Alloy 800 H/HT
Alloy 800H/800HT® are solid solution strengthened, iron- nickel-chromium alloys, typically offered as one, dual certified alloy meeting the elemental requirements of both alloys. The principle difference between alloy 800H and 800HT® material is the restricted aluminum and titanium content in 800HT®, which results in higher creep and stress rupture properties. Both Alloy 800H and 800HT® are superior to Alloy 800, having greater creep and stress rupture properties. The alloy is typically used for applications involving long-term exposure to high-temperatures where resistance is needed from oxidation, carburization and other types of high-temperature corrosion. Hydrocarbon processing, heat-treat furnaces and power generation are some of the most common applications where 800H & 800HT® is utilized. Pressure vessels and vessel components constructed from 800H and 800HT® are approved under the ASME, Boiler and Pressure Vessel Code, Section VIII, Division 1.
Incoloy® alloys 800H and 800HT® have significantly higher creep and rupture strength than Incoloy® Alloy 800. The three alloys have nearly identical chemical composition limits. However, the chemical composition limits vary with carbon, aluminum and titanium. The carbon content of Incoloy® alloy 800 (UNS n08800) is 0.10% max with no limit on the lower end. The carbon content for Incoloy® alloy 800H (UNS N08810) is 0.05% to 0.10% which is the upper end of the 0.10% maximum specified for Incoloy® alloy 800. The chemical limits for Incoloy® 800HT® (UNS08811) are even more restrictive yet still within the limits specified for Incoloy® 800H. The carbon content for Incoloy® alloy 800HT® is further restricted to 0.06%-0.10%. Additionally the Al plus Ti content of Incoloy® alloy 800HT® is restricted to 0.85%-1.20%. Note that the chemical composition for Incoloy® alloy 800HT® will always be within the limits of Incoloy® alloy 800H. Note also that the limits for Incoloy® alloy 800H may or may not be within the limits of Incoloy® alloy 800HT®.
Resistance to Corrosion
High nickel and chromium contents in both alloy 800H and 800HT® result in excellent resistance to oxidation, carburization and sulfidation. The high nickel content also increases the resistance to nitriding, although not as good as other alloys such as Alloy 600, which contains a higher percentage of nickel.
Fabrication and Heat Treatment
Hot-working temperatures should be between 1600°F and 2200°F with heavy forming to be performed at temperatures above 1850°F. No forming should be performed between 1200°F and 1600°F and preheating of tools and dies to 500°F is suggested to avoid chill. Cooling after hot working should be as quick as possible, avoiding extensive time at temperatures between 1000°F and 1400°F. Cold working should be performed on material in the annealed condition. Stress relief or annealing should be considered depending on the total amount of strain induced by fabrication taking into consideration the intended service of the material.
Because excessive grain growth can negatively affect mechanical properties, care must be taken in selecting an annealing temperature and time at temperature for the process. If material is to be deformed more than 20% and a final anneal is desired, fine-grain material should be considered for the starting stock.
Stress relief is performed between 1000°F and 1600°F and should be at temperature for 1 hour per inch of material or for a minimum of 11⁄2 hours at 1600°F, whichever is greater. Recrystallization anneal is achieved at temperatures between 2100°F and 2200°F.
Common Trade Names:
INCOLOY® and 800HT® are registered trademark of the Special Metals Corporation group of companies.