Incoloy 800HT: High-Temperature Creep Resistance & Furnace Applications

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Overview

Incoloy 800HT (UNS N08811 / W.Nr. 1.4959) is a controlled-composition version of Incoloy 800H, specifically optimized for enhanced creep-rupture strength in high-temperature service. While Incoloy 800H (UNS N08810) is limited to approximately 593°C (1100°F) for long-term creep applications, Incoloy 800HT extends the useful service range up to approximately 1100°C (2000°F) by tightening the aluminum and titanium chemistry range to 0.85–1.20% combined (Al+Ti). This controlled precipitation of fine gamma-prime [Ni₃(Al,Ti)] phase significantly improves long-term creep resistance without compromising oxidation resistance or fabricability.

The alloy is a nickel-iron-chromium grade with deliberately restricted carbon (0.06–0.10%) to ensure optimum high-temperature properties. Like its predecessors Incoloy 800 and 800H, it offers excellent resistance to oxidation, carburization, and sulfidation at elevated temperatures, combined with good aqueous corrosion resistance in many media. The "HT" designation specifically denotes the high-temperature grade with certified creep-rupture properties, making it the material of choice for critical furnace components in petrochemical, heat treatment, and thermal processing industries.

Incoloy 800HT is produced by electric arc melting or vacuum induction melting (VIM) followed by electroslag remelting (ESR) for higher cleanliness requirements. The material is typically solution-annealed at 1150–1200°C to dissolve precipitates and control grain size, which is critical for creep performance. Hangbo Alloy Group supplies Incoloy 800HT in seamless tubing, plate, sheet, and bar forms, with full creep-rupture certification per ASTM B408, B409, and AMS 5871.

Chemical Composition (ASTM B408 / UNS N08811)

Incoloy 800HT is distinguished from 800H by its tightly controlled aluminum plus titanium content (0.85–1.20%), which ensures reproducible creep-rupture properties. Carbon is also controlled in a narrower range (0.06–0.10%) compared to 800H (0.05–0.10%).

Element	Min %	Max %
Nickel (Ni)	30.0	35.0
Chromium (Cr)	19.0	23.0
Iron (Fe)	39.5	Balance
Carbon (C)	0.06	0.10
Aluminum (Al)	0.15	0.60
Titanium (Ti)	0.25	0.60
Al + Ti	0.85	1.20
Manganese (Mn)	—	1.50
Silicon (Si)	—	1.00
Copper (Cu)	—	0.75
Phosphorus (P)	—	0.045
Sulfur (S)	—	0.015

Physical Properties

The physical properties of Incoloy 800HT are essentially the same as Incoloy 800 and 800H, with the exception of long-term microstructural stability which is superior in the 800HT grade due to its controlled Al+Ti content.

Property	Value	Unit
Density	7.94	g/cm³
Melting Range	1357 – 1385	°C
Specific Heat (21°C)	460	J/kg·K
Thermal Conductivity (21°C)	11.5	W/m·K
Electrical Resistivity (21°C)	1.01	μΩ·m
Modulus of Elasticity (21°C)	197	GPa
Mean CTE (21–1000°C)	14.4 – 17.9	μm/m·°C

Mechanical Properties at Room Temperature

Incoloy 800HT is supplied in the solution-annealed condition with certified minimum mechanical properties. The alloy also carries certified minimum creep-rupture properties at 10,000 hours and 100,000 hours, which is the key differentiator from Incoloy 800 and 800H.

Property	ASTM B408 Minimum
Tensile Strength	≥450 MPa (65 ksi)
Yield Strength (0.2% offset)	≥170 MPa (25 ksi)
Elongation in 2 inches	≥30%
Hardness	139 – 179 HB

Creep-Rupture Properties

The defining advantage of Incoloy 800HT is its certified creep-rupture strength. ASTM B408 requires minimum creep-rupture values at 10,000 hours and 100,000 hours, which are guaranteed by the controlled Al+Ti chemistry and grain size control during solution annealing.

Temperature (°C)	100,000h Rupture Strength (MPa)
600	59
650	44
700	33
750	25
800	18
870	12
950	7

These certified values give designers confidence to use Incoloy 800HT at higher stresses and temperatures than Incoloy 800H, which does not carry guaranteed creep-rupture strength in the ASTM specification.

High-Temperature Performance

Incoloy 800HT is designed for continuous service in the 593–1100°C (1100–2000°F) range. Its high-temperature capabilities stem from the stable austenitic matrix reinforced by controlled gamma-prime precipitation. The alloy resists oxidation through the formation of a continuous, adherent Cr₂O₃ scale that self-heals if mechanically damaged at temperature.

Carburization resistance is another critical high-temperature property. Incoloy 800HT forms a protective oxide scale that acts as a barrier to carbon ingress, making it suitable for ethylene cracking furnaces, reformer tubes, and other carburizing atmospheres. Sulfidation resistance is also good in reducing sulfur-bearing gases, provided the environment is not strongly oxidizing.

