Alloy System
Ni-Fe-Cr + γ′
Strengthening
Solid Solution + γ′ + Carbide
What is Incoloy 800HT? Incoloy 800HT (UNS N08811 / W.Nr. 1.4959) is the highest-creep variant of the Incoloy 800 family, engineered for sustained-load pressure-bearing service between 760°C and 982°C. The defining difference from 800H is the elevated Al+Ti content (0.85–1.20% combined, with both Al and Ti individually held ≥0.25%). This chemistry enables precipitation of fine γ′-phase (Ni₃(Al,Ti)) particles during service, providing additional strengthening that becomes critical in the 760–982°C range where carbide pinning alone is insufficient. Combined with controlled carbon (0.06–0.10%) and coarse grain size (ASTM ≥5), 800HT delivers the highest stress-rupture strength of any wrought Fe-Ni-Cr alloy. It is the standard material for ethylene cracker radiant coils, steam-methane reformer tubes, and other furnace components where 100,000-hour design life at 800–950°C is required.
Al + Ti ★
0.85 – 1.20%
KEY — γ′ precipitation
Aluminum (Al) ★
0.25 – 0.60%
Higher than 800H (0.15)
Titanium (Ti) ★
0.25 – 0.60%
Higher than 800H (0.15)
Carbon (C)
0.06 – 0.10%
Same as 800H
Tensile Strength (min)
≥450 MPa
65 ksi
Yield Strength (min)
≥170 MPa
25 ksi
Elongation (min)
≥30%
Ductile in annealed state
Density
7.94 g/cm³
0.287 lb/in³
Melting Range
1357–1385°C
2475–2525°F
Incoloy 800HT: γ′ Precipitation Strengthening for the Highest Service Temperatures
Incoloy 800HT (UNS N08811) was developed to extend the service envelope of the 800 family into the 760–982°C range where 800H’s carbide strengthening alone is insufficient. The metallurgical innovation is the elevation of Al+Ti combined content to 0.85–1.20% (vs 0.30–1.20% for 800H), with both aluminum and titanium individually held at minimum 0.25%.
During high-temperature service, the Al and Ti atoms combine with nickel to form fine, coherent γ′-phase (Ni₃(Al,Ti)) precipitates — the same strengthening mechanism used in nickel-based superalloys like Inconel 718. These sub-micron particles impede dislocation motion, dramatically increasing creep resistance. The γ′ precipitates form gradually during the first few thousand hours of service and remain stable for the design life of the component.
The triple strengthening mechanism — solid-solution (Ni-Cr-Fe matrix) + carbide grain-boundary pinning (controlled C 0.06–0.10%) + γ′ precipitation (elevated Al+Ti) — gives 800HT the highest stress-rupture strength in the 800 family. Combined with coarse grain (ASTM ≥5) achieved through high-temperature solution annealing (≥1149°C / 2100°F), 800HT is the standard material for ethylene cracker radiant coils, reformer tubes, and other furnace components designed for 100,000-hour service life at 800–950°C.
Five Defining Advantages of Incoloy 800HT
1. Highest Creep Strength in 800 Family
γ′ precipitation + carbide + coarse grain triple strengthening. Up to 1.5× the stress-rupture life of 800H at 850°C.
2. ASME Code Approved to 982°C
Approved for pressure vessel construction to 982°C (1800°F) per ASME Code Case 1325/1326. Stress values published.
3. 100,000-hr Design Life
Enables 100,000-hour creep-rupture design at 800–950°C. Standard for ethylene cracker and reformer tube design.
4. Oxidation + Carburization Resistance
Same Cr₂O₃ + Al₂O₃ protective scale as 800 family. Withstands carburizing hydrocarbon atmospheres to 980°C.
5. Cost-Effective vs Nickel Superalloys
Delivers comparable creep performance to Inconel 625 at 760–982°C at lower cost due to iron balance.
