Carburization

Last update: Jan 13, 2017

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.

 

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Managing Fired Furnace Tubes in Refineries
January/February 2014 Inspectioneering Journal
By Antonio Seijas at Phillips 66 Company

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Partner Content

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July/August 2004 Inspectioneering Journal
By John Reynolds at Intertek

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 in API RP 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants.

July/August 2004 Inspectioneering Journal
By John Reynolds at Intertek

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 stream.