# Alloy 800HT (UNS N08811): Properties and Applications
## Introduction to Alloy 800HT
Alloy 800HT, also known by its UNS designation N08811, is a high-temperature austenitic nickel-iron-chromium alloy. This material is widely recognized for its excellent resistance to oxidation and carburization at elevated temperatures, making it a preferred choice for various industrial applications.
## Chemical Composition
The chemical composition of Alloy 800HT plays a crucial role in its performance characteristics. The alloy primarily consists of:
– Nickel (Ni): 30-35%
– Chromium (Cr): 19-23%
– Iron (Fe): Balance
– Carbon (C): 0.06-0.10%
– Aluminum (Al): 0.15-0.60%
– Titanium (Ti): 0.15-0.60%
This specific composition provides the alloy with its unique combination of strength and corrosion resistance.
## Mechanical Properties
Alloy 800HT exhibits impressive mechanical properties, especially at high temperatures:
– Tensile Strength: 550-760 MPa
– Yield Strength: 210-350 MPa
– Elongation: 30-45%
– Hardness: 150-200 HB
These properties remain stable even under prolonged exposure to high temperatures, making it suitable for demanding applications.
## Physical Properties
The physical characteristics of Alloy 800HT include:
– Density: 7.94 g/cm³
– Melting Range: 1350-1400°C
– Thermal Conductivity: 11.2 W/m·K at 100°C
– Electrical Resistivity: 1.02 μΩ·m at 20°C
These properties contribute to the alloy’s excellent thermal stability and electrical performance.
## Key Advantages
Alloy 800HT offers several significant advantages:
– Exceptional high-temperature strength
– Outstanding resistance to oxidation and carburization
– Good resistance to sulfidation
– Excellent creep-rupture properties
– Maintains structural integrity in cyclic heating conditions
## Common Applications
Due to its remarkable properties, Alloy 800HT finds applications in various industries:
Keyword: Alloy 800HT UNS N08811
### Petrochemical Industry
– Steam methane reformer tubes
– Ethylene pyrolysis furnaces
– Hydrocarbon cracking equipment
### Power Generation
– Heat exchangers
– Superheater tubes
– Boiler components
### Industrial Furnaces
– Radiant tubes
– Retorts
– Muffles
### Nuclear Industry
– Heat exchangers
– Steam generator tubing
## Heat Treatment
Proper heat treatment enhances the performance of Alloy 800HT:
– Solution annealing is typically performed at 1100-1200°C
– Rapid cooling is essential to maintain the desired microstructure
– Stress relieving may be performed at 870-925°C for specific applications
## Welding Considerations
Alloy 800HT can be welded using common techniques:
– Gas tungsten arc welding (GTAW)
– Gas metal arc welding (GMAW)
– Shielded metal arc welding (SMAW)
– Submerged arc welding (SAW)
Post-weld heat treatment is often recommended to restore optimal properties.
## Comparison with Similar Alloys
While similar to Alloy 800 and Alloy 800H, 800HT offers:
– Higher creep strength
– Improved high-temperature stability
– Better resistance to thermal fatigue
– More consistent performance in cyclic conditions
## Conclusion
Alloy 800HT (UNS N08811) stands out as a superior material for high-temperature applications where strength, oxidation resistance, and carburization resistance are critical. Its balanced composition and excellent mechanical properties make it indispensable in industries ranging from petrochemical processing to power generation. When selecting materials for demanding thermal environments, Alloy 800HT should be strongly considered for its proven performance and reliability.
