Incoloy 800H Carburization Resistance

Carburization is the dominant degradation mode for Incoloy 800H (UNS N08810) in hydrocarbon furnace tube service, heat-treating retorts and reformer outlet pigtails where the process atmosphere contains carbon monoxide, hydrogen and methane at carbon activities approaching 1.0. Carbon ingress into the Cr-rich matrix precipitates internal M7C3 and M23C6 carbides in a saturation profile that propagates inward from the metal-atmosphere interface. The resulting embrittlement reduces the room-temperature elongation from approximately 44 percent to below 5 percent in the carburized zone, and the volumetric expansion accompanying the carbide precipitation generates internal stresses that crack the substrate. Incoloy 800H delivers approximately 0.5 mm carbon ingress depth at 1000 hours, 982 deg C and aC = 1.0, compared to approximately 2.0 mm for 310H stainless under the same exposure. The Al2O3 sublayer supported by the 0.15 to 0.60 percent Al window is what slows the carbon ingress relative to 310H. This page documents the carbon ingress curves, the carbon activity threshold and the comparison to 310H and standard 800.

The driving force for carbon ingress is the carbon activity (aC) of the atmosphere relative to the chromium carbide stability boundary in the alloy. The 23 percent Cr content of Incoloy 800H sets the carbide stability boundary at approximately aC = 0.05 at 982 deg C. Below this activity the chromium scale is the equilibrium phase and no carburization occurs. Above this activity carbon dissolves in the matrix and precipitates Cr-rich carbides at the grain boundaries, then in the grain interior at higher activities. Practical atmosphere control on hydrocarbon cracking furnaces uses oxygen partial pressure (PO2 above approximately 10^-19 atm) to push the equilibrium chromium oxidation forward and suppress the carburization. This is the basis of steam decoking cycles on ethylene cracker tubes, where the steam injection oxidises the Cr surface back to Cr2O3 and resets the carburization clock.

At carbon activity above 1.0 and temperature between 400 and 800 deg C, Incoloy 800H is susceptible to metal dusting, a catastrophic disintegration of the alloy into a graphite-metal powder mixture. The mechanism is carbon supersaturation followed by graphite nucleation at the grain boundaries that disrupts the alloy lattice. The atmosphere envelope is typical of synthesis gas, coal gasification and direct iron reduction processes. The 800H resistance to metal dusting is approximately equal to 800HT and substantially better than 304H, 310H and carbon steel, but worse than Inconel 600 + 601 + 690 which have nickel content above 50 percent. For severe metal dusting service, the alloy specification is Inconel 690 or 693 rather than 800H.

Laboratory carburization testing is run by exposure to a controlled atmosphere (CO + H2 + CH4 with the carbon activity set via the CO/CO2 ratio) at the test temperature, followed by metallographic cross-section + microhardness traverse to measure the carbon ingress depth. The published Special Metals technical bulletin data is the reference for design. Service-side monitoring on installed assets uses periodic UT wall thickness combined with metallographic sampling at the in-service inspection windows specified by API 510 / API 570 for pressure vessels and piping. The carbide-precipitated zone is identifiable by the increased eddy-current response (NDT) and the increased magnetic permeability (relative to the non-magnetic base austenite).