If you are sizing furnace tubes, reformer outlet pigtails, or high-temperature heat-exchanger coils, the Incoloy 800 vs 800H vs 800HT question usually surfaces after 321H or 347H stainless steel has run out of creep life. All three grades share the same nickel-iron-chromium base, and their room-temperature mechanical properties are nearly identical.
The differences - carbon range, aluminum plus titanium content, and required grain size - only become important once the metal spends years above 600 °C under sustained load. Choose the wrong grade and you either overpay for chemistry you do not need, or you design a tube that creeps to failure before the next turnaround.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
What the Standards Say: UNS, ASTM, and EN Equivalents
Pipe and tube forms are covered by ASTM B407 (seamless pipe and tube), B409 (plate), and B408 (rod and bar). The three grades are distinguished at the UNS level and by Werkstoff numbers in Europe.
| Grade | UNS | Werkstoff | ASTM Pipe/Tube | Typical Product Forms | Common European Reference |
|---|---|---|---|---|---|
| Incoloy 800 | N08800 | 1.4876 | ASTM B407 / ASME SB407 | Seamless pipe, tube, heat-exchanger tubing | EN 10095 / AD 2000 W2 |
| Incoloy 800H | N08810 | 1.4958 | ASTM B407 / ASME SB407 | Seamless pipe, tube, furnace tubes | EN 10095 / AD 2000 W2 |
| Incoloy 800HT | N08811 | 1.4959 | ASTM B407 / ASME SB407 | Seamless pipe, tube, high-temperature furnace components | EN 10095 / AD 2000 W2 |
A practical procurement note: most mills now supply material dual-certified as 800H/800HT because the 800HT chemistry envelope sits inside the tighter 800H limits. That dual ticket is useful, but it is not a substitute for confirming the actual carbon, aluminum, titanium, and grain-size values on the EN 10204 3.1 mill test certificate.
Chemistry and Grain Size: Why the Small Numbers Matter
| Element | Incoloy 800 (N08800) | Incoloy 800H (N08810) | Incoloy 800HT (N08811) | Engineering Effect |
|---|---|---|---|---|
| Carbon (C) | 0.10 % max | 0.05 – 0.10 % | 0.06 – 0.10 % | Higher C stabilizes carbides and raises creep strength at temperature |
| Aluminum (Al) | 0.15 – 0.60 % | 0.15 – 0.60 % | 0.85 – 1.20 % | Al + Ti form controlled precipitates that strengthen the matrix |
| Titanium (Ti) | 0.15 – 0.60 % | 0.15 – 0.60 % | 0.85 – 1.20 % | Combined Al+Ti ≥ 0.85 % is the key identifier for 800HT |
| Nickel (Ni) | 30.0 – 35.0 % | 30.0 – 35.0 % | 30.0 – 35.0 % | Austenitic matrix; oxidation and carburization resistance |
| Chromium (Cr) | 19.0 – 23.0 % | 19.0 – 23.0 % | 19.0 – 23.0 % | Oxidation and nitridation resistance |
| Iron (Fe) | Balance | Balance | Balance | Cost base; keeps alloy cheaper than pure nickel grades |
| Grain size | No requirement | ASTM 5 or coarser | ASTM 5 or coarser | Coarse grain = longer creep life |
The real split is not the nickel or chromium; it is the carbon floor and the aluminum-plus-titanium band. Incoloy 800 can be fine-grained and is not required to meet a minimum solution-anneal temperature. Incoloy 800H and 800HT must be solution-annealed at roughly 1 120 – 1 180 °C and quenched to produce an ASTM 5 or coarser grain structure. That coarse grain is what gives the H and HT grades their longer creep-rupture life at temperature.
ASME Allowable Stress: The Data That Drives Wall Thickness
Here is the value most generic supplier pages skip. The table below gives indicative ASME BPVC Section II-D allowable stress values for seamless pipe/tube under ASTM B407. Always verify against the code edition used in your project, but the relative advantage of 800H/HT over 800 is consistent.
| Design Temperature | Incoloy 800 (N08800) | Incoloy 800H/HT (N08810/N08811) | 800H/HT Advantage |
|---|---|---|---|
| 538 °C / 1 000 °F | 58.6 MPa (8.5 ksi) | 68.9 MPa (10.0 ksi) | +18 % |
| 593 °C / 1 100 °F | 44.8 MPa (6.5 ksi) | 55.2 MPa (8.0 ksi) | +23 % |
| 649 °C / 1 200 °F | 32.4 MPa (4.7 ksi) | 41.4 MPa (6.0 ksi) | +28 % |
| 704 °C / 1 300 °F | 20.7 MPa (3.0 ksi) | 29.0 MPa (4.2 ksi) | +40 % |
| 760 °C / 1 400 °F | 12.4 MPa (1.8 ksi) | 17.9 MPa (2.6 ksi) | +44 % |
| 816 °C / 1 500 °F | 6.9 MPa (1.0 ksi) | 9.7 MPa (1.4 ksi) | +41 % |
What this means in practice: at 760 °C, a tube designed to 800H/HT can carry the same pressure with roughly 30 % less wall thickness than a tube designed to Incoloy 800. In a large ethylene cracking furnace with hundreds of radiant tubes, that thickness reduction pays for the material upgrade many times over. Below 593 °C, the advantage shrinks to the point where standard 800 is usually the more economical choice.