Chemical Composition — ASTM B409 UNS N08811
800HT’s defining difference from 800H is the elevated Al+Ti content. Both aluminum and titanium are held at minimum 0.25% (vs 0.15% for 800H), and the combined Al+Ti is controlled to 0.85–1.20% (vs 0.30–1.20% for 800H). This elevated chemistry is essential for γ′-phase precipitation:
| Element |
Min (%) |
Max (%) |
Role |
| Aluminum + Titanium (Al+Ti) ★ |
0.85 |
1.20 |
KEY DIFFERENCE FROM 800H. Elevated combined content forms γ′-phase (Ni₃(Al,Ti)) precipitates during service. Provides precipitation strengthening critical for 760–982°C creep. |
| Aluminum (Al) ★ |
0.25 |
0.60 |
Higher min than 800H (0.15). Active γ′ former. Also forms Al₂O₃ sub-layer for oxidation resistance. |
| Titanium (Ti) ★ |
0.25 |
0.60 |
Higher min than 800H (0.15). Active γ′ former. Also stabilizes against sensitization (TiC formation). |
| Carbon (C) |
0.06 |
0.10 |
Controlled range (same as 800H). Forms grain-boundary carbides for creep resistance. |
| Nickel (Ni) |
30.0 |
35.0 |
Austenitic stabilizer. Combines with Al+Ti to form γ′ precipitates. |
| Chromium (Cr) |
19.0 |
23.0 |
Forms protective Cr₂O₃ film. Oxidation and carburization resistance. |
| Iron (Fe) |
39.5 |
Balance |
Cost-effective balance element. |
| Manganese (Mn) |
— |
1.50 |
Deoxidizer; improves hot workability. |
| Silicon (Si) |
— |
1.00 |
Deoxidizer. |
| Copper (Cu) |
— |
0.75 |
Residual. |
| Sulfur (S) |
— |
0.015 |
Ultra-low for hot workability. |
★ The elevated Al+Ti (0.85–1.20%) is the defining difference from 800H (0.30–1.20%). This chemistry enables γ′-phase precipitation strengthening.
γ′ (Gamma-Prime) Precipitation Strengthening Mechanism: During service at 650–850°C, the elevated Al+Ti combines with nickel to form coherent L1₂-structured Ni₃(Al,Ti) precipitates. These sub-micron (50–200 nm) ordered particles impede dislocation motion through antiphase boundary energy and coherency strain fields. The precipitates nucleate homogeneously in the matrix and grow slowly, providing stable strengthening for the 100,000-hour component design life. This is the same strengthening mechanism used in nickel superalloys (Inconel 718, Waspaloy) — 800HT brings it to the iron-nickel-chromium system at lower cost.
Mechanical Properties — Room Temperature (Solution-Annealed, Coarse Grain)
800HT’s room-temperature properties match 800H — the differentiation is at elevated temperature where γ′ precipitation activates. After service exposure at 650–850°C, room-temperature hardness and strength increase slightly due to γ′ formation (age hardening).
| Condition |
UTS (MPa) |
YS (MPa) |
Elongation (%) |
Hardness |
| Annealed (min, ASTM) |
≥450 |
≥170 |
≥30 |
≤200 HB |
| Typical (20°C, annealed) |
~552 |
~241 |
~37 |
~150 HB |
| After Service Exposure (aged) |
~620 |
~310 |
~25 |
~180 HB |
Elevated-Temperature Stress-Rupture Strength (100,000 hr) — 800HT vs 800H
| Temperature |
800HT Rupture |
800H Rupture |
Improvement |
| 760°C (1400°F) |
~22 MPa |
~17 MPa |
+29% |
| 816°C (1500°F) |
~14 MPa |
~10 MPa |
+40% |
| 871°C (1600°F) |
~9 MPa |
~6 MPa |
+50% |
| 927°C (1700°F) |
~5.5 MPa |
~3.5 MPa |
+57% |
| 982°C (1800°F) |
~3.2 MPa |
— |
(800H not rated) |
Values are typical — design per ASME Section VIII Code Case 1325/1326 published stress values.
Elastic Constants (Annealed at 20°C)
| Elastic Constant |
Value |
Unit |
| Modulus of Elasticity (E) |
197 |
GPa |
| Shear Modulus (G) |
76 |
GPa |
| Poisson’s Ratio (ν) |
0.29 |
— |
Physical Properties — UNS N08811
800HT’s physical properties match those of 800 and 800H since the chemistry differs only in Al+Ti content. The alloy is non-magnetic in all conditions.
| Property |
Value |
Unit |
| Density |
7.94 |
g/cm³ (0.287 lb/in³) |
| Melting Range |
1357 – 1385 |
°C (2475 – 2525 °F) |
| Thermal Conductivity (at 100°C) |
11.4 |
W/m·K |
| Specific Heat (at 20°C) |
460 |
J/kg·K |
| Electrical Resistivity (at 20°C) |
0.989 |
μΩ·m |
| Coeff. of Thermal Expansion (20–100°C) |
13.1 |
μm/m·°C |
| Magnetic Permeability |
< 1.001 |
Non-magnetic |
Heat Treatment Procedures
800HT requires the highest solution-anneal temperature in the 800 family (≥1149°C / 2100°F) to fully dissolve Al+Ti into solution and grow coarse grains. This ensures γ′ precipitation occurs homogeneously during service rather than forming coarse, brittle particles during heat treatment.
