99 Diseases of Pressure Equipment: Carburization

Though oxidation and sulfidation are quite prevalent high temperature corrosion mechanisms in many of our process units, we now come to a few that are not very common, but still deserve some attention to make sure they don’t lead to unexpected failures. Carburization involves the absorption of carbon into the steel or alloy from the high temperature operating environment, typically above temperatures of 1100F (593C). That means that the carburization is usually only a problem with furnace tubes. The carbonaceous environment is typically one with high gas phase carbon activity involving methane, ethane, carbon monoxide or dioxide, and/or hydrocarbon processes with coking tendencies. Furnace tube decoking activities often produce a carburizing environment. Another way carburization can occur is when furnace firing is increased to compensate for a heavy coke deposit on the ID of the tube. That increase in temperature and the coke adjacent to the tube wall can produce a highly carburizing environment. Carbon from the carburizing environment enters the surface of the steel by what we metallurgists call diffusion causing the metal to become embrittled and lose creep resistance and toughness. The result is loss of corrosion resistance and strength, and becoming more prone to cracking type failures.

At this mature stage of our industry, most facilities know if they are susceptible to carburization problems, and are inspecting for them with eddy current techniques and/or techniques based on measurement of increasing ferromagnetism of austenitic steels. If tubes are cut out for metallographic examination, then hardness testing as well as metallography can be employed to detect carburization in a cross section of the tube. In the more advanced stages, users can apply ultrasonics and radiography looking for fissuring and cracks. Some high Cr-Ni alloys resist carburization better than lower Cr-Ni alloys in ethylene pyrolysis furnaces. There are some proprietary surface coatings and micro-finished weld overlays that are showing promise in inhibiting coking and carburization. An associated problem occurs when trying to replace heavily carburized tubes because the carburization reduces weldability, though very specialized welding procedures can result in sound repair welds.

Do you know if any of your furnaces’ tubes are subject to carburization and how to find the problem before it results in tube failures?