Incoloy 800H Radiant Tubes and Furnace Components

Indirect-fired furnaces use radiant tubes to separate the combustion products of the burner from the process atmosphere inside the furnace. The radiant tube wall transfers heat by radiation to the workpiece while keeping the protective atmosphere (nitrogen, hydrogen, endothermic gas or vacuum) isolated from oxygen and water vapour in the combustion side. Incoloy 800H is the workhorse alloy for U-tube, W-tube, P-tube and single-ended recuperative (SER) radiant-tube geometries operating with combustion-side temperatures from 950 to 1150 deg C and process-side atmosphere temperatures from 800 to 1100 deg C. The same material specification covers heat-treating retorts, hearth plates, basket and tray fixtures, muffle linings and burner inserts inside batch box furnaces, continuous mesh-belt furnaces, pusher furnaces, roller hearth furnaces and bell furnaces. This page documents the geometry catalogue, the typical service envelope by furnace type and the failure modes that drive material selection for indirect-fired furnace internals.

Why Indirect-Fired Furnace Internals Specify Incoloy 800H

Furnace radiant tubes and retorts are loaded by three concurrent mechanisms: thermal cycling between idle and operating temperature (often daily), creep deformation under self-weight and any suspended workload, and atmosphere attack from the protective gas or any cracked-ammonia hydrogen. The combination of creep-rupture strength at 815 to 982 deg C, moderate thermal expansion that survives the daily start-stop cycle, and the Cr2O3 oxidation film that protects both the combustion and process sides is the reason 800H outperforms 309 / 310 / 446 stainless and 600-series wrought nickel alloys in this duty. The carbon and Al + Ti chemistry windows that make 800H a creep-rupture alloy also support the dimensional stability needed by retorts and trays that must hold flatness through hundreds of charge cycles.

Geometry Catalogue

ComponentTypical OD x wallLengthService temp (combustion / process)Furnace types
U-tube radiant (gas-fired)100-200 mm OD x 5-12 mm wall2-5 m per leg1100-1200 / 850-1050 deg CPusher, roller hearth, batch box
W-tube radiant (gas-fired)100-180 mm OD x 5-10 mm wall2-4 m per leg1100-1200 / 850-1050 deg CRoller hearth, mesh belt
P-tube radiant (gas-fired)120-200 mm OD x 6-12 mm wall3-6 m loop1100-1200 / 850-1050 deg CPusher, bell, vacuum-purge
Single-ended recuperative (SER)150-250 mm OD x 6-14 mm wall1.5-3 m1150-1250 / 900-1100 deg CForging reheat, continuous strip
Heat-treating retort500-1500 mm ID x 8-25 mm wall1-4 mn/a / 800-1050 deg CBell, pit, batch box
Hearth plate (cast or wrought)20-50 mm plate1-3 m squaren/a / 850-1100 deg CPusher, walking-beam
Basket / tray / fixture3-12 mm plate or 8-25 mm barup to 1.5 m envelopen/a / 800-1050 deg CMesh belt, pusher, vacuum
Muffle lining (workchamber)4-10 mm sheet rolled to chamber profileup to 4 mn/a / 800-1050 deg CMesh belt, continuous

Documented Failure Modes

  • Sag of horizontal radiant tubes: self-weight creep at the mid-span causes progressive bow over 5 to 8 years; trigger for replacement is sag greater than 1 percent of length, set by manufacturer drawing.
  • Burner-flame impingement burn-through: misaligned or fouled burners produce a hot spot on the combustion-side tube wall; localised wall thinning + creep rupture occurs within 6 to 18 months.
  • Thermal-cycle cracking at tube-to-flange bimetallic weld: the weld between the wrought 800H tube body and the carbon-steel mounting flange concentrates differential thermal expansion stress and cracks after 500 to 1500 thermal cycles.
  • Retort distortion + dishing: hearth plates and retort bottoms bow under combined self-weight, charge load and creep; flatness tolerance drift triggers replacement.
  • Carbon pickup in carburising / nitriding furnaces: retorts exposed to endothermic carrier gas + cracked-ammonia atmospheres pick up carbon and nitrogen at the ID; surface chemistry shifts beyond the 0.10 carbon ceiling, embrittling the workpiece contact surface.
  • Spalling of the Cr2O3 scale during shutdown: rapid cooling causes oxide spallation; cumulative chromium loss reduces the chromium reservoir until breakaway oxidation initiates.

