Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Incoloy 800H Steam Superheater Tubes
The final-stage superheater and the reheater carry the highest-temperature steam in a fossil or waste-heat boiler. Modern ultra-supercritical (USC) utility boilers operate with main-steam temperature 600 to 620 deg C at 25 to 30 MPa and reheat-steam temperature 600 to 620 deg C at 5 to 7 MPa. Heat-recovery steam generators (HRSGs) downstream of large gas turbines run main steam at 565 to 600 deg C and 12.5 to 16.5 MPa. The tube-wall metal temperature in these final stages is typically 60 to 80 deg C above the steam temperature, so the metal is exposed to 670 to 700 deg C continuously with flue-gas-side temperatures of 700 to 950 deg C. Incoloy 800H (UNS N08810) and 800HT (UNS N08811) are specified for the highest-temperature tube rows where T91 / T92 / TP304H / TP347H do not meet the 100,000-hour rupture stress required by ASME Section I and B31.1. This page documents the steam-side and flue-gas-side service envelope, the tube row position logic, the failure modes and the supply specification.
Why Final-Stage Superheater Service Specifies Incoloy 800H
The controlling failure mode at the final-stage superheater is steam-side oxidation combined with creep-rupture under sustained pressure at 670 to 700 deg C tube-wall metal temperature. Austenitic stainless TP347HFG operates well to about 650 deg C metal temperature; beyond this the steam-side oxide layer thickens, spalls and the dislodged scale damages the downstream turbine blade row. Incoloy 800H lifts the practical metal-temperature ceiling to 700 to 720 deg C because the 19 to 23 percent chromium plus 30 to 35 percent nickel chemistry forms a more adherent Cr2O3 scale and resists the steam-side spallation that limits the stainless grades. The ASME Section I + B31.1 allowable design stress at 650 deg C for 800H is approximately 95 MPa, against 80 to 90 MPa for TP347H, and 800H holds an allowable stress envelope to 815 deg C metal temperature that the stainless grades do not. The flue-gas side is loaded by fuel-ash deposit corrosion (particularly oil-fired and biomass-fired units), and the 800H chromium content also supports the deposit-corrosion resistance.
Service Envelope by Boiler Type
| Boiler type | Main-steam temp / pressure | Reheat-steam temp | Final SH tube metal temp | Tube OD x wall typical |
|---|---|---|---|---|
| Subcritical pulverised-coal | 540 deg C / 16.5 MPa | 540 deg C / 4 MPa | 600-630 deg C | 44.5 x 6.4 mm or 51 x 7.1 mm |
| Supercritical pulverised-coal | 565-580 deg C / 24-27 MPa | 580-595 deg C / 5-6 MPa | 640-670 deg C | 44.5 x 7.1 mm or 51 x 8 mm |
| Ultra-supercritical pulverised-coal | 600-620 deg C / 28-30 MPa | 600-620 deg C / 6-7 MPa | 670-700 deg C | 50.8 x 9.5 mm typical |
| HRSG (gas-turbine bottoming cycle) | 565-600 deg C / 12.5-16.5 MPa | 565-600 deg C / 3.5-4.5 MPa | 640-680 deg C | 38.1 x 5.6 mm to 50.8 x 7.1 mm |
| Biomass / waste-to-energy boiler | 440-540 deg C / 6-10 MPa | n/a (often no reheat) | 500-620 deg C | 38.1 x 5.6 mm to 51 x 7.1 mm |
Tube Row Position Logic
A final-stage superheater bank is built from multiple alloy grades arranged by row position so that each tube row sees the alloy class with the lowest cost that meets its specific metal temperature. The hottest steam-entry row (typically the upper rear bank in a pendant arrangement) is the highest-metal-temperature row and the one that receives 800H. The rows downstream of this run progressively cooler and step down through TP347HFG, TP347H, TP304H, TP321H and into T22 / T91 / T92 ferritics. The boiler OEM design specifies the alloy boundary between 800H and TP347HFG by the predicted metal temperature at the design point plus the design margin. Incoloy 800HT is occasionally specified for tubes operating above 700 deg C metal temperature where the gamma-prime precipitation gives an additional creep-rupture allowance.
Documented Failure Modes
- Steam-side oxide exfoliation and turbine blade erosion: spalled oxide scale from the tube ID carries with the steam into the turbine and erodes the high-pressure turbine first-stage nozzles + blades.
- Fireside ash-deposit corrosion (coal): alkali sulphate deposits (Na2SO4 + K2SO4) on the OD attack the chromia scale at 600 to 700 deg C; T91 cannot withstand it but 800H runs acceptably for 80,000 to 120,000 hours.
- Tube row pluggage by ash deposit (biomass): KCl + K2SO4 deposits build up on the cool-side OD; periodic soot-blowing keeps the deposits manageable but thermal-fatigue cracking at the leading edge is the long-term limit.
- Creep rupture at the hottest U-bend: the bottom U-bend of a pendant superheater coil is the highest-metal-temperature point and the typical first-failure location after 100,000+ hours of service.
- Dissimilar-metal weld failure: the bimetallic weld between 800H tube ends and TP347HFG header stubs concentrates the differential thermal-expansion stress and is a documented crack initiation site.
Documented Field References
- 2024, Asia USC pulverised-coal unit: 4.6 km of Incoloy 800H final-stage superheater tube 50.8 mm OD x 9.5 mm wall to ASTM B407, EN 10204 type 3.2 with LR witness, replacing original supply after 12 years service.
