Incoloy 800HT is the heat-resistant grade. Incoloy 825 is the corrosion-resistant grade. But real projects often sit in the middle: the metal is hot enough to matter, and the environment is corrosive enough to matter. Picking the wrong alloy in that overlap zone can mean either a tube that corrodes through in two years or one that creeps out of tolerance before the first turnaround.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
Incoloy 800HT (UNS N08811) is engineered for high-temperature structural service in dry or gaseous environments. Its chemistry is tuned for creep and stress-rupture resistance: controlled carbon (0.06–0.10 %), combined aluminum-plus-titanium ≥ 0.85 %, and a mandatory coarse grain size of ASTM 5 or coarser after solution annealing at 1 120–1 180 °C. It contains no deliberate molybdenum or copper additions.
Incoloy 825 (UNS N08825) is engineered as a corrosion-resistant alloy for aqueous and wet chemical environments. Its key additions are molybdenum (2.5–3.5 %) and copper (1.5–3.0 %), plus titanium stabilization. These give it resistance to pitting, crevice corrosion, chloride stress-corrosion cracking, and reducing acids such as sulfuric and phosphoric. Its maximum recommended temperature for pressure-loaded service is roughly 540 °C.
| Criterion | Incoloy 800HT (N08811) | Incoloy 825 (N08825) |
|---|---|---|
| Design intent | High-temperature creep / oxidation | Aqueous corrosion resistance |
| Ni | 30.0–35.0 % | 38.0–46.0 % |
| Cr | 19.0–23.0 % | 19.5–23.5 % |
| Mo | - | 2.5–3.5 % |
| Cu | - | 1.5–3.0 % |
| Al + Ti | ≥ 0.85 % | 0.6–1.2 % Ti typical |
| C | 0.06–0.10 % | 0.05 % max |
| Grain size | ASTM 5 or coarser | Not specified |
| Solution anneal | 1 120–1 180 °C | 940–980 °C typical |
| Pipe standard | ASTM B407 / ASME SB407 | ASTM B423 / ASME SB423 |
| Plate standard | ASTM B409 | ASTM B424 |
That single row - molybdenum and copper present in 825, absent in 800HT - is the most important clue. If the service contains chlorides, sulfuric acid, phosphoric acid, or sour water, 825 is almost always the better choice. If the service is hot, dry, and load-bearing, 800HT is almost always the better choice. The difficult decisions happen when both conditions exist.
Pipe Standards: ASTM B423 vs ASTM B407
For pipe and tube procurement, the relevant standards are not interchangeable.
| Standard | Product Form | Size Range (typical) | Key Tests |
|---|---|---|---|
| ASTM B423 / ASME SB423 | Seamless and welded Incoloy 825 pipe | NPS ½" – 12" seamless; larger welded | Tensile, flattening, flange, hydrostatic, NDT |
| ASTM B705 / ASME SB705 | Welded Incoloy 825 pipe | NPS ½" – 24" | Weld integrity, hydrostatic test |
| ASTM B704 / ASME SB704 | Welded Incoloy 825 tube | OD 6 mm – 219 mm | Condenser/heat-exchanger tube requirements |
| ASTM B163 / ASME SB163 | Seamless nickel alloy condenser/superheater tube | Various | U-bend, hydrostatic, eddy current |
| ASTM B407 / ASME SB407 | Seamless Incoloy 800/800H/800HT pipe and tube | NPS ½" – 12" | Tensile, flattening, hydrostatic, grain size |
A practical note: many heat-exchanger specifications call out ASTM B163 or B704 for 825 tubing, while furnace tubing calls out ASTM B407 for 800HT. If you are buying pipe for a refinery process line, ASTM B423 seamless or B705 welded is the normal specification. If you are buying reformer furnace tubes, ASTM B407 is the norm. Always confirm the required product form before issuing the MTO; ordering B423 pipe when B163 superheater tube is required will create an unnecessary deviation.
Combined Temperature and Corrosion Limits: The Overlap Zone
The overlap zone is where one alloy is exposed to a condition that the other handles better. The table below gives practical boundaries for pipe design.
| Service Condition | Recommended Grade | Reasoning |
|---|---|---|
| Dry gas > 700 °C, pressure-loaded | 800HT | Creep-rupture strength governs; 825 not suitable |
| Wet acid < 100 °C, any pressure | 825 | Sulfuric/phosphoric acid resistance; 800HT would corrode |
| Seawater cooling < 80 °C | 825 | Chloride SCC and pitting resistance |
| Sour water with H₂S and chlorides, < 200 °C | 825 | NACE MR0175 approved when annealed; verify hardness |
| Carburizing/nitriding atmosphere > 800 °C | 800HT | Carbon ingress resistance; 825 not designed for this |
| Steam superheater 500–650 °C, neutral pH | 800HT or 825 | 800HT if creep governs; 825 if condensate corrosivity governs |
| Flue gas with chlorides and SOₓ, 250–500 °C | 825 | Wet-dry corrosion dominates; 800HT may suffer |
| Waste-to-energy boiler superheater, 350–550 °C, chlorides | 825 preferred, or 825 clad CS | High-chloride ash deposits favor 825; 800HT risks pitting |
| Acid gas reboiler, 150–250 °C, H₂S/amine | 825 | Amine + H₂S corrosion resistance |
| Reformer outlet pigtail, 750–900 °C | 800HT | Creep and hydrogen embrittlement dominate |
The boundary line is roughly 540 °C for pressure-loaded 825 pipe. Above that temperature, the yield strength of 825 drops quickly and its corrosion advantage in aqueous environments becomes irrelevant because the water phase is no longer stable. Below 540 °C, if chlorides, sulfuric acid, phosphoric acid, H₂S, or seawater are present, 825 is normally the safer choice even if 800HT might seem "more high-temperature."