Documented Field References

  • 2024, Western Europe forging reheat plant: 18 single-ended recuperative radiant tubes 220 mm OD x 12 mm wall x 2.4 m, wrought 800H to ASTM B407, replacing 310 SS tubes after 6 years of service.
  • 2025, North America heat-treat job shop: 64 W-tube radiant assemblies 160 mm OD x 8 mm wall x 3.0 m per leg, wrought 800H to ASTM B407 with type 3.2 SGS witness, full-furnace bank changeout.
  • 2023, South Asia automotive carburising plant: 12 retorts 1100 mm ID x 12 mm wall x 1.8 m to ASTM B409, 800H plate rolled and seam-welded, replacing HK-40 castings.
  • 2024, Middle East roller-hearth annealing line: 240 hearth plates 30 mm x 1200 mm x 1200 mm from 800H plate to ASTM B409, EN 10204 type 3.1 with mill spectrographic chemistry on every plate.

Sizing + Design Notes

  • Radiant-tube wall thickness: set by the combustion-side design temperature and the ASME design allowable creep-rupture stress at that temperature; not internal pressure (radiant tubes run near atmospheric on the combustion side).
  • Tube support spacing: 1 to 1.5 m maximum to limit mid-span creep sag below the 1 percent of length acceptance.
  • Burner alignment tolerance: flame axis within plus-or-minus 2 deg of the tube centreline at the burner end; misalignment greater than this risks impingement burn-through.
  • Retort + hearth plate corner radius: minimum 5 x wall thickness at all internal corners to avoid stress concentration during thermal cycling.
  • Fixture chemistry: wrought 800H is preferred over cast HU / HX for fixtures that must hold flatness through 300+ thermal cycles, because the wrought-alloy ductility is more forgiving of charge-handling impacts.
  • Welding: GTAW with ERNiCrFe-7 filler for tube-to-tube butt joints; GMAW with ENiCrFe-2 for retort longitudinal seams; PWHT not normally required for furnace component service.

Standards That Govern

  • ASTM B407: wrought 800H seamless pipe + tube for radiant-tube geometry.
  • ASTM B409: 800H plate, sheet and strip for retorts, hearth plates and muffle linings.
  • ASTM B408: 800H rod and bar for tray and fixture frame stock.
  • ASTM B515 / B516 (welded tube + pipe): for fabricated radiant-tube geometries where seamless is not commercially available in the OD x wall.
  • ASME Section IX: WPS / PQR for the welded retort and tube-to-flange joints.
  • NFPA 86 (combustion safety): burner control + interlock requirements for the combustion side.

Inspection + Documentation Expected

  • EN 10204 type 3.1 mill certificate on every component as default; type 3.2 with third-party witness on call-out for OEM furnace builders that require it.
  • Hydrostatic test on radiant tubes per ASTM B407 paragraph 11 (radiant tubes are routinely hydro-tested even though service pressure is near atmospheric, to verify weld integrity).
  • PT on all retort seam welds and tube-to-flange welds.
  • Visual inspection at every burner port and tube support land for fit-up and freedom from sharp burrs.
  • Photographic record of each assembled radiant tube with serial number stamped on the burner-side flange.

Companion Components

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

Why are radiant tubes used instead of direct firing?

Indirect firing isolates the combustion products (CO2, H2O, residual O2 and NOx) from the workpiece atmosphere. This protects bright-finish heat treatments and any process that requires a controlled reducing or inert atmosphere from oxidation, scaling or carbon contamination by the burner flue gas.

What is the typical service life of an Incoloy 800H radiant tube?

5 to 10 years in well-aligned, well-maintained service. Replacement is triggered by sag greater than 1 percent of length, by visible burner-side wall thinning on UT inspection, or by visible cracking at the tube-to-flange weld.

Can wrought 800H replace centrifugally cast HU or HX radiant tubes?

In many cases yes, where the operating temperature stays below 1050 deg C combustion-side and the customer accepts wrought-alloy creep performance. Wrought 800H offers better thermal-shock resistance and easier on-site weld repair than cast HU / HX. Above 1050 deg C combustion-side, centrifugally cast HU / HX or HP-Modified retain a creep-strength advantage.

Should I use 800H or 800HT for retort service?

800H is normally adequate for retort body service to 1000 deg C; 800HT is specified where the retort runs above 1000 deg C continuously or where dimensional creep-deflection of the retort floor is the controlling failure mode.

Are 800H fixtures suitable for cracked-ammonia (dissociated NH3) atmospheres?

Yes for service to about 1000 deg C. Above this, nitrogen pickup at the fixture surface lowers the room-temperature ductility after cooling and the fixture eventually cracks under charge handling. Periodic surface metallography during scheduled maintenance flags fixtures approaching the end of useful life.

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