Heat treatment of 800 series high-nickel alloys

The heat treatment of 800 series nickel alloys (e.g., 800H/HT) primarily involves high-temperature solution annealing (approximately 1150-1200°C/2100-2200°F) followed by rapid cooling. This may be followed by aging treatment (650-750°C/1200-1380°F) to control carbide precipitation, thereby optimizing strength, grain size (ASTM Grade 5 or coarser), creep properties, and fracture toughness, which are crucial for high-temperature corrosive environments.

What are the temperature limits for Incoloy 800 alloy?

For Incoloy 800H and 800HT alloys, the hot working temperature range is 1740–2190°F (950–1200°C) if the deformation is 5% or greater. If the degree of hot deformation is less than 5%, the recommended hot working temperature range is 1560–1920°F (850–1050°C).

Incoloy 800H/HT Key Processing Steps
Solution Annealing (Primary):
o Purpose: To dissolve precipitates, homogenize the microstructure, and form a coarse-grained structure to improve creep strength.
o Temperature: 1150-1200°C (2100-2200°F), suitable for 800H/HT alloys.
Cooling: Rapid cooling (water quenching or rapid air cooling) to avoid uncontrolled precipitation.

Aging Treatment (Optional/Specific):
o Purpose: To promote the formation of fine, stable γ' phase precipitates to enhance creep resistance.
o Temperature: 650-750°C (1200-1380°F), for 1-24 hours.
Stress Relief: Low-temperature treatment to balance stresses without recrystallization.

Incoloy 800H/800HT Alloy Heat Treatment
Heat treatment of Incoloy 800 alloys is crucial for achieving optimal high-temperature performance. Both nickel alloys 800H and 800HT require specific heat treatments to achieve optimal properties.

Solution Treatment: The initial solution treatment is performed at 1150-1180°C (2100-2150°F) to dissolve existing precipitates and homogenize the microstructure. This is followed by rapid cooling to prevent uncontrolled precipitation during cooling.

Aging Treatment: A key step in improving high-temperature strength is aging treatment at 650-750°C (1200-1380°F) for 1-24 hours. This treatment promotes the controlled precipitation of γ' phase particles, thereby improving creep resistance. The specific treatment time and temperature are adjusted according to the specific application requirements.

High-Temperature Performance
The excellent high-temperature performance of nickel alloys 800H and 800HT makes them ideal for applications requiring prolonged exposure to high temperatures and mechanical stress.

Creep Resistance: Both Incoloy 800 alloys exhibit excellent creep resistance, with 800HT performing even better due to its higher aluminum and titanium content. The γ' phase precipitates formed during heat treatment provide excellent resistance to deformation under long-term high-temperature loading. Typical creep rupture strength at 760°C (1400°F) for 100,000 hours is 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 stable chromium oxide layer formed protects the underlying metal from further oxidation, ensuring long-term reliability in high-temperature applications.

Carburization Resistance: Both grades of alloy exhibit good resistance to carburization in hydrocarbon environments, making them suitable for petrochemical applications involving exposure to carbon-containing gases at high temperatures. Their iron content contributes to resistance against carbon absorption compared to higher nickel alloys.

Mechanical Properties at High Temperatures:
800H at 760°C: Tensile strength ~170 MPa, Yield strength ~70 MPa
800HT at 760°C: Tensile strength ~200 MPa, Yield strength ~90 MPa
Nickel alloy 800HT material exhibits higher strength at elevated temperatures, making it an ideal choice for the most demanding high-temperature structural applications.

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