contact us market@htpipe.com for more information and a free quote!
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
Metallurgical Composition
| Element | UNS N06625 (Inconel 625) | UNS N08825 (Incoloy 825) |
| Nickel (Ni) | 58.0% min | 38.0% – 46.0% |
| Chromium (Cr) | 20.0% – 23.0% | 19.5% – 23.5% |
| Molybdenum (Mo) | 8.0% – 10.0% | 2.5% – 3.5% |
| Iron (Fe) | 5.0% max | 22.0% min (Balance) |
| Niobium (Nb) | 3.15% – 4.15% | - |
| Copper (Cu) | - | 1.5% – 3.0% |
| Titanium (Ti) | 0.40% max | 0.6% – 1.2% |
Mechanical Properties for Welded Pipe
| Property | Inconel 625 Welded Pipe (ASTM B705) | Incoloy 825 Welded Pipe (ASTM B705) |
|---|---|---|
| Tensile Strength, min | 827 MPa (120 ksi) | 586 MPa (85 ksi) |
| Yield Strength (0.2% offset), min | 414 MPa (60 ksi) | 241 MPa (35 ksi) |
| Elongation in 2 in., min | 30% | 30% |
| Density | 8.44 g/cm³ | 8.14 g/cm³ |
| Max recommended service temp | ~980°C (1800°F) | ~540°C (1000°F) |
| Melting range | 1290–1350°C | 1370–1400°C |
Corrosion Dynamics in Welded Environments
For welded pipes, the weld seam itself is often the most vulnerable point for localized corrosion. High-temperature thermal cycles during welding can cause elemental segregation, leaving the weld joint susceptible to pitting, crevice attack, or stress corrosion cracking (SCC) if the material composition is not selected carefully.
Pitting and Crevice Corrosion in Chloride Services
The Pitting Resistance Equivalent Number (PREN) provides a reliable indicator of an alloy's performance in chloride-rich environments:
Because Inconel 625 contains up to 10% molybdenum, its PREN value typically exceeds 50, allowing welded pipes to withstand warm, highly aerated seawater and brackish water up to 80°C without localized breakdown.
Incoloy 825, with a modest molybdenum content of 2.5% to 3.5%, yields a PREN of approximately 31. This makes welded 825 pipes vulnerable to pitting in warm seawater environments above 30°C, meaning they should be reserved for milder chloride services or systems where chemical inhibition is constantly maintained.
Reducing and Oxidizing Acids
Incoloy 825 outperforms Inconel 625 in pure reducing acid systems. The synergistic effect of its copper and nickel content provides superior resistance to sulfuric acid solutions across a broad range of concentrations.
When transporting phosphoric acid or nitric acid mixtures, the iron-nickel matrix of Alloy 825 offers a more predictable passivation layer, making it the preferred and more economical piping choice for fertilizer production and acid pickling plants.
ASTM Specifications: Getting Your PO Right
Mixing up ASTM specifications is one of the most common procurement errors we see. Both alloys share some specification numbers but differ on others. Here's the definitive mapping:
| Product Form | Inconel 625 | Incoloy 825 | Notes |
|---|---|---|---|
| Seamless pipe & tube | ASTM B444 / SB444 | ASTM B423 / SB423 | High-pressure critical service |
| Welded pipe | ASTM B705 / SB705 | ASTM B705 / SB705 | Same spec covers both alloys. UNS designation differentiates. |
| Welded tube | ASTM B704 / SB704 | ASTM B704 / SB704 | Smaller OD (typically ≤ 4"), heat exchanger service |
| Plate & sheet | ASTM B443 / SB443 | ASTM B424 / SB424 | Source material for welded pipe fabrication |
| Bar & rod | ASTM B446 / SB446 | ASTM B425 / SB425 | For forged flanges and fittings |
| Butt-weld fittings | ASTM B366 / SB366 | ASTM B366 / SB366 | Specify grade: WPNCMC for 625, WPNCMC for 825 |
| Forgings | ASTM B564 / SB564 | ASTM B564 / SB564 | Flanges, forged fittings |
Note that ASTM B705 and B704 cover both UNS N06625 and UNS N08825. The UNS number on your purchase order is what differentiates the alloy - not the specification number alone. We've seen POs that list "ASTM B705 welded pipe" without a UNS designation, leading to shipments of the wrong material. Always include both.
Weldability and Heat-Affected Zone (HAZ) Integrity
Welding thin-to-medium wall pipes requires precise control over heat input to prevent microstructural degradation. Both alloys exhibit good overall weldability, but their actual behavior under an arc differs.
Inconel 625 Welded Pipe Fabrication
Inconel 625 is highly tolerant to variations in welding parameters. It is typically welded using ERNiCrMo-3 filler metal.
The weld metal solidifies into a fine dendritic structure that naturally resists hot cracking. Because the alloy is stabilized by niobium, it does not form chromium-depleted carbide networks in the HAZ, eliminating the need for post-weld heat treatment (PWHT) in most standard pressure piping applications.