Long-term thermal exposure can cause the precipitation of secondary phases including sigma, chi, and eta phases, particularly in the 600–900°C range. Incoloy 800HT's controlled chemistry minimizes the kinetics of these precipitation reactions, maintaining ductility and toughness over extended service lives. Solution annealing at 1150–1200°C restores the alloy to its optimum condition if previously exposed to intermediate temperatures.

Corrosion Resistance

In addition to oxidation and carburization resistance at high temperature, Incoloy 800HT offers useful aqueous corrosion resistance in many media, similar to Incoloy 800 and 800H.

• Oxidation Resistance: Excellent up to approximately 1100°C in air and combustion gases. The high chromium content forms a stable protective scale.
• Carburization Resistance: Superior to many heat-resistant stainless steels in carburizing atmospheres typical of ethylene furnaces and reformer units.
• Sulfidation Resistance: Good resistance to sulfur-bearing gases in reducing environments; performance is optimized by maintaining a low oxygen partial pressure.
• Aqueous Corrosion: Resistant to many organic acids, nitric acid at moderate concentrations, and atmospheric corrosion. Not recommended for strongly reducing acids such as hydrochloric or sulfuric.
• Stress-Corrosion Cracking (SCC): Highly resistant to chloride-induced SCC, far superior to austenitic stainless steels in this regard.

Applications

Incoloy 800HT is specified for critical high-temperature components where creep-rupture strength, oxidation resistance, and long service life are essential.

• Petrochemical Furnaces: Ethylene cracking tubes, reformer tubes, and outlet piping in steam methane reforming (SMR) units. The alloy's carburization resistance and creep strength make it the industry standard for these applications.
• Heat Treatment Equipment: Radiant tubes, muffles, retorts, and baskets in annealing, carburizing, and sintering furnaces operating up to 1100°C.
• Power Generation: Superheater and reheater tubes in coal-fired and waste-to-energy boilers where oxidation and creep are limiting factors.
• Chemical Processing: High-temperature reactors, heat exchanger tubing, and furnace components in nitric acid, urea, and caprolactam production.
• Ceramic and Glass Industries: Kiln components, burner nozzles, and high-temperature fixtures exposed to cyclic heating and aggressive atmospheres.

Available Product Forms

Hangbo Alloy Group manufactures and supplies Incoloy 800HT in product forms that meet the grain size and creep-rupture certification requirements of ASTM B408, B409, and B410.

• Seamless Tubes: ASTM B407 / ASME SB-407, OD 12.7 mm to 273 mm, wall thickness 1.65 mm to 28 mm, for furnace tubes and heat exchanger applications. Grain size typically ASTM 5 or coarser per customer specification.
• Plates & Sheets: ASTM B409 / ASME SB-409, thickness 2 mm to 100 mm, for furnace casings, ducting, and fabricated components.
• Round Bars: ASTM B408, diameters 10 mm to 300 mm, for machined components, flanges, and fasteners.
• Forgings: Open-die and ring-rolled forgings, custom shapes, with grain size control and creep certification available.
• Welding Wire: AWS A5.14 ERNiCrCoMo-1 (for overlay) or matching composition wires for joining Incoloy 800HT components.

Related Standards

Standard	Description
ASTM B407	Seamless Pipe and Tube
ASTM B408	Rod and Bar (with creep-rupture certification)
ASTM B409	Plate, Sheet, and Strip
ASTM B410	Strip (Cold-Rolled)
ASME SB-407 / SB-408 / SB-409	Boiler and Pressure Vessel Code
AMS 5871	Plate, Sheet, and Strip
DIN 1.4959	W.Nr. specification (X8NiCrAlTi32-21)

Incoloy 800 vs 800H vs 800HT — Which to Choose?

The selection between Incoloy 800, 800H, and 800HT depends primarily on service temperature and design life requirements:

• Incoloy 800: General-purpose grade for service up to approximately 593°C. No certified creep properties. Used in heat exchangers, piping, and moderate-temperature applications.
• Incoloy 800H: Higher carbon (0.05–0.10%) than 800 for improved creep strength up to approximately 593°C. May carry some creep data but not certified to ASTM B408 minimums.
• Incoloy 800HT: Optimized Al+Ti (0.85–1.20%) with certified minimum creep-rupture strength. Required for design temperatures above 593°C and for critical furnace tube applications where long-term reliability is essential.

Contact Us for Incoloy 800HT

Hangbo Alloy Group supplies Incoloy 800HT seamless tubes, plates, bars, and forgings with full creep-rupture certification and grain size reports. Our technical team assists with material selection, design stress calculations, welding procedure development, and export documentation. We serve petrochemical, heat treatment, power generation, and chemical processing industries worldwide.

For quotations, material certifications, or technical consultation, contact our sales team or call +86-136-1165-6360. We typically respond within 10 minutes.