Incoloy 800H: Controlled Carbon + Coarse Grain = Higher Creep Strength

Incoloy 800H (UNS N08810) was developed in the 1960s to address a specific limitation of standard Incoloy 800: insufficient creep-rupture strength for sustained-load service above 600°C. While standard 800 with its ≤0.10% carbon specification performs well at moderate temperatures, its grain structure tends to be fine and variable, and its carbon content is not controlled tightly enough to optimize carbide precipitation for creep resistance.

800H solves this through two metallurgical modifications. First, carbon is controlled to a tight 0.05–0.10% range — enough to form grain-boundary carbides that pin grain boundaries and resist sliding under sustained stress at elevated temperature. Second, the alloy is solution-annealed at a higher temperature (≥1121°C / 2050°F) to produce coarse grain size (ASTM 5 or coarser). Coarse grains reduce the total grain-boundary area, slowing the diffusional creep mechanism that dominates at high temperature.

The result: 800H delivers approximately 2× the stress-rupture strength of standard 800 at 700°C and 100,000 hours. For refinery heater tubes designed for 100,000-hour service life, this translates directly to thinner walls, lighter weights, and lower material cost. For the highest creep-rupture requirement above 760°C, Incoloy 800HT (N08811) with elevated Al+Ti (0.85–1.20%) provides additional γ′-phase precipitation strengthening.

Five Defining Advantages of Incoloy 800H

Chemical Composition — ASTM B409 UNS N08810

Incoloy 800H shares the 800-family base chemistry (Ni 30–35%, Cr 19–23%, Fe balance) but tightens the carbon specification to a 0.05–0.10% range. This controlled carbon level ensures sufficient carbide formation for grain-boundary pinning without risking excessive sensitization:

★ The controlled carbon range is the defining difference from standard Incoloy 800. Al+Ti range matches standard 800; for elevated Al+Ti, specify 800HT.

Mechanical Properties — Room Temperature (Solution-Annealed, Coarse Grain)

800H’s room-temperature properties are slightly lower than standard 800 (due to coarser grain), but its elevated-temperature creep-rupture strength is significantly higher. The design value is the 100,000-hour stress-rupture strength at service temperature, per ASME Section VIII Code Case 1325.

Elevated-Temperature Stress-Rupture Strength (100,000 hr)

Values are typical — design per ASME Section VIII Code Case 1325 published stress values.

Elastic Constants (Annealed at 20°C)

Physical Properties — UNS N08810

800H’s physical properties match those of standard Incoloy 800 since the chemistry differs only in carbon control and grain size. The alloy is non-magnetic in all conditions.

Heat Treatment Procedures

800H’s heat treatment is critical to its performance. The solution-anneal temperature must be high enough to dissolve carbides and grow grains to ASTM 5 or coarser. Insufficient annealing produces fine grains and forfeits the creep-rupture advantage that justifies specifying 800H over standard 800.

Corrosion Resistance — Same Excellent Performance as 800 Family

800H’s corrosion resistance is essentially identical to standard Incoloy 800. The Ni-Fe-Cr base chemistry with Ti/Al stabilization provides the same excellent oxidation, carburization, and chloride SCC resistance. The controlled carbon does slightly increase susceptibility to sensitization, but Ti/Al stabilization mitigates this in the as-welded condition.

Oxidation Resistance ★

Excellent oxidation resistance to 1100°C (2010°F) in air and combustion atmospheres. The Cr‍₂O‍₃ + Al‍₂O‍₃ composite scale provides superior spallation resistance during thermal cycling compared to 300-series stainless steels.

Carburization Resistance ★

Outstanding resistance to carburization in petrochemical environments to 980°C. 800H is widely specified for ethylene cracker coils, reformer tubes, and other furnace components handling hydrocarbon process streams at high temperature.

Chloride SCC Resistance

The 30–35% Ni content provides superior chloride SCC resistance compared to austenitic stainless steels. Suitable for steam-generator tubing and water-handling service where chloride contamination is a concern.

Intergranular Corrosion (IGC)

The controlled 0.05–0.10% carbon slightly increases sensitization risk compared to standard 800. However, Ti/Al stabilization (Ti/C > 7:1) preferentially forms TiC and prevents harmful Cr‍₂‍₃C‍₆ precipitation. Passes ASTM A262 Practice A and E in solution-annealed condition. Post-weld solution anneal recommended for severe IGC service.

Incoloy 800 Family Selection Guide — 800H Position

800H is positioned between standard 800 and 800HT for the 600–760°C service band where intermediate creep strength is required at moderate cost premium:

Key Applications

800H is the workhorse material for refinery and petrochemical furnace tubing where sustained operation at 600–760°C requires ASME Code-approved creep-rupture design:

Applicable Standards & Specifications

Available Product Forms from HT PIPE

We supply Incoloy 800H in solution-annealed condition (annealing temperature ≥1121°C) with grain size ASTM 5 or coarser verified on every MTC. EN 10204 3.1 certification includes chemical analysis, mechanical testing, grain size, and heat-treatment records. ASTM A262 IGC testing per heat available on specification.

HT PIPE Project Experience — Incoloy 800H

For additional project references for refinery, petrochemical, or power generation applications, contact our technical sales team.

Frequently Asked Questions — Incoloy 800H