Incoloy 800H Creep-Rupture Strength

Creep-rupture strength is the single property that justifies the Incoloy 800H price premium over 304H, 316H, 321H and 310H stainless above 650 deg C (1200 deg F). The controlled 0.05 to 0.10 percent carbon window combined with the ASTM grain size 5 or coarser practice precipitates a stable M23C6 carbide population at the grain boundaries that locks creep cavity nucleation and coalescence. Incoloy 800HT (UNS N08811) adds the 0.85 to 1.20 percent combined Al + Ti window to precipitate gamma-prime Ni3(Al,Ti) on long-term exposure, lifting the 100,000-hour rupture stress at 815 deg C by approximately 40 percent over plain 800H. The published Larson-Miller parameter curves from the Special Metals technical bulletin Tables 6 and 7 cover the full design envelope from 540 deg C (1000 deg F) to 870 deg C (1600 deg F) at 1,000, 10,000 and 100,000 hours. These curves are the foundation of the ASME Section II Part D Table 1A allowable design stress for SB-407 / SB-408 / SB-409 / SB-564 above 649 deg C.

The Larson-Miller parameter collapses time and temperature onto a single curve for engineering extrapolation. The standard form is LMP = T (C + log t), with T in Kelvin, t in hours and C taken as 20 for Incoloy 800H per the Special Metals bulletin practice. The published 800H rupture curve runs from LMP 25,000 (high stress, short life) to LMP 33,000 (low stress, long life). The 100,000-hour design point at 815 deg C corresponds to LMP = 1088 (20 + log 100,000) = 1088 x 25 = 27,200. The corresponding rupture stress on the master curve is approximately 22 MPa (3.2 ksi), consistent with the table above. Engineering extrapolation outside the test range is limited to plus or minus 50 deg C without explicit ASME Code Case approval.

The dominant creep mechanism in 800H above 650 deg C is grain-boundary sliding accommodated by dislocation climb in the grain interior. The M23C6 carbides precipitated by the controlled carbon window resist grain-boundary sliding by pinning the boundary, while the coarse grain size (ASTM 5 or coarser) reduces the total grain-boundary area available for sliding per unit volume. The combined effect is a creep-rupture strength approximately 2 to 3 times higher than standard Incoloy 800 (UNS N08800) at 815 deg C and 100,000 hours. In 800HT the gamma-prime Ni3(Al,Ti) precipitate in the grain interior adds dispersion strengthening that resists dislocation motion and further lifts the rupture stress by 30 to 50 percent. Service exposure above 540 deg C is what triggers the gamma-prime precipitation, so the 800HT advantage is realised only after the first 1,000 to 10,000 hours of service.

Creep-rupture testing is run to ASTM E139 with the test specimen held at constant tensile stress at constant temperature until rupture. The test temperature is held to plus or minus 2 deg C across the gauge length over the full test duration. Strain is measured continuously by extensometer to derive the minimum creep rate. Rupture stress is the applied stress at the recorded time to failure. Test durations for the published 100,000-hour design points are typically extrapolated from 10,000-hour and 30,000-hour test runs using the Larson-Miller parameter. Heat-specific creep testing is not standard practice for production lots; the ASME Section II Part D allowable design stress is taken from the published master curves on the Special Metals technical bulletin and confirmed against the heat-specific chemistry + grain size + room-temperature mechanical properties.