Incoloy 800H Ethylene Cracker + Hydrocarbon Cracking Tubes

Ethylene cracking is the largest single application for Incoloy 800H (UNS N08810) and 800HT (UNS N08811) by tonnage. A modern world-scale steam-cracker furnace processes ethane, propane, naphtha or gas-oil at 850 to 1100 deg C tube-wall temperature and 0.5 to 1.5 MPa internal pressure, with steam-to-hydrocarbon ratios from 0.3 to 1.0 controlling the coke deposition rate. The radiant section tubes carry the cracking reaction, the pigtails route effluent from the radiant tubes to the transfer-line exchanger, and the convection bundles preheat the feed and superheat the dilution steam. Each section has a different combination of temperature, pressure, coke deposition rate and decoking-cycle frequency that drives a different alloy selection. Incoloy 800H is specified for the pigtails and the upper convection bundles; centrifugally cast HK-40, HP-40, HP-Modified and 35Cr-45Ni alloys dominate the radiant section itself; centrifugally cast 800H is specified where the customer requires the wrought-alloy chemistry in a cast geometry. This page documents the operating envelope, the failure modes, the decoking-cycle endurance and the design references for steam-cracker furnace service.

Three failure modes drive material selection in steam-cracker pigtails and convection bundles: (1) creep rupture under sustained pressure at 850 to 950 deg C tube-wall temperature, (2) carburization from the cracking-side carbon activity that ranges from 0.1 to over 1.0 depending on steam ratio, and (3) thermal-shock cracking from the steam decoking cycles every 30 to 90 run-days. The 0.05 to 0.10 percent carbon window combined with ASTM grain size 5 or coarser delivers the creep-rupture strength required at 815 deg C pigtail wall temperature, while the 19 to 23 percent chromium content supports the Cr2O3 scale that slows the carbon ingress to approximately 0.5 mm at 1000 hours at 982 deg C compared to approximately 2 mm for 310H stainless. The thermal-shock endurance derives from the moderate thermal expansion coefficient of 17.8 um/m/K and the elevated elevated-temperature ductility that lifts the elongation to over 70 percent at 815 deg C. The 800H + 800HT pigtail and convection-section service life is typically 8 to 12 years before scheduled replacement, against approximately 5 to 7 years for 310H stainless.

The radiant section of a steam-cracker furnace is historically dominated by centrifugally cast tubes, HK-40 (25Cr-20Ni), HP-40 (25Cr-35Ni) and HP-Modified (25Cr-35Ni-Nb), because the cast process delivers tube geometry with the rough internal-surface required for the radiant-flux heat transfer plus the columnar grain structure that resists radial creep. Wrought Incoloy 800H is the alternative when the customer requires the wrought-alloy chemistry envelope (carbon 0.05 to 0.10 percent guaranteed, no cast porosity, no shrinkage cavities) and accepts the wrought-alloy creep performance below the cast HP-Modified curve at the 1050 to 1100 deg C extreme service temperature. TorqBolt supplies wrought Incoloy 800H pigtails and convection tubes; for the radiant section, centrifugally cast 800H is supplied via centrifugally cast tube product range manufactured to ASTM A608.

Steam decoking removes the coke layer that deposits on the tube ID during the cracking run by combustion with air-steam mixture at 800 to 900 deg C. The cycle is typically: shut off feed, purge with steam, ramp to decoking temperature, controlled air addition for 24 to 72 hours, monitor combustion via CO2 + CO + O2 analyser at the decoke outlet, hot rinse with steam to remove residual coke, return to feed. Each decoking cycle imposes a thermal shock as the tube cools from cracking temperature to decoking temperature and back; the Cr2O3 scale spalls during the rapid heat-up under air, exposing fresh metal to oxidation; the carburized zone from the run is partially oxidised back to Cr2O3 and chromium loss accelerates. The practical decoking-cycle endurance of Incoloy 800H pigtails is approximately 200 to 300 cycles, corresponding to 8 to 12 years of plant service at 30 to 60 day run lengths.

The same Incoloy 800H pigtail + convection-bundle design extends to propylene units (propane dehydrogenation), aromatics units (BTX reformer outlets) and styrene units (ethylbenzene dehydrogenation). Each variant shifts the operating envelope: propylene runs slightly hotter (1100 to 1150 deg C peak radiant) but at lower pressure, aromatics runs cooler (700 to 850 deg C) but with hydrogen partial pressure that lifts the hydrogen embrittlement risk on bolt-up flanges, styrene runs cooler still (550 to 650 deg C) but with the catalyst-coke deposition rate well above ethylene. The 800H + 800HT material selection logic transfers cleanly to all four hydrocarbon-cracking variants because the controlling failure modes are the same combination of creep, carburization and thermal-shock cracking.

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.