Incoloy 800H Pipe Fittings: The High-Temperature Grade for Reformers and Pyrolysis
Incoloy 800H (UNS N08810, EN 1.4876) is the high-carbon, grain-coarsened variant of the Incoloy 800 family, specifically designed to maximise creep strength and rupture resistance above 600°C. It is the dominant material for steam reformer outlet manifolds, ethylene pyrolysis transfer lines, and high-temperature heat exchangers where creep governs the design life.
Incoloy 800 / 800H / 800HT — What Is the Difference?
| Grade | UNS | EN | C range | Al+Ti | Grain size | Use |
|---|---|---|---|---|---|---|
| Alloy 800 | N08800 | 1.4876 | ≤0.10% | 0.30–1.20% | Any | General oxidation/carburisation |
| Alloy 800H | N08810 | 1.4876 (H) | 0.05–0.10% | 0.15–0.60% | ASTM No. 5 or coarser | Creep and rupture above 600°C |
| Alloy 800HT | N08811 | 1.4876 (HT) | 0.06–0.10% | 0.25–0.60% each | ASTM No. 5 or coarser | Highest creep service — very high temp |
The controlled carbon range (0.05–0.10%) and minimum grain size requirement (ASTM No. 5) distinguish 800H from standard 800. Carbon contributes to grain boundary strengthening at elevated temperature; coarse grain (lower ASTM number = larger grains) resists grain boundary sliding — the mechanism of creep. Fine-grained material would be weaker in creep even with identical chemistry.
ASME Code Coverage
Incoloy 800H is covered by ASME Boiler and Pressure Vessel Code Section II, Part D as an allowable material up to 982°C (1800°F). This is higher than any austenitic stainless steel (316H: up to ~816°C in Section I code cases). In Europe, 1.4876 is listed in EN 10253-4 for high-temperature service and in AD 2000 Merkblatt W2. ASME P-Number is P-No. 45 (same group as Incoloy 825, but note that 825 is a lower-temperature corrosion-resistant grade — the two are NOT interchangeable despite sharing the P-Number group).
High-Temperature Performance
Incoloy 800H retains useful strength at temperatures where austenitic stainless grades (316H, 321, 347) are no longer creep-adequate. The alloy is also resistant to oxidation and carburisation — critical in reformer service where CO/CO₂ process gas would cause metal dusting in carbon steel or low-alloy grades. The aluminium and titanium content (controlled tightly in 800H) forms a protective Al₂O₃/TiO₂ layer that resists sulphidation and carburising atmospheres.
Applications
- Steam methane reformer (SMR) outlet manifolds and pigtails — 800–950°C process gas
- Ethylene cracking transfer lines — rapid quench service with cyclic thermal loading
- Ammonia reformer loops — high temperature hydrogen/nitrogen process
- Superheater header tubes and outlet headers — supercritical boilers
- High-temperature heat exchanger headers where shell-side reaches above 700°C
- Calciner and rotary kiln inlet/outlet nozzles
Incoloy 800H vs Inconel 625 for High-Temperature Service
| Property | Incoloy 800H | Inconel 625 |
|---|---|---|
| Max useful service temp | ~982°C (creep governs above 760°C) | ~980°C oxidising; 650°C creep limit |
| Creep strength at 800°C | Superior (designed for creep) | Moderate — 625 is a corrosion alloy |
| Corrosion resistance | Good at high temperature; not suited for HCl/seawater | Excellent in aqueous corrosion |
| Filler metal | ERNiCr-3 (FM82) | ERNiCrMo-3 (FM625) |
| ASME P-No. | P-No. 45 | P-No. 43 |
| Relative cost | Lower | Higher (Mo content) |
Welding Incoloy 800H
Filler: ERNiCr-3 (Inconel FM82) is the standard. ERNiCrMo-3 (FM625) is used as an alternative where higher corrosion resistance of the weld deposit is needed. Preheat is not required. Incoloy 800H is susceptible to hot cracking if the weld pool is contaminated with sulphur (S) or lead (Pb) — all surfaces must be thoroughly degreased and cleaned. Interpass temperature ≤150°C. Post-weld annealing at 1150°C / 30 min is recommended for high-temperature creep service to coarsen the grain structure in the HAZ back to a condition consistent with the base metal.
What to Specify on the Purchase Order
ASTM B366 WPNI (Alloy 800H, high-temperature grade), fitting type, NPS, schedule, EN 10204 3.1 with confirmed grain size ASTM No. 5 or coarser and carbon 0.05–0.10%. If 800H is being substituted for 800 (basic grade): always verify the heat number grain size — 800 and 800H have overlapping EN numbers (both called 1.4876) but the grain size and carbon minimum requirements differ and must be explicitly confirmed in the certificate.