The Incoloy 800 alloys family represents a series of iron-nickel-chromium superalloys specifically engineered for high-temperature service applications. Among these, nickel alloy 800H and nickel alloy 800HT material stand out as premium grades designed for the most demanding elevated temperature environments. These advanced materials combine excellent oxidation resistance, superior creep strength, and exceptional thermal stability, making them indispensable in power generation, petrochemical processing, and other critical high-temperature applications.
The evolution from the basic Incoloy 800 to the enhanced 800H and 800HT grades reflects decades of metallurgical advancement aimed at optimizing performance in increasingly severe operating conditions. While maintaining the fundamental iron-nickel-chromium composition that provides excellent corrosion resistance, these upgraded versions incorporate controlled chemistry and specialized heat treatments to deliver superior high-temperature mechanical properties and extended service life.
Chemical Composition and Metallurgical Design
The chemical composition of nickel alloy 800H and 800HT is carefully optimized to achieve maximum high-temperature performance while maintaining excellent fabricability and weldability. Both grades share similar base compositions but differ in their carbon, aluminum, and titanium contents, which significantly influence their high-temperature properties.
Nickel Alloy 800H Chemical Composition (% by weight):
- Nickel (Ni): 30.0-35.0%
- Chromium (Cr): 19.0-23.0%
- Iron (Fe): Balance (≥39.5%)
- Carbon (C): 0.05-0.10%
- Aluminum (Al): 0.15-0.60%
- Titanium (Ti): 0.15-0.60%
- Silicon (Si): 1.0% maximum
- Manganese (Mn): 1.5% maximum
- Sulfur (S): 0.015% maximum
Nickel Alloy 800HT Material Composition (% by weight):
- Nickel (Ni): 30.0-35.0%
- Chromium (Cr): 19.0-23.0%
- Iron (Fe): Balance (≥39.5%)
- Carbon (C): 0.05-0.10%
- Aluminum (Al): 0.15-0.60%
- Titanium (Ti): 0.15-0.60%
- Al + Ti: 0.85-1.20% (Higher than 800H)
- Silicon (Si): 1.0% maximum
- Manganese (Mn): 1.5% maximum
- Sulfur (S): 0.015% maximum
The key difference between these Incoloy 800 alloys lies in the combined aluminum and titanium content. Nickel alloy 800HT material contains higher levels of these elements (Al + Ti = 0.85-1.20%), compared to 800H, which provides enhanced creep resistance and improved high-temperature strength through precipitation hardening mechanisms.
The iron-nickel-chromium matrix provides excellent oxidation resistance and structural stability, while the controlled addition of aluminum and titanium enables the formation of gamma-prime (γ') precipitates that significantly strengthen the alloy at elevated temperatures. This precipitation hardening mechanism is what distinguishes these advanced grades from the basic Incoloy 800.
High-Temperature Properties and Performance
The exceptional high-temperature performance of nickel alloy 800H and 800HT makes them superior choices for applications involving sustained exposure to elevated temperatures and mechanical stress.
Creep Resistance: Both Incoloy 800 alloys exhibit outstanding creep resistance, with 800HT showing superior performance due to its higher aluminum and titanium content. The gamma-prime precipitates formed during heat treatment provide excellent resistance to deformation under long-term loading at high temperatures. Typical creep rupture strengths at 760°C (1400°F) for 100,000 hours are approximately 20-25 MPa for 800H and 25-30 MPa for 800HT.
Oxidation Resistance: The chromium content in these nickel alloy 800H and 800HT materials provides excellent oxidation resistance in air and other oxidizing environments up to 1150°C (2100°F). The formation of a stable chromium oxide scale protects the underlying metal from further oxidation, ensuring long-term reliability in high-temperature service.
Carburization Resistance: Both grades demonstrate good resistance to carburization in hydrocarbon environments, making them suitable for petrochemical applications involving exposure to carbonaceous gases at elevated temperatures. The iron content helps resist carbon pickup compared to higher nickel alloys.
Mechanical Properties at Elevated Temperature:
- 800H at 760°C: Tensile Strength ~170 MPa, Yield Strength ~70 MPa
- 800HT at 760°C: Tensile Strength ~200 MPa, Yield Strength ~90 MPa
The superior strength of nickel alloy 800HT material at elevated temperatures makes it the preferred choice for the most demanding high-temperature structural applications.
Heat Treatment and Microstructural Development
The heat treatment of Incoloy 800 alloys is crucial for developing optimal high-temperature properties. Both nickel alloy 800H and 800HT require specific thermal treatments to achieve maximum performance.
Solution Treatment: Initial solution treatment at 1150-1180°C (2100-2150°F) dissolves existing precipitates and homogenizes the microstructure. This is followed by rapid cooling to prevent uncontrolled precipitation during cooling.
Aging Treatment: The critical step for developing high-temperature strength is the aging treatment at 650-750°C (1200-1380°F) for 1-24 hours. This treatment promotes the controlled precipitation of gamma-prime particles that provide creep resistance. The exact time and temperature are tailored to the specific application requirements.
