Incoloy 800H Thermal Expansion

Incoloy 800H (UNS N08810) has a mean linear thermal expansion coefficient (alpha) of 14.4 micrometres per metre per Kelvin from 21 to 100 deg C, rising approximately monotonically to 17.8 um/m/K at the 21 to 815 deg C interval. The 800HT variant (UNS N08811) shares the same alpha curve within the engineering uncertainty. The expansion coefficient is approximately 15 percent below 304H stainless and approximately 10 percent above Inconel 600. The dimensional mismatch with adjacent piping materials is the controlling design constraint at expansion joints, tube-to-tubesheet welds and bimetallic flange connections in high-temperature service. This page documents the full alpha curve from 21 to 982 deg C, the cumulative expansion at the four design points (540, 650, 815, 982 deg C) and the mismatch versus the four common piping-side materials (304H, 316H, Inconel 600, Inconel 625).

The expansion mismatch generates residual stress on every thermal cycle in a bimetallic joint. At the 540 deg C piping interface temperature, the mismatch between 800H and 304H over a 1-metre joint length is approximately 1 mm of differential expansion. This translates to approximately 200 MPa of bending stress in a fully-constrained butt weld, well above the yield stress at temperature. Practical design solutions include: (1) installing an expansion bellows or omega-loop between the two materials, (2) using a transition piece (typically Inconel 625 weld overlay or a forged transition forging) to spread the mismatch over a longer geometric path, (3) selecting the closest expansion match, 310H stainless or Hastelloy X, when the metallurgical requirements allow. The third option is the design-of-choice for reformer outlet pigtail connections to the catalyst-tube manifold.

The published values above are MEAN alpha from 21 deg C to the test temperature. For piping flexibility analysis the relevant value is the INSTANTANEOUS alpha at the operating temperature, which is typically 5 to 10 percent higher than the mean alpha to the same temperature. For example, the instantaneous alpha at 815 deg C is approximately 19.0 um/m/K versus the 17.8 um/m/K mean alpha from 21 to 815 deg C. Piping flexibility codes (ASME B31.1, B31.3) require the MEAN alpha from the installation temperature to the operating temperature, which approximates the instantaneous alpha at the operating temperature for installations near room ambient.

Thermal expansion testing is run to ASTM E228 by push-rod dilatometry from ambient to the test temperature in inert atmosphere or vacuum. Heat-specific dilatometry is not standard practice for production lots; the published Special Metals data is the design reference. Dilatometric measurements on the heat-specific sample are supplied on call-out for prototype qualification of code-stamped pressure vessels with cryogenic-to-high-temperature service envelope.