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The short answer: Incoloy 800HT (UNS N08811 / W.Nr. 1.4959) can withstand continuous oxidation in air or combustion gases up to approximately 870°C (1600°F) without excessive scale formation, and can tolerate intermittent / cyclic exposure as high as 980–1100°C (1800–2012°F) for limited periods. Beyond the continuous-service limit, protective Cr₂O₃ scale begins to spall or diffuse away under sustained high-temperature conditions, and metal loss by oxidation accelerates.
This article breaks down the oxidation limit by mechanism, compares it with 300-series stainless steels and other Incoloy 800 grades, and explains when you should—and shouldn't—specify 800HT on the basis of oxidation resistance alone.
Why Incoloy 800HT Resists High-Temperature Oxidation
Oxidation resistance comes from the 19–23% Chromium content, which forms a continuous, adherent Cr₂O₃ (chromia) scale on the alloy surface when exposed to oxygen-containing atmospheres (air, flue gas, products of combustion).
The Al (≥0.25%) and Ti (≥0.25%, Al+Ti ≥0.85%) in 800HT do notprimarily improve oxidation resistance (unlike in alumina-forming alloys such as Inconel 601), but they stabilize the austenitic grain structure during long-term high-temp exposure, slowing chromia-scale spallation under thermal cycling.
The coarse grain (ASTM ≥5) from the ≥1149°C solution anneal also improves scale adherence compared with fine-grained material.
In oxygen-poor (reducing) atmospheres, Cr₂O₃ cannot form reliably—oxidation resistance becomes irrelevant, and sulfidation or carburization becomes the governing damage mechanism.
Continuous vs. Intermittent Oxidation Temperature Limits
|
Condition |
Max. Recommended Temperature |
Oxidation Behavior |
|---|---|---|
|
Continuous service in air / oxidizing combustion gas |
870°C (1600°F) |
Thin, stable Cr₂O₃ scale; metal loss < 0.025 mm/year in clean air |
|
Extended exposure up to… |
900°C |
Scale thickens gradually; still acceptable for many furnace parts |
|
Intermittent / cyclic heating (start–stop furnaces) |
980–1100°C (1800–2012°F) |
Scale forms but may spall on cooling; re-forms on reheating. Periodic descaling/inspection advised |
|
Above ~1150°C |
Not recommended |
Cr₂O₃ volatile above ~1100–1150°C in some gas compositions; rapid attack |
For pressure-design codes: ASME Section I allows 800HT to 982°C (1800°F), but this is a creep/strengthallowance—not a guarantee of zero oxidation. In dirty or sulfur-bearing flue gases, the practical oxidation-limited temperature may be 30–50°C lower.
Comparison: Incoloy 800HT vs. 800H vs. 800 vs. 310S Stainless
|
Alloy |
Continuous Oxidation Limit (air) |
Notes |
|---|---|---|
|
Incoloy 800HT (N08811) |
~870°C (good to 980°C intermittent) |
Same as 800H/800 chemically; Al+Ti control & coarse grain improve scale retention on cycling |
|
Incoloy 800H (N08810) |
~870°C |
Identical oxidation behavior to 800HT; difference is creep strength |
|
Incoloy 800 (N08800) |
~870°C |
Same Cr₂O₃ protection; finer grain may spall more on cycling |
|
Type 310S (25Cr-20Ni SS) |
~1000–1050°C (continuous), up to 1150°C intermittently |
Higher Cr gives slightly better peak oxidation resistance, but nocreep allowance > ~650°C and prone to sigma phase embrittlement |
|
Type 304/316 SS |
~750–800°C (max) |
Rapid scaling above 850°C; not comparable for high-heat furnace parts |
Bottom line: If your selection driver is onlyoxidation up to ~870°C in clean air, 800/800H/800HT are equivalent. Choose 800HT when oxidation + creep + carburization all matter (e.g., ethylene cracker tubes, reformer outlets).
When Oxidation Limit Is NOT the Governing Factor
Reducing / Sulfur-Bearing Atmospheres (low pO₂, high H₂S/SO₂): Cr₂O₃ may not be stable → sulfidation attack. For pS₂ above critical values at >650°C, consider Inconel 601 (23Cr-12Al) or higher-Cr Ni-base alloys.
Severe Carburization (high C activity, CO/CH₄): Even if oxidation is limited, carbon ingress is the damage mode → 800HT is preferred becauseof carburization resistance, not oxidation resistance per se.
Strong Reducing Acids (H₂SO₄, HCl): Irrelevant to oxidation—800HT is notacid-resistant; use Incoloy 825 or C-276.
