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Carburization involves the absorption of carbon into steel or alloy during operation in a high temperature environment, typically above 1100F (593C). Carbon from the carborizing environment enters the surface of the steel by diffusion, causing the metal to become embrittled and lose creep resistance and toughness. The result is a loss of corrosion resistance and strength and an increased susceptibility to cracking type failures. Because it occurs in high temperature environments, carburization often affects furnace tubes. It typically occurs when furnace firing is increased to compensate for a heavy coke deposit on the ID of the tube. The increase in temperature and the coke adjacent to the tube wall can produce a highly carburizing environment, in which carbon can enter steel via diffusion.

The conditions behind carburization are well understood, thus it is fairly easy to detect once it has occurred. This can be done by using eddy current or other techniques measuring increased ferromagnetism of austenitic steels. Ultrasonics and radiography can also be used to detect fissuring or cracks in more advanced stages of carburization. To help prevent carburization, industrial coatings are often applied to materials to prevent carbon in the environment from being absorbed into the material. This topic is covered in more detail in API RP 571 - Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.

Metal Dusting

Metal Dusting, also known as catastrophic carburization, is simply a severe form of carburization in which the extensive carbides formed as a result of carburization lead to grains of metal falling out of the tube or piping and being swept away in the process stream. This form of degradation leaves a heavily pitted looking structure along with thinning of the tube/pipe.

Metal dusting generally affects materials in high carbon atmospheres; primarily those that operate at temperatures ranging from 300°C to 850°C. Thus, it is most commonly found in reforming units and cracker furnaces.

It is therefore important to establish an effective inspection program for these areas in particular. Proper insulation can also help protect equipment from high carbon atmospheres, thereby reducing the risk of metal dusting.

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Articles about Carburization
January/February 2014 Inspectioneering Journal

Fired furnaces in the petrochemical and refining industry are critical pieces of equipment that can have a major impact on process unit safety, reliability, and economics. They are complex pieces of equipment, where tubes and other pressure boundary...

Authors: Antonio Seijas
July/August 2004 Inspectioneering Journal

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...

Authors: John Reynolds
July/August 2004 Inspectioneering Journal

Decarburization is the antithesis of carburization and rarely results in equipment failure. However, surface decarburization is often a sign that something more serious is going on, ie high temperature hydrogen attack (HTHA), which is well covered...

Authors: John Reynolds
July/August 2004 Inspectioneering Journal

Metal dusting is simply a severe form or extension of carburization in which the extensive carbides that form as a result of carburization lead to grains of metal falling out of the tube or piping and being swept away by the process...

Authors: John Reynolds
Partner Content

Traditional pressure vessel inspections often require taking assets out of service. However, MISTRAS’ Non-Intrusive Inspection (NII) programs provide data as good or better than internal inspections, while assets remain in service. NII is...

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