1.4876 X10NiCrAlTi32-21 X10NiCrAlTi32-20 Alloy 800 | UNS N08800

"Alloy 800" is an austenitic nickel-iron-chromium alloy with good resistance to oxidation, carburization and high-temperature corrosion. The material family comprises three standardized variants: Alloy 800, Alloy 800H and Alloy 800HT. In English the family is also frequently listed under the registered trademark Incoloy® 800 / 800H / 800HT. Tenders also feature the collective designation "Alloy 800(H/HT)" and Valbruna's own designation AN1.

The three variants are based on the same basic composition – the differences in the C content as well as the Al and Ti contents significantly influence the high-temperature behavior. Alloy 800 (1.4876): low carbon content (≤ 0.10 %), no special requirements regarding grain size or Al + Ti. The classic choice for medium temperatures up to about 600 °C. Alloy 800H (1.4958): defined carbon content range (0.05–0.10 %) and a prescribed minimum grain size. This results in significantly better creep and stress-rupture strength above 600 °C. Alloy 800HT (1.4959): like 800H, additionally with a clearly defined range for the sum of the precipitation-forming elements aluminum and titanium (0.85–1.20 %). The result is a precipitation hardening that is stable up to high temperatures, raising the creep strength up to high temperatures (up to approx. 1,000 °C). Rule of thumb: below 600 °C, 800 is sufficient. Between 600 and 800 °C, 800H is usually chosen. Above 800 °C – e.g. for reformer or pyrolysis tubes – 800HT is the right material.

The reference analysis is typically in the following ranges (mass fractions): nickel (Ni) 30.0–35.0 %, chromium (Cr) 19.0–23.0 %, iron (Fe) at least 39.5 % (balance), aluminum (Al) 0.15–0.60 %, titanium (Ti) 0.15–0.60 %, manganese (Mn) up to 1.5 %, silicon (Si) up to 1.0 %, copper (Cu) up to 0.75 %, carbon (C) up to 0.10 % (800) / 0.05–0.10 % (800H) / 0.06–0.10 % (800HT). For 800HT, a range for the sum Al + Ti between 0.85 and 1.20 % also applies. The precipitation structure resulting after appropriate heat treatment is the actual lever for the increased stress-rupture strength of the HT variant.

At room temperature, 1.4876 as solution-annealed bar or wire material usually achieves a tensile strength from 500 MPa, a 0.2% proof stress from 210 MPa and an elongation at break of at least 30 %. More interesting, however, is the range above this: the alloy retains its strength well beyond 600 °C without embrittling, and shows good stress-rupture and creep strength. This is exactly where 800H and 800HT come in – they are specially optimized for long service lives under constant high-temperature loading. The family is also insensitive to σ-phase embrittlement, which can occur in some austenitic stainless steels at service temperatures in the range 600–900 °C.

The material family has been established for decades wherever permanent heat meets chemically demanding atmospheres. Common fields of application: petrochemicals and refineries (reformer and pyrolysis tubes, cracker internals, convection bundles – predominantly 800H/HT); plant construction for heat treatment (furnace internals, muffles, support grates, annealing boxes); heating element technology (heating element tubes, heating cartridges, industrial furnaces); energy and power plant construction (steam superheaters, heat exchangers, components in high-temperature reactors); chemical industry (molten salts, acid vessels, vessels with oxidizing and reducing atmospheres). For connecting and fastening elements in such plants, bar and wire materials made of 1.4876 / 1.4958 / 1.4959 are sourced as semi-finished products.

Three points make the alloy particularly attractive at high temperatures. First, the high chromium content results in a stable, firmly adhering oxide layer that makes the material scale-resistant in air up to approx. 1,000 °C. Second, the high nickel content reduces diffusion and thus the tendency toward carburization and nitriding – an important advantage in atmospheres containing hydrocarbons or ammonia. Third, Alloy 800 is largely resistant to chloride-induced stress corrosion cracking at room and medium temperatures, where conventional austenitic steels can fail. The material reaches its limits in atmospheres with high sulfur partial pressure (sulfidation) as well as in strongly oxidizing acids at high temperatures – here higher-alloyed nickel-based materials are the better choice.

A rough guide for continuous applications: Alloy 800 (1.4876): suitable up to approx. 600 °C; above this limit the creep strength drops significantly. Alloy 800H (1.4958): standard range 600–800 °C. Alloy 800HT (1.4959): approx. 1,000 °C, and higher for short periods. Towards low temperatures the use is not limited; the grades are also used in cryogenic applications. An important design boundary condition is thermal expansion: it lies between austenitic stainless steels and pure nickel-based materials, which facilitates combination with different materials in plants.

Alloy 800 can be processed well with the fusion welding processes TIG, MIG/MAG, plasma and SAW; manual arc welding (MMA) with covered stick electrodes is also possible. As filler material for welding wire and wire electrodes, a NiCr variant such as ERNiCr-3 / SNi6082 is usually used, since it is easy to process and the weld metal has good corrosion resistance. Post-weld heat treatment of the weld seam is normally not required; however, for thick-walled components or highly stressed 800H/HT structures, solution annealing should be planned. When machining, the alloy behaves similarly demandingly to austenitic stainless steels: it tends to work-harden, which is why stable machines, sharp and wear-resistant indexable inserts, high feed rates and continuous cooling are important. Valbruna supplies the material family under the internal designation AN1 as bar (round, hexagonal, square), as wire rod and drawn wire as well as drawn profile. For tenders it is advisable to specify, in addition to the material number (1.4876 or 1.4958 / 1.4959), also the UNS number with reference to a concrete material standard such as ASTM B408 for bars and wires.