Incoloy 800H
Incoloy 800H (UNS N08810) is a controlled-chemistry variant of the Alloy 800 family. It has a narrowed carbon range of 0.05-0.10% and a minimum grain size of ASTM No. 5 or coarser. These two requirements, which are absent in standard 800, substantially improve creep and stress-rupture resistance in elevated-temperature service above 600 degrees C.
1. Chemical Composition
The table below defines the elemental limits for Incoloy 800H. The controlled carbon range and grain size requirement differentiate it from standard Incoloy 800.
Grain Size Requirement: Average grain size of ASTM No. 5 or coarser (per ASTM E112) is mandatory for all product forms.
| GRADE | UNS Designation |
Standard (Pipe) |
Chemical Requirement (Max) | |||||||||||
| C | Mn | P | S | Si | Ni | Cr | Mo | Cu | Fe | Ti | Al | |||
| Incoloy 800H | N08810 | B407 | 0.05-0.10 | ≤1.50 | ≤0.045 | ≤0.015 | ≤1.00 | 30.0-35.0 | 19.0-23.0 | / | ≤0.75 | ≥ 39.5 | 0.15-0.6 | 0.15-0.6 |
2. Mechanical Properties
The table below presents the mechanical properties of Incoloy 800H in the solution-annealed condition. Its creep performance is better than standard Alloy 800, though room-temperature values are similar.
| Grade | Condition & Size | Standard | Tensile Strength , MPa (ksi) |
Yield Strength, MPa (ksi) |
Elongation , % |
Hardness |
| N08810 | hot-finished annealed or cold-worked annealed | B407 | ≥450（65） | ≥170（25） | ≥30 | — |
Note: While room-temperature values are similar to Alloy 800, elevated-temperature creep and stress-rupture lives are substantially higher because of the coarse-grain structure.
3. Equivalent Grade
The table below provides international designations of Incoloy 800H (UNS N08810) across EN/DIN, JIS, ISO, GB/T, and GOST systems.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| Incoloy 800H | N08810 | - | NS1102 | NCF800H | - | 1.4958 | - |
4. Key Technical Advantages
- Coarse Grain Structure for Creep Resistance: The mandatory ASTM grain size No. 5 or coarser in Incoloy 800H is achieved by annealing above 1150 °C. Coarser grains reduce the grain boundary area, thereby minimizing grain boundary sliding and enhancing resistance to creep deformation. This results in a significantly longer stress-rupture life at temperatures between 600 °C and 900 °C.
- Controlled Carbon for Grain Boundary Carbide Precipitation: The minimum carbon of 0.05% provides enough M23C6 carbide precipitation at grain boundaries during service. These carbides pin boundaries against sliding and further improve creep and stress-rupture performance, a clear metallurgical advantage over the carbon-unrestricted standard 800 grade.
- ASME Code-Recognised High-Temperature Grade: Incoloy 800H is recognised in ASME Boiler and Pressure Vessel Code Section I and Section VIII Division 1 with allowable stress values at elevated temperature. This makes it a code-compliant choice for pressure equipment operating above 540 °C where Alloy 800 has no listed allowables.
Technical Note: Incoloy 800H carries UNS N08810, while 800HT carries UNS N08811. 800HT imposes a tighter Al+Ti range of 0.85-1.20% compared to 800H's 0.30-1.20%. Specifying the correct sub-designation is necessary in procurement documents to get the correct microstructural condition from the mill.
5. Common Manufacturing Standards
ASTM B407: Standard Specification for Seamless Pipe and Tube, Nickel-Iron-Chromium Alloy (UNS N08800, N08810, N08811). This is the governing specification for Incoloy 800H seamless pipe and tube, including grain size verification requirements.
ASME Standards (Non-linked):
- ASME SB-407: ASME Boiler Code equivalent of ASTM B407 for pressure vessel applications
- ASME Code Case 1325: Elevated-temperature allowable stresses for 800H in Section I applications
DIN/EN Standards:
- EN 10216-5: Seamless steel tubes for pressure purposes, nickel alloy grades (Material No. 1.4958)
- 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
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 Pyrolysis Furnace Tubes: Radiant coil and convection section tubing in steam cracker furnaces where metal temperatures reach 900-1000 °C and long creep life under thermal cycling is mandatory.
- Steam Reformer Tubes: Outlet manifolds and transfer lines in ammonia and methanol plants where the alloy must withstand stress at metal temperatures up to 870 °C in hydrogen-rich reformed gas service.
- Supercritical Steam Boilers: Superheater and reheater tubes in large utility boilers operating at high steam pressures and temperatures, where ASME code allowable stresses govern tube wall thickness design.
- Heat Treatment Furnace Equipment: Structural components, hangers, and radiant tubes in continuous annealing furnaces operating in the 700-900 °C range, where creep deformation under dead load is a design constraint.
- High-Temperature Flue Gas Systems: Transfer ducts and expansion joints in process heater flue gas circuits where the iron-nickel-chromium base resists oxidising combustion products at elevated wall temperatures.