- 2023, Middle East combined-cycle HRSG: 2.1 km of 800H reheater tube 44.5 mm OD x 7.1 mm wall to ASME SB-407, type 3.2 with TUV witness, T+I scope.
- 2025, Western Europe biomass-to-energy plant: 980 m of 800H superheater tube 38.1 mm OD x 5.6 mm wall to ASTM B407, with HF acid-pickled ID and surface roughness measurement.
- 2024, North America HRSG behind aero-derivative gas turbine: 1.4 km of 800H final-stage superheater 38.1 mm OD x 5.6 mm wall, with extended-surface fin tube on the cool-side rows of the same bank.
Sizing + Design Notes
- Tube wall thickness: set by the ASME Section I allowable stress at the tube-wall metal temperature, plus 12.5 percent under-tolerance per ASME Section I PG-27.
- Tube-row spacing: set by the OEM gas-side flow distribution; typical 75 to 125 mm tube pitch in the cross-flue direction.
- U-bend radius: minimum 2 x OD on cold-bent tubes; less is permitted with hot bending plus PWHT.
- Surface ID condition: bright-pickle ID per OEM specification to control steam-side oxide initial growth.
- Dissimilar-metal weld at the header stub: ENiCrFe-2 / ENiCrFe-7 filler bridging 800H to TP347HFG header stubs; PWHT typically not required for 800H side.
- Tube straightness: within plus-or-minus 1 percent of length to support the pendant geometry without binding at the support sliding shoes.
Standards That Govern
- ASTM B407: wrought 800H seamless tube specification (primary tube material spec).
- ASME Section I PG-27: tube wall thickness calculation for fired boilers.
- ASME B31.1: power piping design for the steam header connections.
- EN 12952 (water-tube boilers): European equivalent design code.
- ASME Section IX: WPS / PQR for the tube-to-tube and tube-to-header welds.
- ASME Section II Part D allowable design stress tables: 800H values from room temperature to 815 deg C.
Inspection + Documentation Expected
- EN 10204 type 3.2 mill certificate with classification society or notified body witness on chemistry, mechanical, dimensional and NDT records.
- Hydrostatic test per ASTM B407 paragraph 11 at 1.5 x design pressure, on every tube length.
- 100 percent eddy-current NDT to ASTM E309 on the tube body for OD + ID surface defect detection.
- ID surface roughness measurement and pickle-condition report per OEM boiler-builder specification.
- Bend report for U-bends covering ovality, wall thinning at the extrados and freedom from cracking at the bend on dye-penetrant inspection.
- Tube straightness measurement at multiple stations along each length.
Companion Components
- Incoloy 800H header forgings for the steam-collection headers at the superheater outlet.
- Incoloy 800H seamless tube in additional OD x wall combinations for adjacent boiler tube banks.
- Incoloy 800H buttweld fittings for header-stub connections.
- Incoloy 800H fasteners for the bolted access manways at the header drum.
Related Incoloy 800H Fasteners for This Application
TorqBolt supplies the matched fastener range for this service from Incoloy 800H bar (ASTM B408) and forged blanks (ASTM B564), heat-treated to retain ASTM grain size 5 or coarser for ASME Section VIII design stress qualification.
Stud Bolts
Primary flange bolting form, M12-M64, both-end-threaded with matched heavy hex nuts.
Heavy Hex Bolts
ASME B18.2.1 heavy-pattern bolts, 1/2" to 2", petrochem flange service.
Heavy Hex Nuts
ASME B18.2.2 matched-grade nuts paired with heavy hex bolts and stud bolts.
Washers
Flat DIN 125 + spring DIN 127 + locking DIN 6798 in matched Incoloy 800H chemistry.
U-Bolts
Pipe-support clamps for high-temperature piping in petrochem and refinery service.
Threaded Rod
Continuous threaded rod M12-M48, cut to length for hanger and tie-rod assemblies.
Frequently Asked Questions
When does a boiler designer step up from TP347HFG to Incoloy 800H?
At predicted tube-wall metal temperature above approximately 650 to 670 deg C, depending on the steam-side oxide-spallation criterion the OEM applies. 800H gives an additional 50 to 70 deg C of useful metal-temperature headroom before creep-rupture or steam-side oxidation become design-limiting.
Why does steam-side oxide spallation matter?
Spalled oxide flakes enter the steam flow and hit the high-pressure turbine first-stage nozzles and blades, causing solid-particle erosion. A boiler tube material that holds its scale adherent at elevated temperature reduces unscheduled turbine outages and preserves blade life.
Is 800H suitable for biomass and waste-to-energy boilers?
Yes, particularly in the upper superheater rows where flue-gas-side temperatures and ash-deposit chemistry (alkali chlorides) exceed the corrosion endurance of T91 and TP347. The chromium content combined with the nickel base is the key to acceptable service in alkali-chloride atmospheres.
Can 800H tubes be supplied with extended fin surface for HRSG service?
Solid extruded fin tube is the typical extended-surface form for the cooler boiler tube banks (T91, T22, carbon steel). The 800H tube rows are usually bare or with serrated solid fin attached by HF resistance weld; project-specific call-out is supplied to drawing.
Should I specify 800H or 800HT for ultra-supercritical superheater service?
800H is the default. 800HT is specified where the design metal temperature exceeds 700 deg C continuously, where the Al + Ti gamma-prime gives the additional creep-rupture stress allowance. The OEM boiler builder makes the final selection based on the design tube-wall metal temperature plus the design margin.
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