We take a look at the INCONEL alloy 800 series, in particular the INCOLOY Alloy 800H and 800HT variants, which offer significantly higher creep and rupture strength over their predecessor

The INCOLOY alloy 800 series is a range of specialist alloys specifically designed to meet a need in the market for a material that can maintain its strength and offer excellent corrosion resistance at high temperatures.

The original metal, INCOLOY Alloy 800, was introduced back in the 1950s as a nickel-iron-chromium alloy to meet the gap in the market for a heat- and corrosion-resistant alloy with a relatively low nickel content and low carbon content. It has been used in the decades since for some steam generation plants, as well as in furnace components and equipment, petrochemical furnace cracker tubes, pigtails and headers, and sheathing for electrical heating elements.

Given its ability to resist oxidation, carburization and other types of high-temperature corrosion, as well as its high strength under these conditions, INCOLOY alloy 800 is still a widely-used material. However, it has been refined to form two newer versions: INCOLOY alloy 800H and INCOLOY alloy 800HT.

INCOLOY alloy 800H

INCOLOY Alloy 800H is a controlled composition derivative of INCOLOY alloy 800, with the particular difference being its more precise carbon level. While INCOLOY alloy 800 offers carbon levels to a maximum of 0.10% (with no lower limit), INCOLOY alloy 800H has a specific carbon range of 0.05-0.10%. The 0.05-0.10% range has been proven to offer higher creep and rupture properties, which is why it is a key specification of INCOLOY alloy 800H, along with an average grain size of ASTM 5 or coarser.

Applications of INCOLOY alloy 800H include petrochemical processing reformer tubing; bellows and flare tips in refinery service; superheater and reheater tubing in power generation; and heat treatment retorts, muffles jigs and fixtures.

INCOLOY alloy 800HT

INCOLOY Alloy 800HT is a controlled composition derivative of INCOLOY alloy 800H, based on the original compositional structure of INCOLOY alloy 800. It has a more precise carbon content of 0.06-0.10%, but the main difference in the HT version of the metal is that it has a stricter range of aluminium and titanium contents. In development, it was found that keeping these elements in the upper portion of the allowable range for INCOLOY alloy 800H ensured higher creep and stress rupture properties. INCOLOY alloy 800HT maintains higher allowable design stresses through a restricted chemistry and requires a heat treatment of 1149°C.

It is used for many of the same applications as INCOLOY alloy 800HT and both remain in use simultaneously. However, in some certain specifications INCOLOY alloy 800H may not be suitable for use, dependent on the service temperatures, in which case INCOLOY alloy 800HT can be employed instead.

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