Excerpt:
Incoloy 800HT (UNS N08811, W.Nr. 1.4959) technical whitepaper from Shanghai Hangbo Alloy Group. Creep-optimized 800-series alloy with controlled Al+Ti of 0.85-1.20% and ASTM grain size No. 5 or coarser for maximum creep-rupture strength above 600 C; ethylene pyrolysis, reformer and power generation service; solution annealed at 1150 C minimum with per-heat quality verification.
Technical Whitepaper: Incoloy 800HT (UNS N08811 / W.Nr. 1.4959) — High-Temperature Creep and Industrial Service | Hangbo Alloy
Introduction
Incoloy 800HT (UNS N08811) is the ultimate evolution of the iron-nickel-chromium 800 series. While Incoloy 800 provides general corrosion resistance and Incoloy 800H offers improved creep strength through controlled carbon and grain size, Incoloy 800HT is engineered for maximum performance in high-temperature creep-limited applications. By strictly controlling the combined Aluminum and Titanium (Al+Ti) content between 0.85% and 1.20% and mandating a high-temperature solution anneal, Hangbo Alloy provides a material that ensures structural integrity in the most demanding hydrocarbon processing and power generation environments.
1. Chemical Composition Comparison
The key to 800HT's superior performance lies in its restricted chemistry. The table below compares 800HT with its siblings, 800 and 800H.
| Element | Incoloy 800 (N08800) | Incoloy 800H (N08810) | Incoloy 800HT (N08811) |
|---|---|---|---|
| Nickel (Ni) | 30.0 - 35.0 | 30.0 - 35.0 | 30.0 - 35.0 |
| Chromium (Cr) | 19.0 - 23.0 | 19.0 - 23.0 | 19.0 - 23.0 |
| Iron (Fe) | 39.5 min | 39.5 min | 39.5 min |
| Carbon (C) | 0.10 max | 0.05 - 0.10 | 0.06 - 0.10 |
| Al + Ti | 0.30 - 1.20 | 0.30 - 1.20 | 0.85 - 1.20 |
| Aluminum (Al) | 0.15 - 0.60 | 0.15 - 0.60 | 0.25 - 0.60 |
| Titanium (Ti) | 0.15 - 0.60 | 0.15 - 0.60 | 0.25 - 0.60 |
Hangbo Alloy strictly controls the Al+Ti range to the upper limit (0.85% min) for 800HT to guarantee maximum creep-rupture life.
2. Physical and Mechanical Properties
Incoloy 800HT maintains excellent mechanical strength at elevated temperatures. Unlike many superalloys that become brittle over time, 800HT remains ductile during long-term service.
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.94 g/cm³ | 0.287 lb/in³ |
| Melting Range | 1357 - 1385 °C | 2475 - 2525 °F |
| Tensile Strength (Annealed) | 520 MPa min | 75 ksi min |
| Yield Strength (0.2% Offset) | 210 MPa min | 30 ksi min |
| Elongation | 30% min | 30% min |
3. High-Temperature Creep-Rupture Strength
The defining characteristic of 800HT is its creep strength. For service above 600°C (1100°F), 800HT provides the highest allowable design stresses of the 800 series.
| Temperature (°C) | Temperature (°F) | 10,000 Hour Rupture Strength (MPa) | 100,000 Hour Rupture Strength (MPa) |
|---|---|---|---|
| 650 | 1200 | 150 | 105 |
| 760 | 1400 | 60 | 38 |
| 870 | 1600 | 25 | 15 |
| 980 | 1800 | 10 | 6 |
4. Engineering Rationale: Why 800HT?
The combination of high nickel and high chromium provides 800HT with excellent resistance to both oxidation and carburization. In hydrocarbon processing, where materials are exposed to carbon-rich gases at high heat, 800HT forms a stable, protective oxide scale that resists carbon penetration.
5. Typical Industrial Applications
- Ethylene Pyrolysis Furnaces: Radiant tubes and pigtails.
- Steam Methane Reformers: Catalyst tubes and headers.
- Nitric Acid Production: Ammonia burner baskets.
- Power Generation: Superheater and reheater tubing in coal-fired plants.
Hangbo Alloy Quality Assurance
Every heat of Incoloy 800HT supplied by Hangbo Alloy Group undergoes:
- Chemical Validation: Verification of Al+Ti ≥ 0.85%.
- Grain Size Control: Confirmation of ASTM grain size No. 5 or coarser (essential for creep).
- Heat Treatment: Full solution anneal at 1150°C (2100°F) minimum.
Technical FAQ
Q1. Can Incoloy 800H be substituted for 800HT?
A: Only if the Al+Ti content of the 800H batch happens to be above 0.85%. Otherwise, 800HT must be used for design calculations requiring higher creep-rupture values.
Q2. What is the maximum service temperature for 800HT?
A: It is frequently used up to 980°C (1800°F) in non-load-bearing applications, though design codes usually limit pressure service to 815°C (1500°F).
Q3. How does 800HT handle carburizing environments?
A: Its high nickel (30-35%) and chromium (19-23%) content provides superior resistance to carburization compared to 300-series stainless steels.
Q4. Is 800HT susceptible to "Green Rot"?
A: It is generally resistant, but exposure to marginally oxidizing/reducing atmospheres should be monitored.
Q5. What welding filler is recommended for 800HT?
A: Inconel Filler Metal 82 or Welding Electrode 182 are typically used for maximum joint integrity.
Q6. Does 800HT become magnetic in service?
A: No, it remains austenitic and non-magnetic across its entire service range.
Q7. How do you verify the grain size?
A: Hangbo performs metallographic examination on every lot to ensure it meets the ASTM No. 5 or coarser requirement.
Q8. Can 800HT be used in sulfuric acid service?
A: It has moderate resistance, but for severe sulfuric acid, alloys like Incoloy 825 or Hastelloy C-276 are preferred.
Q9. What is the difference in cost between 800H and 800HT?
A: The cost is similar, but availability of 800HT is often more restricted due to the tighter chemistry requirements.
Q10. Why is Hangbo Alloy a preferred supplier for 800HT?
A: We maintain deep inventory of large-diameter bars and heavy plates with certified 0.85% min Al+Ti chemistry, saving customers long mill lead times.
This page is part of the technical guide series published by Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com). Data presented are typical engineering values compiled from recognized industry and producer sources and are provided for material selection guidance; the governing documents for any purchase are the applicable ASTM/ASME/AMS specifications and the certified mill test report. Contact Hangbo Alloy for current stock, mill certificates, PMI verification, and application engineering support.
