Incoloy 800HT
Incoloy 800HT (UNS N08811) is the most tightly controlled variant of the Alloy 800 family. It has an aluminum plus titanium range of 0.85-1.20%, well above the 0.30-1.20% allowed in 800H. It also keeps the mandatory ASTM No. 5 coarse grain size. Together, these produce the highest creep and stress-rupture strength in the family for service above 900 °C.
1. Chemical Composition
The table below specifies the elemental limits for Incoloy 800HT. The raised Al+Ti minimum of 0.85% and the coarse grain requirement are what set it apart from 800H.
| GRADE | UNS Designation |
Standard (Pipe) |
Chemical Requirement (Max) | ||||||||||||
| C | Mn | P | S | Si | Ni | Cr | Mo | Cu | Fe | Ti | Al | Al+Ti | |||
| Incoloy 800HT | N08811 | B407 | 0.06-0.10 | ≤1.50 | ≤0.045 | ≤0.015 | ≤1.00 | 30.0-35.0 | 19.0-23.0 | / | ≤0.75 | ≥ 39.5 | 0.25-0.6 | 0.25-0.6 | 0.85-1.20 |
Grain Size Requirement: Average grain size of ASTM No. 5 or coarser (per ASTM E112) is mandatory. This, combined with the elevated Al+Ti floor, defines the 800HT microstructural condition.
2. Mechanical Properties
The values below apply to Incoloy 800HT in the solution-annealed and coarse-grained condition. At equivalent stress levels above 900 °C, the creep rupture life for 800HT exceeds that of 800H.
| Grade | Condition & Size | Standard | Tensile Strength , MPa (ksi) |
Yield Strength, MPa (ksi) |
Elongation , % |
Hardness |
| N08811 | hot-finished annealed or cold-worked annealed | B407 | ≥450（65） | ≥170（25） | ≥30 | — |
Note: Room-temperature minimums are equivalent to 800 and 800H. The advantage of 800HT shows up only in elevated-temperature creep, stress-rupture, and long-term oxidation tests above 850 °C.
3. Equivalent Grade
The table below maps Incoloy 800HT (UNS N08811) to its corresponding designations in international material standards. 800H and 800HT share the UNS N0881x series but have distinct ASTM sub-designations.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| Incoloy 800HT | N08811 | - | NS112 | NCF800HT | - | 1.4859 | - |
4. Key Technical Advantages
- Maximized Gamma-Prime Precipitation for Creep Resistance: The elevated minimum Al+Ti of 0.85% in Incoloy 800HT produces a higher volume fraction of gamma-prime precipitates during high-temperature service than 800H does. These coherent precipitates block dislocation movement and grain boundary sliding, extending creep rupture life at metal temperatures above 900 °C.
- Better Oxidation Resistance from Increased Aluminum Content: The higher aluminum floor in 800HT promotes formation of an alumina-enriched inner oxide layer beneath the chromia outer scale. This improves scale adhesion and reduces metal recession rate during prolonged exposure above 900 °C in air and steam-containing atmospheres.
- ASME Code Allowable Stresses at Extreme Temperatures: Incoloy 800HT has the highest ASME elevated-temperature allowable stresses of the Alloy 800 sub-family. It is the code-mandated choice for pressure components operating at metal temperatures between 760 °C and 980 °C in ASME Section I and VIII Division 1 designs.
- Technical Note: Despite sharing nominal composition ranges with 800H, Incoloy 800HT is not interchangeable in ASME code applications. Each has separately tabulated allowable stresses. Ordering documentation must clearly specify UNS N08811 to make sure the correct Al+Ti floor is met in mill certification.
5. Common Manufacturing Standards
ASTM B407: Standard Specification for Seamless Pipe and Tube, Nickel-Iron-Chromium Alloy (UNS N08800, N08810, N08811). Covers Incoloy 800HT in all seamless tubular forms, with requirements for chemical composition, grain size, and mechanical testing.
ASTM B423: Standard Specification for Seamless Pipe and Tube, Nickel-Iron-Chromium-Molybdenum-Copper Alloy (for related Incoloy family products).
ASME Standards (Non-linked):
- ASME SB-407: ASME Boiler Code equivalent of ASTM B407
- ASME Section II Part B: Material specification with elevated-temperature allowable stress tables for N08811
DIN/EN Standards:
- EN 10216-5: Seamless steel tubes for pressure purposes, nickel alloy grades (Material No. 1.4959)
- DIN 17459: Seamless circular tubes made of nickel alloys
JIS Standards:
- JIS H 4552: Nickel and nickel alloy seamless pipes and tubes
GB/T Standards:
- GB/T 15007: Corrosion-resistant alloy grades (NS1102HT designation)
Summary Table — Standard Equivalents:
| ASTM | ASME | EN/DIN | JIS | GB/T |
| ASTM B407 | ASME SB-407 | EN 10216-5 / DIN 17459 | JIS H 4552 | GB/T 15007 |
6. Primary Applications
- Ethylene and Propylene Cracker Radiant Tubes: This is the most demanding tube position in steam cracker furnaces. Metal temperatures can reach 1000-1050 °C and tube life depends on creep rupture. It is the primary design case for which 800HT was developed.
- High-Temperature Hydrogen Reforming: Outlet pigtails and transfer line tubing in steam methane reformers where combined creep load, carburizing reformed gas, and cyclic thermal stresses create one of the harshest operating environments in the petrochemical industry.
- Nuclear Process Heat Applications: Intermediate heat exchanger tubing in high-temperature gas-cooled reactors (HTGR) and very high-temperature reactor (VHTR) designs where ASME code requires the verified high-creep-strength of N08811.
- Superheater Tubes in Waste-to-Energy Boilers: Tubing in the highest-temperature stages of municipal solid waste incinerators and biomass boilers, where fireside corrosion, oxidation, and creep act simultaneously as design constraints.
- Direct-Reduction Iron (DRI) Shaft Furnaces: Gas distribution headers and internal structural tubing in direct reduction plants operating with reducing reformed gas at metal temperatures exceeding 850 °C.