Incoloy 800 vs 800H vs 800HT: Choosing the Right Grade for High-Temperature Creep Service
Incoloy 800, 800H, and 800HT are three variants of the same base Fe-Ni-Cr alloy (EN 1.4876 / ASTM B366 grade family) that are frequently specified interchangeably — an error that can lead to significant under-performance in creep service above 600°C. Each variant has a specific carbon content range and minimum grain size requirement that determines its creep strength. Specifying the base 800 grade for reformer or furnace piping expecting 800H or 800HT performance is one of the most common alloy family specification errors in the high-temperature piping industry.
The Three Grades
| Grade | UNS | EN W.Nr. | Carbon % | Grain Size | Max Service Temp |
|---|---|---|---|---|---|
| Incoloy 800 | N08800 | 1.4876 | ≤0.10% | No requirement | ~600°C |
| Incoloy 800H | N08810 | 1.4876 (H) | 0.05–0.10% | ASTM No. 5 min (coarse) | ~900°C |
| Incoloy 800HT | N08811 | 1.4876 (HT) | 0.06–0.10% | ASTM No. 5 min (coarse) | ~1000°C |
Why Carbon and Grain Size Control Creep Strength
Creep strength in austenitic alloys above 600°C depends on grain boundary carbide precipitation — carbides at grain boundaries impede grain boundary sliding, the dominant creep mechanism at elevated temperature. A controlled minimum carbon (0.05–0.10% for 800H/HT) ensures adequate carbide formation. A coarse grain size (ASTM No. 5 or coarser — larger grains) means fewer grain boundaries per unit volume, reducing the creep rate. Fine-grained 800 (no grain size control) has many more grain boundaries and significantly lower creep rupture life at the same temperature and stress.
The difference in 100,000-hour creep rupture strength at 650°C between 800 and 800H/HT can be 30–50 MPa — equivalent to 20–30% of the design stress. In a long-service reformer or ethylene cracker, this difference determines whether the fitting lasts 100,000 hours or fails in 40,000 hours.
Application Guide
- Below 600°C: Incoloy 800 is acceptable — creep is not the controlling mechanism. Used for hot sulphide and oxidation service in petroleum refining.
- 600–800°C: Specify Incoloy 800H minimum. This covers steam reformer outlet headers, ethylene pyrolysis transfer lines at the cooler end, and thermal oxidiser interconnects.
- 800–1000°C: Specify Incoloy 800HT — the additional grain size and carbon controls provide maximum creep strength. Used for reformer process outlet pigtails, hot gas manifolds, and secondary reformer outlet lines.
- Above 1000°C: Step up to Inconel 601 (Al₂O₃ film for oxidation resistance) or cast HK/HP alloys for fixed reformer tubes.
Ordering Note
The EN designation 1.4876 covers 800, 800H, and 800HT — you cannot distinguish them by EN number alone. Always state the UNS number (N08800 / N08810 / N08811) on the PO alongside the EN designation, and require the EN 10204 3.1 certificate to state carbon analysis, Al+Ti content (0.85–1.20% for 800HT), and grain size (ASTM No. 5 or coarser for 800H and 800HT). A certificate that only states "1.4876" without these details does not confirm which variant was supplied.