Step 1 — Solution Anneal (CRITICAL)
≥1149 °C (2100 °F) + Rapid WQ/AC
Hold ~1 hr per 25 mm section thickness. Higher than 800H (≥1121°C) to fully dissolve Al+Ti. Water quench or rapid air cool. Grain size ASTM ≥5 mandatory on MTC.
Step 2 — γ′ Precipitation (Service-Induced)
650 – 850 °C during first 1,000–5,000 hr service
Not a heat-treatment step — occurs naturally during service. Fine γ′ precipitates nucleate homogeneously and grow slowly, providing progressive strengthening.
Step 3 — Post-Weld
Solution Anneal Strongly Recommended
Welding produces fine grain and may cause localized γ′ coarsening. Full post-weld solution anneal (≥1149°C) restores coarse grain and dissolves precipitates for re-precipitation in service.
CRITICAL WARNINGS:
Solution anneal below 1149°C forfeits 800HT properties. Insufficient temperature leaves Al+Ti undissolved, forming coarse γ′ that is brittle rather than strengthening. Resulting material performs like 800H, not 800HT.
Do not subject 800HT to extended dwelling at 650–850°C outside of service. This is the γ′ precipitation range. Heat-treatment exposure here forms coarse over-aged precipitates that reduce toughness without providing creep benefit.
Post-weld solution anneal is essential for pressure-bearing service. The HAZ has fine grain and altered precipitate structure that must be restored through full re-anneal.
Corrosion Resistance — Same Excellent 800-Family Performance
800HT’s corrosion resistance is essentially identical to 800H and standard 800. The elevated Al+Ti does not significantly change aqueous corrosion behavior but does enhance oxidation resistance through the additional Al₂O₃ sub-layer contribution.
Oxidation Resistance ★
Excellent oxidation resistance to 1100°C. The higher Al content (0.25–0.60%) provides additional Al₂O₃ sub-layer beneath the Cr₂O₃ film, slightly improving spallation resistance compared to 800H during thermal cycling.
Carburization Resistance ★ — DEFINING APPLICATION
Outstanding carburization resistance to 980°C in ethylene cracker and reformer environments. 800HT is the industry standard for ethylene pyrolysis furnace radiant coils where carbon uptake from hydrocarbon cracking would embrittle conventional stainless steels within months.
Chloride SCC Resistance
Same superior chloride SCC resistance as 800 family due to 30–35% Ni content. Suitable for steam-generator tubing applications.
Intergranular Corrosion (IGC)
Ti/Al stabilization provides IGC resistance in the as-welded condition. The elevated Ti (0.25% min) further enhances stabilization compared to 800H. Passes ASTM A262 Practice A and E. Post-weld solution anneal recommended for the most severe IGC service.
Incoloy 800 Family Selection Guide — 800HT Position
800HT is positioned as the premium choice for 760–982°C service where the highest creep-rupture strength is essential. The elevated Al+Ti chemistry and higher solution-anneal temperature justify the cost premium:
| Alloy |
Carbon |
Al + Ti |
Grain Size |
Creep Strength |
Relative Cost |
| Incoloy 800 |
≤0.10% |
0.30–1.20% |
Not specified |
★★ |
$$ |
| Incoloy 800H |
0.05–0.10% |
0.30–1.20% |
ASTM ≥5 |
★★★★ |
$$$ |
| Incoloy 800HT |
0.06–0.10% |
0.85–1.20% |
ASTM ≥5 |
★★★★★ |
$$$$ |
| Best For |
<600°C |
600–760°C |
760–982°C |
Service temperature |
Trade-off |
800HT Positioning: "The Premium Choice for 760–982°C"
Choose 800HT when service temperature exceeds 760°C and the design requires maximum creep-rupture strength for 100,000-hour life. Standard applications: ethylene cracker radiant coils, steam-methane reformer tubes, high-temperature furnace radiant tubes. For 600–760°C service,
800H is more economical. For <600°C without creep requirements, choose
standard 800.
