Hangbo Alloy Group
Shanghai · Est. 2012
Why nickel alloys resist carburization in ethylene furnaces and reformers: alloy chemistry, grade selection, and inspection for HP Modified, alloy 602CA, and 601.
Carburization is the quiet failure mode of high-temperature furnaces. Carbon from the process gas diffuses into the metal, precipitates as chromium-rich carbides at the grain boundaries, and turns a ductile tube into a brittle, cracked shell. It is the single biggest life limiter in ethylene pyrolysis coils and steam reformers. This guide explains the mechanism, the chemistry that fights it, and how plants pick and inspect the alloys that survive.
At 800–1100°C in a carbon-rich atmosphere, atomic carbon dissolves into the alloy surface and migrates inward along grain boundaries. There it reacts with chromium to form M7C3 and M23C6 carbides. Because chromium is tied up as carbides, the matrix loses corrosion resistance and the boundary zone becomes hard and brittle. Cracks then initiate at the carburized layer and propagate under thermal cycling. The tube does not lose wall thickness — it loses toughness.
Two elements earn their keep. Chromium forms a protective scale and, in high enough amount, slows carbon entry. Silicon (1.5–2.5%) is especially effective: it promotes a silica-rich layer that blocks carbon diffusion, which is why the best carburization-resistant casts carry deliberate silicon additions. A high nickel base keeps the austenite stable and resistant to sigma-phase embrittlement. Cast HP Modified (25Cr-35Ni plus Nb and Si) and wrought alloy 602CA (high Cr plus yttrium) sit at the top of the resistance chart.
Ethylene pyrolysis furnaces run the hottest and most carburizing, so coil outlets use HP Modified or alloy 602CA. Steam reformers and methanol syngas tubes use similar Fe-Ni-Cr casts. Heat-treatment furnace rollers, rails, and fixture alloys see lower carbon potentials but long dwell times. Haynes 230 and Hastelloy X are common where fabricability and oxidation resistance also matter.
Carburization depth is measured metallographically on a cut sample — typical acceptance limits run a few millimetres depending on the code. A rising carburization rate or surface cracking signals end of life. Plants extend service by lowering process-gas carbon activity, reducing thermal cycling, and switching the most exposed sections to a higher-silicon or higher-chromium grade at the next turnaround. Reclaiming or weld-repairing a heavily carburized tube is rarely worthwhile because the embrittlement is through-wall.
For wrought product, alloy 602CA leads thanks to its high chromium and yttrium addition. For cast furnace tubing, HP Modified (often with intentional silicon) is the workhorse. Haynes 230 is a strong fabricable alternative for fabricated hardware.
Oxidation removes metal by forming a surface scale you can see; carburization adds carbon below the surface and makes the metal brittle without much metal loss. They often occur together in furnaces, but carburization is the one that ends tube life through cracking rather than thinning.
HP Modified is a centrifugally cast 25Cr-35Ni austenitic grade with niobium and a deliberate silicon bump for carburization resistance. Its balance of creep strength and carburization resistance at 900–1000°C makes it the default for steam reformer and ethylene furnace tubes — see our furnace tube selection guide.
Yes. Silicon (commonly 1.5–2.5% in the best grades) forms a silica-rich barrier at the surface that slows carbon diffusion far more than chromium alone. That is why high-silicon variants of HP Modified and alloy 602CA outlast plain 25-20 or 35Ni casts in the same furnace.
As a dedicated nickel alloy manufacturer serving chemical processing, power generation, oil & gas sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.