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Incoloy 800HT (UNS N08811 / W.Nr. 1.4959 / EN X8NiCrAlTi32-21) is the high-creep-strength variant of the Incoloy 800 family, engineered specifically for severe high-temperature pressure service such as ethylene cracking furnace radiant tubes and steam methane reformer (SMR) catalyst tubes. Compared with Incoloy 800 (N08800) and 800H (N08810), 800HT has tighter aluminum and titanium controls (Al+Ti ≥ 0.85%), a minimum carbon content of 0.06%, and is solution-annealed at ≥ 1149°C to produce a coarse grain size (ASTM 5 or coarser). These metallurgical refinements promote γ′ precipitate (Ni₃(Al,Ti)) strengthening and deliver the highest allowable stress and creep-rupture life in the 800 series. Below is a complete technical overview for engineers, purchasers, and QA/QC.
Chemical Composition (ASTM B409 / B564 / B407)
|
Element |
Min (%) |
Max (%) |
Significance |
|---|---|---|---|
|
Nickel (Ni) |
30.0 |
35.0 |
Austenite stabilizer; resists Cl⁻ SCC |
|
Chromium (Cr) |
19.0 |
23.0 |
Forms protective Cr₂O₃ scale (oxidation resistance) |
|
Iron (Fe) |
39.5 |
Balance |
Matrix element |
|
Carbon (C) |
0.06 |
0.10 |
Higher min. vs. 800; ensures grain-boundary carbide pinning |
|
Aluminum (Al) |
0.25 |
0.60 |
Raised min. to support γ′ formation |
|
Titanium (Ti) |
0.25 |
0.60 |
Raised min.; Al + Ti total = 0.85–1.20% (mandatory) |
|
Manganese (Mn) |
— |
1.50 |
— |
|
Silicon (Si) |
— |
1.00 |
— |
|
Copper (Cu) |
— |
0.75 |
— |
|
Sulfur (S) |
— |
0.015 |
— |
Key distinction: Incoloy 800H (N08810) requires only Al+Ti ≥ 0.30%; Incoloy 800HT (N08811) mandates Al+Ti ≥ 0.85% and coarse-grain anneal ≥ 1149°C. Many mills supply "Dual Certified" 800H/800HT material meeting both specs.
Mechanical & Physical Properties (Solution Annealed, Room Temp.)
|
Property |
Value (Typical / Minimum per ASTM) |
|---|---|
|
Tensile Strength (Rm) |
≥ 450 MPa (65 ksi); typical 480–560 MPa |
|
Yield Strength (Rp0.2) |
≥ 170 MPa (25 ksi); typical 200–240 MPa |
|
Elongation (A₅₀) |
≥ 30%; typical 35–42% |
|
Hardness |
≤ 90 HRB (typical 70–85 HRB); ~125–160 HB |
|
Density |
7.94–8.00 g/cm³ (use 8.0 g/cm³ for calculations) |
|
Melting Range |
1357–1385°C (2475–2525°F) |
|
Modulus of Elasticity |
~196–200 GPa |
|
Magnetic |
Non-magnetic in annealed condition |
Creep & High-Temperature Performance
Creep-Rupture Strength: 800HT exhibits ~15–25% higher 100,000-hour rupture strength than 800H at 750–850°C due to γ′ (Ni₃(Al,Ti)) precipitation and coarse grain structure.
ASME Allowable Stress: Permitted for pressure design up to 982°C (1800°F) per ASME Section I; Section VIII Div.1 lists allowable stresses to ~899°C.
Recommended Service Range:
800 (N08800): ≤ 538°C (1000°F) for pressure design; general heat-resisting to ~600°C.
800H (N08810): 593–760°C creep-governed service.
800HT (N08811): 760–982°C in severe creep / carburizing / reformer applications.
Corrosion / Environmental Resistance Summary
|
Environment |
Performance |
Note |
|---|---|---|
|
High-Temp Oxidation (air/combustion gas) |
★★★★☆ |
Good to ~870°C continuous; scale-resistant to 980°C intermittently |
|
Carburization / Metal Dusting |
★★★★ |
Superior to 300-series SS; preferred for ethylene cracker radiant tubes |
|
H₂ + CO Reforming Gas |
★★★★☆ |
Classic SMR converter tube & outlet manifold alloy |
|
Cl⁻ SCC (aqueous, ≤200°C) |
★★★★★ vs. 304/316 |
Suitable for nuclear secondary-side / feedwater heaters |
|
Reducing Acids (H₂SO₄, HCl) |
✗ Not recommended |
Use Incoloy 825 or Hastelloy C-276 |
|
Seawater Pitting |
★★ Moderate |
PREN ≈ 19–23 (no Mo); less resistant than 904L or duplex |
Typical Applications
Ethylene pyrolysis (cracker) furnace radiant tubes (800–950°C, carburizing atmosphere)
Steam Methane Reformer (SMR) catalyst tubes, outlet pigtails, and transfer lines
Secondary reformer transition sections in ammonia / hydrogen plants
High-temperature furnace muffles, retorts, and radiant-cast supports (carburizing / nitro-carburizing)
Petrochemical reformer piping where creep is design-governing above 760°C
Grade Equivalents & Standards
|
Standard |
Designation |
|---|---|
|
UNS |
N08811 |
|
ASTM / ASME |
B409 (plate/sheet), B408 (bar), B407 / B163 (tube), B564 (forging) |
|
DIN / EN |
1.4959 — X8NiCrAlTi32-21 (EN 10095) |
|
GB (China) |
NS112 (with supplemental tech. clause for C≥0.05%, Al+Ti≥0.85%, coarse grain) |
|
JIS |
NCF 800HT (referenced in JIS G4901/G4311 context) |