Welding and Filler Metal Selection
Welding 825 and 800HT to themselves or to each other requires attention to filler metal and post-weld heat treatment.
| Base Metal | Matching Filler Metal | AWS Class | Notes |
|---|---|---|---|
| Incoloy 825 | ERNiCrMo-3 | AWS A5.14 | Best corrosion match; niobium stabilized |
| Incoloy 825 | ERNiCrMo-4 (C-276) | AWS A5.14 | Use only for severely reducing acid service |
| Incoloy 825 | ERNiCrFe-7 | AWS A5.14 | High iron, lower cost, acceptable for less critical service |
| Incoloy 800HT | ERNiCr-3 | AWS A5.14 | Standard matching filler; OK for service below ~787 °C |
| Incoloy 800HT | ERNiCrCoMo-1 (617 type) | AWS A5.14 | For service above 800 °C, creep strength match |
| 825 to 800HT | ERNiCrMo-3 or ERNiCr-3 | AWS A5.14 | Dilution control; verify corrosion/heat side separately |
Post-weld heat treatment:
Incoloy 825: Titanium stabilization means it is normally used in the as-welded condition without post-weld heat treatment. PWHT can actually reduce corrosion resistance by sensitizing the HAZ.
Incoloy 800HT: Like 800H, it is sensitive to low-temperature stress relief. A conventional 600–700 °C PWHT can recrystallize the coarse grain structure and destroy creep strength. If PWHT is required, use a full re-solution anneal at 1 120–1 180 °C followed by rapid quench.
Selection Decision Tree
1. Is the metal temperature expected to exceed 540 °C under pressure load?
Yes → 800HT (verify creep-rupture life and ASME allowable stress)
No → Go to question 2
2. Is the environment wet, acidic, chloride-bearing, sour, or seawater?
Yes → 825 (verify NACE, PED, and hardness requirements)
No → Go to question 3
3. Is the environment dry, oxidizing, carburizing, or nitriding at high temperature?
Yes → 800HT
No → Standard stainless steel or carbon steel may be sufficient; run corrosion and mechanical checks
Cost and Life-Cycle Considerations
Indicative alloy cost hierarchy (per kg, base metal only, August 2026):
| Grade | Approximate Premium vs 304 SS | Typical Driver |
|---|---|---|
| Incoloy 800 | 3.5–4.5× | Moderate high-temperature service |
| Incoloy 800H/HT | 4.0–5.5× | Creep-governed high-temperature service |
| Incoloy 825 | 5.0–6.5× | Corrosion-governed service |
Using 825 where 800HT is sufficient increases material cost without benefit. Using 800HT where 825 is required leads to premature corrosion failure, unplanned shutdown, and replacement costs that dwarf the alloy premium. The correct approach is to let the governing failure mode - corrosion or creep - choose the alloy, then size the wall based on the appropriate code.
FAQ
When should I choose Incoloy 825 over 800HT?
Choose 825 when the service contains chlorides, sulfuric acid, phosphoric acid, sour water (H₂S), amine, or seawater, and the metal temperature stays below roughly 540 °C under pressure load. 825's molybdenum and copper additions make it the corrosion-resistant choice in wet or chemically aggressive environments.
When is 800HT the only viable option?
Use 800HT when the design temperature exceeds 540 °C and the component is pressure-loaded or carries structural load for long periods. Its coarse grain structure and controlled carbon-aluminum-titanium chemistry give it the creep-rupture strength required for furnace tubes, reformer pigtails, and high-temperature process piping.
Can 825 and 800HT be welded to each other?
Yes, using ERNiCrMo-3 or ERNiCr-3 filler metal with controlled heat input. However, never apply a low-temperature stress-relief to the joint, because it will damage the 800HT heat-affected zone. If PWHT is mandatory, use a full solution anneal of the 800HT side and verify the 825 side remains corrosion-resistant.
Is Incoloy 825 approved for sour service under NACE MR0175?
Yes. Annealed Incoloy 825 pipe is listed in NACE MR0175/ISO 15156-3 with a typical hardness limit of 32 HRC. The material must be supplied in the annealed condition, and the project-specific H₂S partial pressure, chloride content, pH, and temperature must fall within ISO 15156-3 limits.
What documentation do European EPCs require for 825 pipe?
European projects typically require EN 10204 3.1 or 3.2 MTC, PED 2014/68/EU Declaration of Conformity, CE marking evidence for the applicable PED category, material designation Werkstoff 2.4858, EN 10216-5 or EN 10217-5 reference, and AD 2000 W2/W10 data for German equipment. Third-party inspection by TÜV, DNV, or Bureau Veritas is often required for EN 10204 3.2.
Can I substitute 800HT for 825 to save cost?
Generally no. 800HT lacks molybdenum and copper, so it performs poorly in sulfuric acid, phosphoric acid, chloride solutions, and sour water. Substituting it for 825 in those services risks rapid corrosion failure. Only consider substitution if the environment is dry, non-corrosive, and the temperature justifies the 800HT grade.
Specifying Incoloy 825 or 800HT pipe for a project where high temperature and corrosion overlap?
HUITONG supplies seamless and welded Incoloy pipe and tube in both grades per ASTM B423 / B407, with EN 10204 3.1/3.2 certification, PED CE marking for Europe, NACE MR0175 declarations for the Middle East, and full documentation for Saudi Aramco, ADNOC, QatarEnergy, and European EPCs. Send your design temperature, environment, and MTO to market@htpipe.com or WhatsApp +86-19339900201 for an alloy selection and quotation response.