Incoloy 825 Welded Pipe Fabrication
Welded 825 pipes are fabricated using ERNiFeCr-1 filler metal. Because of the lower nickel content and the presence of titanium-rich phases, the weld pool is more viscous and less fluid than that of 625.
Welding engineers must strictly control the heat input to prevent titanium from volatilizing or forming coarse carbonitride clusters in the fusion zone, which can lower ductility and impact toughness. Post-weld cooling must be fast enough to avoid sensitization in heavy-walled designs.
Decision Framework: Which Alloy for Which Project?
After hundreds of pipe supply projects across the Middle East, Europe, and Asia, the selection logic boils down to a few key questions. Work through these in order:
1. What is the service temperature?
Above 540°C → 625. No exceptions. 825 loses strength rapidly above this threshold and is not rated for continuous service at these temperatures.
2. Is the service sour (H₂S present)?
Yes, with high H₂S partial pressure (> 0.1 MPa) or high temperature (> 65°C) → 625. Yes, but moderate conditions → 825 is acceptable and significantly cheaper.
3. Is the primary corrosive medium a reducing acid (sulfuric, phosphoric)?
Yes, at moderate temperature (< 80°C) → 825. The copper addition gives it an edge over 625 here, at lower cost. If chlorides are also present or temperature is higher → 625.
4. Is it seawater service above 60°C?
Yes → 625. The PREN gap is too large to ignore. No (ambient seawater or freshwater) → 825 is adequate.
5. Is field welding a major factor?
If the project involves extensive field welding of large-diameter pipe in hot climates (Middle East, tropical Asia), 625's no-PWHT requirement and easier weldability give it a practical edge. 825's potential need for local solution anneal in severe service adds complexity to field construction.
6. What does the budget allow?
If you've answered "825" to questions 1–5 but the project has budget flexibility, consider upgrading to 625 for critical sections (wellhead piping, high-temperature process lines) while using 825 for less severe sections. Mixed-alloy piping systems are common and both alloys can be welded to each other using ERNiCrMo-3 filler.
Frequently Asked Questions
Q: Can I weld Inconel 825 pipe to Inconel 625 pipe?
Yes. The two alloys have excellent metallurgical compatibility. Use ERNiCrMo-3 (Inconel 625) filler metal for the dissimilar joint. The weld metal will have pitting resistance equal to the 625 side, protecting the joint. This is common practice in mixed-alloy piping systems where 625 is used for high-severity sections and 825 for less aggressive service.
Q: What is the difference between Inconel and Incoloy?
Inconel is a registered trademark of Special Metals Corporation for nickel-chromium-based alloys with nickel as the primary element (typically ≥50%). Incoloy is the same company's brand for nickel-iron-chromium alloys where iron is a significant constituent. Inconel 625 contains ≥58% nickel; Incoloy 825 contains 38–46% nickel with iron as the balance. The naming convention reflects the compositional difference, which drives the performance and cost differences discussed in this guide.
Q: Is welded pipe acceptable for sour service under NACE MR0175?
Yes. Both Inconel 625 and Incoloy 825 welded pipe (ASTM B705) are qualified for sour service under NACE MR0175 / ISO 15156-3. The standard requires that the base metal, weld metal, and HAZ all meet specified hardness limits (≤ 35 HRC for nickel alloys). Welded pipe for sour service must have qualified welding procedures (WPQR) that include hardness testing of the weld cross-section.
Q: What wall schedule should I specify for nickel alloy welded pipe?
This depends on the design pressure, temperature, and corrosion allowance. For 625, the higher yield strength (414 MPa vs. 241 MPa for 825) often allows a lighter schedule. Common schedules for process piping are Sch 10S, Sch 40S, and Sch 80S. For corrosive service where some wall loss is expected over the design life, add a corrosion allowance of 1.5–3.0 mm to the calculated minimum wall thickness.
Q: How do I verify that the welded pipe I received is actually Inconel 625 or 825?
Three methods: (1) Review the EN 10204 Type 3.1 or 3.2 MTC for the heat analysis - verify nickel, chromium, molybdenum, and niobium (for 625) or copper and titanium (for 825) content. (2) Perform PMI using a handheld XRF analyzer on both the pipe body and weld seam. (3) For critical service, send a sample to a certified laboratory for OES (optical emission spectroscopy) full chemical analysis. Never rely on visual identification or stencil markings alone.
Q: What is the typical lead time for Inconel 625 or 825 welded pipe?
For standard sizes (2"–12" NB, Sch 10S–Sch 40S) in stock at Asian mills, lead time is typically 2–4 weeks for production plus 2–3 weeks for ocean freight to Middle East or European destinations. Non-standard sizes, special schedules, or material requiring additional testing (NACE, PED 3.2) can add 4–8 weeks. Always confirm lead time in writing and build contingency into your project schedule.