Key Applications
800HT is specified exclusively for the most demanding high-temperature applications where service above 760°C with sustained loads requires the highest creep-rupture strength available in the Fe-Ni-Cr system:
| Application |
Components & Conditions |
| Ethylene Cracker Radiant Coils ★ |
Pyrolysis furnace radiant coil tubes. Operating 820–950°C with severe carburization, thermal cycling, and 100,000-hour design life. 800HT is the industry-standard material for this service. |
| Steam-Methane Reformer Tubes ★ |
Catalyst tubes for hydrogen and ammonia plant reformers. Operating 800–950°C with 35–50 bar internal pressure. Centrifugally cast or wrought 800HT. |
| High-Temperature Furnace Components |
Radiant tubes, muffles, retorts, and furnace structural components operating above 760°C with sustained loads. |
| Superheater/Reheater Tubing |
Advanced utility boiler superheater and reheater tubing for ultra-supercritical steam conditions operating above 760°C. |
| Petrochemical Transfer Piping |
High-temperature transfer lines between process furnaces and reactors. Hot-gas piping operating 760–900°C. |
| Waste Heat Recovery Systems |
Tubing and components in high-temperature waste heat boilers and recovery units operating above 760°C. |
Applicable Standards & Specifications
| Standard |
Product Form / Scope |
Notes |
| ASTM B409 / ASME SB-409 |
Plate, Sheet, Strip — UNS N08811 |
Grain size ≥ ASTM 5 verified on MTC. |
| ASTM B407 / ASME SB-407 |
Seamless Pipe & Tube — UNS N08811 |
Seamless pipe primary spec |
| ASTM B408 / ASME SB-408 |
Rod, Bar — UNS N08811 |
Round bar and wire products |
| ASTM B514 / B515 |
Welded Pipe & Tube — UNS N08811 |
Welded pipe/tube products |
| ASTM B366 / ASME SB-366 |
Wrought Welded Fittings |
Butt-weld and socket-weld fittings |
| ASTM B564 / ASME SB-564 |
Nickel Alloy Forgings |
Flanges, fittings, pressure parts |
| ASME Code Case 1325 / 1326 |
Pressure Vessel Construction to 982°C |
Stress values published for design |
| DIN W.Nr. 1.4959 |
X8NiCrAlTi32-21 — European Werkstoff |
European project specification |
Available Product Forms from HT PIPE
We supply Incoloy 800HT in solution-annealed condition (annealing temperature ≥1149°C) with grain size ASTM 5 or coarser and Al+Ti verification on every MTC. EN 10204 3.1 certification includes chemical analysis (Al, Ti, and Al+Ti verified), mechanical testing, grain size, and heat-treatment records.
| Product Form |
Specification & Range |
| Seamless Pipes & Tubes |
½″–12″ NB, Sch 10S–Sch 80S. ASTM B407 UNS N08811. Solution-annealed ≥1149°C. Grain ASTM ≥5. Al+Ti 0.85–1.20% verified. |
| Plates & Sheets |
3–80 mm thickness. ASTM B409 UNS N08811. Solution-annealed. Grain ASTM ≥5 verified. |
| Welded Pipes |
ASTM B514/B515 UNS N08811. Full range. Solution-annealed after welding. |
| Forged Fittings |
ASTM B366 UNS N08811. Butt-weld and socket-weld. Elbows, tees, reducers, caps, stub ends. |
| Flanges |
ASTM B564 UNS N08811. 150#–1500#. Weld neck, blind, slip-on, lap joint, long weld neck, orifice. |
| Round Bars |
6–300 mm diameter. ASTM B408 UNS N08811. Hot-finished and cold-drawn. Solution-annealed. |
HT PIPE Project Experience — Incoloy 800HT
Case Study: Ethylene Cracker Radiant Coil Replacement — 8.5 Tons Incoloy 800HT Seamless Tube
HT PIPE supplied 8.5 tons of Incoloy 800HT seamless tubes (ASTM B407 UNS N08811, OD 141.3 mm × WT 8.0 mm, lengths 14 m) to a Northeast Asian ethylene plant for pyrolysis furnace radiant coil replacement in 2025. The coils operate at 880°C tube-skin temperature with 4 bar internal pressure, designed for 100,000-hour creep-rupture life per ASME Code Case 1325.