Environment-by-Environment Selection Matrix
| Service Condition | Recommended Grade | Reason |
|---|---|---|
| ≤ 593 °C, no creep governing | Incoloy 800 | Fine grain gives better thermal fatigue resistance; lowest cost |
| 600 – 760 °C, sustained pressure | Incoloy 800H | Coarse grain + carbon control for reliable creep life |
| 760 – 980 °C, long-term service | Incoloy 800HT | Highest Al+Ti for precipitation stability and maximum creep strength |
| Frequent thermal cycling below 650 °C | Incoloy 800 | Finer grain handles thermal fatigue better |
| Ethylene cracking furnace radiant tubes | 800H or 800HT | Design temperatures 820 – 950 °C; creep dominates |
| Steam methane reformer outlet pigtails | 800HT | Highest metal temperature and longest required life |
| Ammonia plant secondary reformer | 800H / 800HT | High temperature + hydrogen/nitrogen atmosphere |
| Heat-treatment furnace rolls and radiant tubes | 800H / 800HT | Long-term dimensional stability |
Welding, Heat Treatment, and the PWHT Trap
The 800-series family is not difficult to weld, but it is easy to ruin the high-temperature properties after welding.
| Parameter | Incoloy 800 | Incoloy 800H / 800HT |
|---|---|---|
| Matching filler metal | ERNiCr-3 (AWS A5.14) | ERNiCr-3 or ENiCrFe-3 |
| Preheat | Generally not required | Generally not required |
| Interpass temperature | < 150 °C recommended | < 150 °C recommended |
| Backing purge | Argon purge on root pass | Argon purge on root pass |
| Post-weld stress relief | May be used for 800 | Not recommended for 800H/HT pressure parts |
| Re-solution anneal if required | 1 010 – 1 090 °C | 1 120 – 1 180 °C + rapid quench |
| Risk of low-temperature PWHT | None significant | Can refine coarse grain and destroy creep strength |
The critical point: if a shop applies a conventional 600 – 700 °C stress-relief heat treatment to an 800H or 800HT furnace tube, the carefully produced coarse grain can recrystallize into a finer grain structure. The tube may still pass room-temperature tensile tests, but its creep life above 700 °C can drop by 50 % or more. For this reason, most furnace-tube specifications explicitly prohibit low-temperature stress relief on 800H/800HT and instead require a full re-solution anneal if the as-welded condition is unacceptable.
Quick-Reference Selection Table
| Parameter | Incoloy 800 (N08800) | Incoloy 800H (N08810) | Incoloy 800HT (N08811) |
|---|---|---|---|
| Best for | Moderate temperature, thermal cycling | 600 – 850 °C sustained load | > 750 °C, maximum creep life |
| Carbon | ≤ 0.10 % | 0.05 – 0.10 % | 0.06 – 0.10 % |
| Al + Ti | 0.30 – 1.20 % | 0.30 – 1.20 % | 0.85 – 1.20 % |
| Grain size | No requirement | ASTM 5 or coarser | ASTM 5 or coarser |
| Solution anneal | 1 010 – 1 090 °C | 1 120 – 1 180 °C | 1 120 – 1 180 °C |
| Approx. max design temp | 650 °C | 815 °C | 980 °C (short term) |
| ASME advantage at 760 °C | Baseline | +44 % allowable stress | +44 % allowable stress |
| Filler metal | ERNiCr-3 | ERNiCr-3 | ERNiCr-3 |
| PWHT risk | Low | High if low-temp stress relief | High if low-temp stress relief |
| Typical cost | Lowest | Medium | 5 – 10 % premium over 800H |
FAQ
What is the real difference between Incoloy 800H and 800HT?
Both grades require the same high-temperature solution anneal and coarse grain. The difference is chemistry: 800HT has a tighter carbon range and a minimum aluminum-plus-titanium content of 0.85 %. That higher Al+Ti promotes more stable precipitation strengthening at temperatures above 750 °C, giving 800HT the best creep-rupture life in the family. Mills often supply material dual-certified as 800H/800HT.
Can I substitute Incoloy 800 for 800H or 800HT?
Only if the design temperature is below roughly 593 °C and creep is not the governing failure mode. Above 600 °C, the lower allowable stress and lack of grain-size control in standard 800 can lead to excessive wall thickness or insufficient creep life. Always re-run the design calculations and confirm the MTC chemistry before substituting.
Why is post-weld stress relief not recommended for 800H/800HT?
A conventional 600 – 700 °C stress-relief treatment can recrystallize the coarse grain structure that was intentionally created by the 1 120 – 1 180 °C solution anneal. The result is a finer grain that passes room-temperature tests but loses creep strength. If heat treatment is required after welding, use a full re-solution anneal followed by rapid cooling.
What documentation should I request for a European PED project?
Request an EN 10204 3.1 or 3.2 mill test certificate, a PED Declaration of Conformity, CE marking evidence for the applicable PED category, AD 2000 W2/W10 data if the project is in Germany, and third-party inspection by TÜV, DNV, or Bureau Veritas when EN 10204 3.2 is specified.
Which grade is best for thermal cycling service?
At moderate temperatures with frequent start-stop cycles, standard Incoloy 800 is often the better choice because its finer grain size provides better thermal-fatigue resistance. If the peak temperature is high enough that creep governs, you may need 800H/800HT and accept the trade-off in thermal-fatigue performance.
Specifying Incoloy 800, 800H, or 800HT pipe for your next furnace or reformer project?
HUITONG supplies seamless and welded Incoloy pipe and tube in all three grades per ASTM B407 / B409 / B408, with EN 10204 3.1/3.2 certification, PED CE marking for Europe, and full documentation for Saudi Aramco, ADNOC, and QatarEnergy projects. Send your design temperature, pressure, and MTO to market@htpipe.com or WhatsApp +86-19339900201 for an alloy selection and quotation response.
