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Carburization


Abstract:
Carburization is the formation of metal carbides in a material as a result of exposure to a carbon containing atmosphere. In some cases, materials are intentionally carburized to impart a high surface hardness and wear resistance. However, in service, it can result in loss of mechanical properties over time in addition to wastage of the material. The corrosion process referred to as "Metal Dusting" is a carburization process which results in surface attack and the formation of a powdery residue. Carburizing environments are commonly found in process piping and furnace tubes that contain an excess of carbon-containing species which includes hydrocarbon gases or carbon monoxide. Pyrolysis piping in ethylene and olefins plants are prime examples of equipment that must be designed to resist severe carburization.
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Carburization is the formation of metal carbides in a material as a result of exposure to a carbon containing atmosphere. In some cases, materials are intentionally carburized to impart a high surface hardness and wear resistance. However, in service, it can result in loss of mechanical properties over time in addition to wastage of the material. The corrosion process referred to as "Metal Dusting" is a carburization process which results in surface attack and the formation of a powdery residue. Carburizing environments are commonly found in process piping and furnace tubes that contain an excess of carbon-containing species which includes hydrocarbon gases or carbon monoxide. Pyrolysis piping in ethylene and olefins plants are prime examples of equipment that must be designed to resist severe carburization.

Resistance to carburization generally centers around two mechanisms:

  • the establishment of an effective barrier on the surface of the materials that limits the ingress of carbon into the material.

  • tying up carbon in the material using alloying elements which are strong carbide formers.

In the first case, the barrier is usually a Cr2O3 layer on the material. This is accomplished through the use of Cr as an alloying element. Secondly alloying additions of Ni also help by reducing carbide diffusion in the material.

Under severe carburizing conditions up to 800 C, the use of Type HK Alloys (Fe-25Cr-20Ni), Type HP (Fe-25Cr-35Ni) and Alloy 800H is common. Si additions have been found to be beneficial in enhancing these alloys through formation of a protective SiO2 layer on the surface of the material over the range of 1 to 2 wt percent Si (See Figure). Additions of niobium, a carbide forming element, is also used to extend the performance of these alloys by tying up carbon in the form of stable carbides to reduce its reaction with Cr.

Carburization can also occur in Cr-Mo steels at low temperatures (450 to 650 C). Several methods of reducing carburization can be utilized. These include the use of barrier coatings, reducing the carbon activity of the environment, lowering the service temperature by cooling and introduction of sulfur or other compounds which poison surface adsorption sites and thereby limit the entry of carbon into the material.




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