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Incoloy 800H Heat Treatment

Incoloy 800H (UNS N08810) heat treatment follows ASTM B408 solution-anneal practice: heat to 1149 deg C (2100 deg F) minimum, hold to soak through-section, then water-quench or rapid air-cool to develop the carbide distribution and grain size that gives 800H its rated creep-rupture strength above 600 deg C (1100 deg F). Hardness limits per ASME II Part D and grain-size ranges per ASTM E112 are reproduced below. TorqBolt supplies Incoloy 800H in the solution-annealed condition with EN 10204 type 3.1 or 3.2 certification confirming heat-treat temperature, hold time, cooling method, and resulting grain size.

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Standard Heat Treatment Procedure (ASTM B408)

The standard solution-anneal cycle for Incoloy 800H is: charge furnace at or below 540 deg C (1000 deg F); heat to a minimum of 1149 deg C (2100 deg F) for the solution-anneal step; hold at temperature for one hour per inch of section thickness (minimum 30 minutes for thin gauges); rapid-cool through the 540 deg C to 870 deg C carbide-sensitisation range via water quench, oil quench, or forced-air cool depending on section size and risk of distortion. Heavy sections over 100 mm thickness may require slow-cool sub-routines to manage residual stress, balanced against the time-at-sensitisation cost. The cycle is captured on the mill traveler and reproduced on the EN 10204 3.1 certificate.

Grain Size Requirement

Per ASTM B408 paragraph 6.1, Incoloy 800H grain size shall be ASTM No. 5 or coarser (average grain diameter at or above 0.056 mm / 0.0022 in). For Incoloy 800HT (UNS N08811), the requirement tightens to ASTM No. 5 or coarser with additional Al plus Ti chemistry control per paragraph 6.1.2 of the standard. Grain size finer than No. 5 disqualifies the material from ASME-coded high-temperature service.

Stress Relief for Welded Components

Welded Incoloy 800H assemblies do not normally require post-weld heat treatment (PWHT) per ASME BPVC IX. When PWHT is specified for very-heavy-section welded vessels or service in hydrogen reformers, the recommended temperature is 885 deg C plus or minus 15 deg C (1625 deg F plus or minus 25 deg F) with a hold of one hour per 25 mm of weld thickness. Higher PWHT temperatures may degrade creep performance by inducing carbide coarsening; avoid PWHT above 982 deg C. See our weldability and welding procedures page for GTAW / GMAW filler selection and joint design.

Hardness Limits

Solution-annealed Incoloy 800H has a hardness specification of Brinell HB 100 to 200 per ASTM B408. The typical mill-supplied condition is 138 to 185 HB. Cold-work raises hardness rapidly; cold-worked material that exceeds 200 HB requires re-solution-annealing before placement in high-temperature service. Hardness is reported per heat lot on the mill test certificate.

Pre-Heat and Inter-Pass Welding Considerations

Welding Incoloy 800H usually requires no preheat for sections under 25 mm. Heavier sections may benefit from a 95 deg C to 150 deg C preheat to manage thermal gradient and reduce hot-crack susceptibility. Interpass temperature should not exceed 175 deg C. Post-weld, the material can be returned to service in the as-welded condition for most applications; high-temperature creep service may benefit from a re-solution-anneal cycle.

Companion Standards

The same heat-treatment philosophy applies across the Incoloy 800H product range: ASTM B407 (seamless pipe and tube), ASTM B408 (rod and bar), ASTM B409 (plate, sheet, and strip), ASTM B564 (forgings). For high-temperature service classification, reference ASME BPVC Section VIII and the ASME II Part D design stress tables for allowable stresses in solution-annealed condition.

Frequently Asked Questions

Why does Incoloy 800H need solution annealing at 1149 deg C minimum?
ASTM B408 sets the minimum to ensure complete carbide dissolution and grain coarsening to ASTM No. 5 or larger. Below 1149 deg C, carbides may not fully redissolve, leading to grain-boundary chromium depletion and reduced creep-rupture strength in service.
Can Incoloy 800H be stress-relieved without full re-annealing?
Yes. Sub-critical stress relief at 540 deg C to 650 deg C for 1 hour per 25 mm relieves residual stress from cold-working or welding without affecting grain size. Above 870 deg C, carbide reprecipitation begins and the material must be re-solution-annealed if returned to high-temp service.
What is the difference in heat treatment between Incoloy 800, 800H, and 800HT?
Incoloy 800 has no grain-size requirement; 800H requires ASTM No. 5 or coarser; 800HT adds an Al plus Ti chemistry window (0.85 to 1.20 percent combined) plus the same grain-size requirement. See 800 vs 800H vs 800HT comparison.
Does TorqBolt supply re-solution-annealed material on request?
Yes. TorqBolt Incoloy 800H is supplied in the solution-annealed condition with EN 10204 type 3.1 or 3.2 certification confirming heat-treatment temperature, hold time, cooling method, and resulting grain size. Type 3.2 with Lloyds / DNV / BV / SGS / TUV witness on call-out.