Incoloy alloy 800HT UNS N08811 ASTM B409 sheet bar tube

Overview

Several other alloy manufacturers entered the alloy 800H (UNS N08810) market and additional creep and rupture data became available. The Metals Property Council for ASME gathered this data and made a new analysis using parametric procedures, involving 87 heats and 1,052 data points. The additional data, from other manufacturers, included results with considerably lower strength, and the new analysis, which reflected the results of all the available data, resulted in a recommendation that the design stresses be revised. These revised values were lower for temperatures of 1100 through 1500°F (593-816°C), and about the same for 1600 and 1650°F (871 and 899°C).

Special Metals knew the importance of maintaining the aluminum and titanium contents in the upper portion of the specified material range. This resulted in higher creep and stress rupture properties than competitive alloy 800H. Therefore, to maintain higher allowable design stresses, the company introduced a variation of INCOLOY alloy 800H which is called INCOLOY alloy 800HT (UNS N08811). INCOLOY alloy 800HT has a restricted chemistry, within the limits of alloy 800H, and requires a heat treatment of 2100°F (1149°C) minimum. The carbon is 0.06 to 0.10% (alloy 800H is 0.05 to 0.10%), the Al + Ti is 0.85 to 1.20% (alloy 800H is 0.30 to 1.20% Al + Ti).

Limiting Chemical Composition, %

Nickel...................................................................................................................................................................................30.00-35.00

Chromium............................................................................................................................................................................19.00-23.00

Iron..........................................................................................................................................................................................39.50 min.

Carbon......................................................................................................................................................................................0.06-0.10

Manganese..............................................................................................................................................................................1.50 max.

Sulfur......................................................................................................................................................................................0.015 max.

Silicon .....................................................................................................................................................................................1.00 max.

Copper......................................................................................................................................................................................0.75 max.

Aluminum................................................................................................................................................................................0.25-0.60

Titanium...................................................................................................................................................................................0.25-0.60

Physical Constants

Mechanical properties

The major differences between alloys 800, 800H and 800HT are mechanical properties. The differences stem from the restricted compositions of alloys 800H and 800HT and the high-temperature anneals used for these alloys. In general, alloy 800 has higher mechanical properties at room temperature and during short-time exposure to elevated temperatures, whereas alloys 800H and 800HT have superior creep and rupture strength during extended hightemperature exposure.

Tensile-properties

Tensile properties and hardness of INCOLOY alloys

800H/800HT at high temperatures

Corrosion Resistance

Alloys 800, 800H and 800HT have the same nickel, chromium, and iron contents and generally display similar corrosion resistance. Since alloys 800H and 800HT are used for their high-temperature strength, corrosive environments to which these alloys are exposed normally involve hightemperature reactions such as oxidation and carburization.

Heating and Pickling

All material to be heated must be clean. Oil, paint, grease, shop soil and other foreign substances must be removed prior to the heating operation.

Heating must be performed in a low-sulfur atmosphere. Open heating must be done with low-sulfur fuel, and the furnace atmosphere must be maintained in a reducing condition to prevent excessive oxidation.

Because of the readiness with which chromium is oxidized into a refractory oxide by air, carbon dioxide or water vapor, 800-series alloys cannot be bright annealed in the usual industrial annealing furnace. Under closely controlled conditions, the alloy can be bright annealed in dry, pure hydrogen (dew point of -73°F (-58°C) or lower, less than 0.004% by volume water, and less than 0.007% by volume air)

INCOLOY alloys 800H and 800HT are normally annealed in box or muffle furnaces using prepared reducing atmospheres.A satisfactory atmosphere is formed by the products of combustion from low-sulfur natural gas burned with a deficiency of air. It produces a thin, adherent, greenblack film of oxide on the material. Oxidizing atmospheres produce a heavy black scale that is difficult to remove. Removal of such scale often requires considerable grinding.

The alloys usually must be pickled after being heated if a bright surface is required. Because of the alloy’s inherent resistance to chemical attack, specialized pickling procedures are needed.

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