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Incoloy 800, 800H, and 800HT share the same base chemistry—approx. 32% Ni, 21% Cr, balance Fe (UNS N08800 / N08810 / N08811; W.Nr. 1.4876 / 1.4958 / 1.4959)—but they are not interchangeable in high-temperature pressure service. The three grades differ only in carbon control, Al+Ti content, and solution-annealing temperature, yet those subtle changes produce a large step-up in creep-rupture strength and allowable ASME design temperature. Below is a practical, engineer-focused breakdown.
Chemical & Heat Treatment Differences (ASTM B409 / B407 / B408)
|
Element / Parameter |
Incoloy 800 (N08800) |
Incoloy 800H (N08810) |
Incoloy 800HT (N08811) |
|---|---|---|---|
|
Carbon (C) |
≤ 0.10% (no min.) |
0.05–0.10% (controlled min.) |
0.06–0.10% (controlled min.) |
|
Aluminum (Al) |
0.15–0.60% |
0.15–0.60% |
0.25–0.60% (lower limit raised) |
|
Titanium (Ti) |
0.15–0.60% |
0.15–0.60% |
0.25–0.60% (lower limit raised) |
|
Al + Ti (total) |
0.30–1.20% |
0.30–1.20% |
0.85–1.20% (min. 0.85%) |
|
Solution Anneal Temp. |
980–1040°C |
≥1121°C (2050°F) |
≥1149°C (2100°F) |
|
Grain Size (ASTM) |
Not specified (typically finer) |
≥ ASTM No. 5 (coarse) |
≥ ASTM No. 5 (coarse) |
All three grades have identical Ni (30–35%), Cr (19–23%), Fe (≥39.5%) and similar room-temperature tensile minima (Rm ≥450–520 MPa, Rp0.2 ≥170–205 MPa, A ≥30%). The performance gap appears only at elevated temperature.
How Carbon, Grain Size & Al+Ti Affect Creep Strength
Controlled Carbon (C ≥0.05% in 800H/HT): Promotes M₂₃C₆ carbide precipitation at grain boundaries during high-temp service, which "pins" boundaries and slows grain-boundary sliding—the dominant creep mechanism above ~600°C. Standard 800 with low C (<0.04%) lacks sufficient boundary carbides and creeps faster.
Coarse Grain (ASTM 5 or coarser): Larger grains mean fewer grain boundaries per unit volume. Since creep in Ni-Fe-Cr alloys is grain-boundary-slide dominated, coarse-grained 800H/HT show 2–4× longer creep-rupture life vs. fine-grained material at the same stress and temperature.
Elevated Al+Ti ≥0.85% (800HT only): During prolonged exposure at 650–850°C, 800HT precipitates fine, coherent γ′ phase—Ni₃(Al,Ti)—which provides dispersion strengthening of the matrix. This gives 800HT approx. 15–25% higher 100,000-hour rupture strength than 800H at 750–850°C.
ASME Section II Part D assigns significantly higher allowable stress values to 800H/800HT above 593°C (1100°F); Incoloy 800 (N08800) is not permitted for creep-based pressure design above 538°C (1000°F) in Section VIII Div.1.
Recommended Service Temperature & Creep Performance Comparison
|
Grade |
Max. Recommended Continuous Creep Service |
ASME Design Temp. (Pressure Vessel)* |
Creep-Rupture Rating |
|---|---|---|---|
|
Incoloy 800 (N08800) |
≤ 593°C (1100°F) — non-creep / low stress |
538°C (1000°F) |
Baseline (lowest) |
|
Incoloy 800H (N08810) |
593–760°C (1100–1400°F) — creep-governed |
899°C (1650°F) |
~2× 800 @ 704°C |
|
Incoloy 800HT (N08811) |
760–982°C (1400–1800°F) — severe creep |
982°C (1800°F) |
~1.15–1.25× 800H @ 760–850°C |
*Per ASME BPVC Section VIII Div.1 / Section I — allowable stress values. Always verify latest code edition.
Typical Applications by Grade
Incoloy 800 (N08800):
Shell-and-tube heat exchanger tubing for nitric acid, caustic, or high-purity water (<500°C process temp)
Electrical heating element sheathing (400–550°C)
Furnace muffles / baskets where load is self-supported (no pressure stress)
Nuclear secondary-side feedwater heater tubing (non-creep design)
Incoloy 800H (N08810):
Reformer outlet pigtails and transfer lines (650–800°C, pressurized)
Pyrolysis furnace radiant coil headers
High-temp process piping in petrochemical plants where creep is design driver
Incoloy 800HT (N08811):
Ethylene cracking furnace radiant tubes (800–950°C, high hoop stress)
Steam methane reformer (SMR) catalyst tubes and outlet manifolds
Hydrogen plant secondary reformer transition sections
High-temp superheater / reheater tubing in specialty boilers
Procurement tip: Many mills supply "Dual Certified" 800H/800HT material meeting both N08810 (carbon range) and N08811 (Al+Ti ≥0.85%, coarse grain, high-temp anneal). For any project >593°C under pressure, specify dual cert. to avoid costly grade confusion.
