Incoloy 800H Chemical Composition (UNS N08810 + UNS N08811)

Incoloy 800H (UNS N08810) chemistry is a controlled subset of the standard Incoloy 800 (UNS N08800) envelope with carbon held to 0.05 to 0.10 percent and a combined aluminium plus titanium window of 0.30 to 1.20 percent. Incoloy 800HT (UNS N08811) further restricts the combined Al + Ti window to 0.85 to 1.20 percent with the same nickel + chromium + iron base. Both variants run 30 to 35 percent nickel and 19 to 23 percent chromium with the balance iron, so they sit in the nickel-iron-chromium family rather than the nickel-base superalloys. The full 11-element envelope below is the ASTM B series + Special Metals technical bulletin reference for all four product forms supplied by TorqBolt: pipe, tube, plate, sheet, bar, forgings and machined fasteners. Element ranges are wt percent unless tagged maximum.

Standard Incoloy 800 (UNS N08800) caps carbon at 0.10 percent maximum with no lower bound. A heat that runs 0.03 percent C still meets 800, but the M23C6 carbide population at the grain boundaries is starved and the creep-rupture strength at 815 deg C (1500 deg F) drops below the ASME Section VIII design table values.

Incoloy 800H (UNS N08810) closes this loophole with a 0.05 percent minimum, guaranteeing the carbide population that controls creep cavity coalescence at the grain boundaries. The 0.10 percent ceiling stays so that intergranular corrosion in aqueous chloride service does not become a service concern. Specifying 800H instead of 800 is the procurement difference between a heat that will hold the published 100,000-hour rupture stress and a heat that will not.

Incoloy 800HT (UNS N08811) lifts the lower bound on the combined Al + Ti from 0.30 to 0.85 percent. Above this threshold, the alloy precipitates a measurable population of gamma-prime Ni3(Al,Ti) coherent precipitates after long-term exposure between 540 deg C and 870 deg C. The precipitate lifts the 100,000-hour rupture stress at 815 deg C from approximately 25 MPa (3.6 ksi) for plain 800H to approximately 35 MPa (5 ksi) for 800HT, and qualifies the alloy for ASME Section VIII design temperatures up to 899 deg C (1650 deg F) versus 815 deg C (1500 deg F) for plain 800H. The trade-off is reduced room-temperature ductility after long-term exposure due to the gamma-prime population, so 800HT is specified only where the service temperature genuinely demands the additional creep strength.

The 800H and 800HT chemistry must be paired with a coarse grain practice. ASTM B407, B408 and B409 require ASTM grain size 5 or coarser after the high-temperature solution anneal at 1149 deg C (2100 deg F) minimum followed by water quench or rapid air cool. A fine-grain heat that meets every element envelope in the table above still fails the 800H qualification if grain size is 6 or finer. Mechanical property certification on every TorqBolt mill test report includes the ASTM E112 grain size measurement.

Each TorqBolt heat is chemistry-tested on the ladle and product sample basis required by ASTM B407 paragraph 8 and ASTM B408 paragraph 8. Element results are reported on the EN 10204 type 3.1 mill test certificate by default, with optical emission spectrometry for major elements (Ni, Cr, Fe, Mn, Si, Cu) and combustion analysis (LECO) for carbon and sulphur. Aluminium and titanium are reported by inductively coupled plasma (ICP) to meet the 0.85 to 1.20 percent combined window resolution required for the 800HT qualification. Type 3.2 certification with Lloyds Register, DNV, BV, SGS or TUV witness on the cast and product analysis is supplied on call-out at the order stage.