Material selection justification: 800HT was specified over 800H based on three factors:
Service temperature 880°C exceeds 760°C threshold where 800HT’s γ′ precipitation strengthening becomes essential;
800HT stress-rupture at 880°C/100,000 hr: ~7.5 MPa (meets design life at 4 bar);
800H stress-rupture at 880°C/100,000 hr: ~5 MPa (insufficient — would require 60% thicker wall, unacceptable for coil weight and thermal response);
Carburization resistance critical — ethylene pyrolysis environment with active carbon uptake;
Delivery: Full EN 10204 3.1 certification including chemical analysis (C 0.08%, Al 0.42%, Ti 0.48%, Al+Ti 0.90% — verified within 0.85–1.20% range), mechanical testing, grain size (ASTM 3–4 verified per heat — coarser than minimum), solution-anneal furnace charts (1155°C, 2 hr hold, water quench), and 100% PMI on every tube. Delivered in 18 weeks. Furnace commissioned and operating at design capacity with coil performance meeting specification.
For ethylene cracker, reformer, or other high-temperature furnace project references, contact our technical sales team.
Frequently Asked Questions — Incoloy 800HT
Q1: What is the difference between 800HT and 800H?
800HT raises the Al+Ti combined content from 0.30–1.20% (800H) to 0.85–1.20%, with both Al and Ti individually held ≥0.25% (vs 0.15% for 800H). This elevated chemistry enables γ′-phase (Ni₃(Al,Ti)) precipitation strengthening during service, providing up to 50% higher stress-rupture strength at 850°C. 800HT also requires higher solution-anneal temperature (≥1149°C vs ≥1121°C for 800H). Choose 800HT for service above 760°C; choose 800H for 600–760°C.
Q2: What is γ′ precipitation strengthening and why does it matter?
γ′ (gamma-prime) is an ordered L1₂-structured Ni₃(Al,Ti) intermetallic phase that precipitates as sub-micron coherent particles during service at 650–850°C. These particles impede dislocation motion through antiphase boundary energy and coherency strain fields, dramatically increasing creep resistance. The mechanism is the same as that used in nickel superalloys (Inconel 718, Waspaloy). 800HT brings this strengthening to the iron-nickel-chromium system at lower cost. The precipitates form gradually during the first 1,000–5,000 hours of service and remain stable for the 100,000-hour design life.
Q3: Can 800HT replace HK40 or HP40 cast tubes in ethylene crackers?
Yes — wrought 800HT is increasingly replacing centrifugally cast HK40 (25Cr/20Ni) and HP40 (25Cr/35Ni) tubes in ethylene cracker radiant coils. 800HT offers several advantages: wrought structure with consistent properties (vs cast segregation), thinner walls (saving weight and improving thermal response), better weldability, and lower carburization rate. The trade-off is higher material cost, but the longer service life and reduced downtime typically justify the investment. Many modern ethylene furnaces specify wrought 800HT for both radiant coils and transfer piping.
Q4: Why does 800HT require higher solution-anneal temperature than 800H?
800HT requires solution annealing at ≥1149°C (2100°F), compared to ≥1121°C (2050°F) for 800H. The higher temperature is needed to fully dissolve the elevated Al+Ti into the austenitic matrix. If Al+Ti remains undissolved, it forms coarse, brittle particles that reduce toughness without providing creep strengthening. The higher anneal temperature also ensures coarse grain growth (ASTM ≥5), which is essential for creep-rupture life. Insufficient anneal temperature is a common failure mode that reduces 800HT to 800H performance.
Q5: What is the maximum service temperature for 800HT?
For oxidation resistance, 800HT can be used to 1100°C. For pressure-bearing creep-rupture service per ASME Code Case 1325/1326, the limit is 982°C (1800°F). Above 982°C, γ′ precipitates over-age and lose strengthening effectiveness, and oxidation rate becomes excessive for pressure-bearing service. For the most demanding service at 950–1100°C, nickel-based superalloys (Inconel 617, 625, or HR-120/160) are specified.
Request a Quote for Incoloy 800HT (UNS N08811)
Send your specification — sizes, quantities, service temperature and pressure, design life requirement, certification needs (EN 10204 3.1/3.2, grain size ASTM ≥5, Al+Ti verification 0.85–1.20%, ASME Code Case 1325/1326). We typically quote within 24 hours with full MTC details, mill lead times, and technical recommendations for your high-temperature creep-rupture design.
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