Microstructural Evolution: The microstructure of properly heat-treated material consists of an austenitic matrix strengthened by fine, uniformly distributed gamma-prime precipitates. These precipitates remain stable at service temperatures, providing long-term strength and creep resistance.
Fabrication and Processing Considerations
The fabrication of nickel alloy 800H and 800HT requires understanding of their unique characteristics and processing requirements.
Hot Working: Hot working should be performed at temperatures between 1040-1180°C (1900-2150°F). The materials exhibit good hot workability, allowing for forging, rolling, and extrusion operations. Proper heating and controlled cooling are essential to prevent cracking and ensure optimal microstructure.
Cold Working: Both Incoloy 800 alloys can be cold worked, though they work harden more rapidly than austenitic stainless steels. Cold working increases strength but may require intermediate annealing for extensive deformation operations.
Welding: Excellent weldability is a key advantage of these materials. Gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) are preferred processes. Matching composition filler metals ensure joint properties comparable to the base material. Post-weld heat treatment may be required for applications requiring maximum creep resistance.
Machining: Standard machining practices apply, though the materials' tendency to work harden requires proper tool selection and cutting parameters. Sharp tools and adequate cooling are essential for efficient machining operations.
Industrial Applications and Market Sectors
The exceptional high-temperature properties of nickel alloy 800H and 800HT make them essential in numerous demanding applications.
Power Generation: Steam generator tubing in nuclear power plants extensively uses these Incoloy 800 alloys. The combination of corrosion resistance, high-temperature strength, and neutron radiation resistance makes them ideal for nuclear steam generators and superheater applications.
Petrochemical Industry: Reformer tubes, cracking furnaces, and other high-temperature processing equipment benefit from the carburization resistance and high-temperature strength of nickel alloy 800H and 800HT material. These applications often involve sustained exposure to temperatures above 800°C in aggressive chemical environments.
Heat Treatment Equipment: Industrial furnaces, annealing equipment, and thermal processing systems utilize these alloys for critical components requiring dimensional stability and oxidation resistance at elevated temperatures.
Chemical Processing: Reactor vessels, heat exchangers, and piping systems in chemical plants handling high-temperature processes rely on the corrosion resistance and thermal stability of these materials.
Aerospace Applications: While less common than in industrial applications, certain aerospace components requiring high-temperature performance in oxidizing environments utilize these Incoloy 800 alloys.
Performance Comparison: 800H vs 800HT
The choice between nickel alloy 800H and 800HT depends on specific application requirements:
Nickel Alloy 800H:
- Good high-temperature strength and creep resistance
- Excellent general-purpose high-temperature performance
- More economical option for moderate temperature applications
- Suitable for temperatures up to 1000°C in many applications
Nickel Alloy 800HT Material:
- Superior creep resistance due to higher Al + Ti content
- Enhanced high-temperature strength for demanding applications
- Preferred for critical applications requiring maximum performance
- Optimized for temperatures above 700°C with sustained loading
Code Compliance and Standards
Both materials comply with major international standards:
- ASME: Section II, Part B (SA-409 for 800H, SA-409 for 800HT)
- ASTM: B407, B408, B409, B564
- EN/DIN: Various European standards for high-temperature alloys
- Nuclear Codes: ASME Section III for nuclear applications
Economic Considerations and Selection Criteria
The selection between different Incoloy 800 alloys involves balancing performance requirements with economic considerations. While nickel alloy 800HT material commands a premium over 800H, the enhanced performance may justify the additional cost in critical applications where failure could result in significant downtime or safety concerns.
Lifecycle cost analysis often favors these premium alloys due to their extended service life and reduced maintenance requirements compared to lower-performance alternatives. The materials' excellent fabricability also contributes to overall cost-effectiveness through reduced manufacturing complexity.
Future Developments and Emerging Applications
Research continues into optimizing these Incoloy 800 alloys for even more demanding applications. Advanced processing techniques, including powder metallurgy and additive manufacturing, are being explored to create components with enhanced properties or complex geometries not achievable through conventional processing.
Environmental concerns and efficiency improvements in power generation and chemical processing are driving demand for materials capable of operating at higher temperatures and pressures, positioning these advanced nickel-based alloys for continued growth in critical applications.
Conclusion
Nickel alloy 800H and 800HT represent the pinnacle of iron-nickel-chromium superalloy technology for high-temperature applications. Their exceptional combination of oxidation resistance, creep strength, and fabricability makes them indispensable for critical applications in power generation, petrochemical processing, and other industries where reliable high-temperature performance is essential.
The choice between these Incoloy 800 alloys depends on specific service requirements, with 800HT offering superior performance for the most demanding applications. Understanding their properties, processing requirements, and application limits ensures optimal material selection and reliable performance in service. As industries continue to push operating temperatures higher in pursuit of efficiency and performance, these advanced materials will remain crucial for enabling technological advancement in high-temperature applications.
