Carburization

The carburization of steels and alloys occurs virtually during any high-temperature process (>593°C) in the presence of carbonaceous compounds, such as various hydrocarbons, CO, or coke. The degradation of metallic structures due to carburization remains difficult to predict accurately, potentially leading to unforeseen failures in high-temperature operating equipment. The forthcoming chapter offers a comprehensive overview of the carburization process, detailing its primary parameters and suggesting mitigation measures.

General Information

Carburization is a form of metallurgical failure/degradation commonly observed in environments containing carbon at elevated temperatures. This type of degradation is frequently associated with metals and alloys exposed to carburizing conditions, where carbon diffuses into the material at high temperatures, leading to the formation of carbides.1 This process can compromise the material’s mechanical properties and structural integrity. Furthermore, it may escalate to metal dusting, particularly when carbon activity exceeds unity, resulting in highly localized carburization and causing catastrophic damage.2 4 Carburized steel tends to become brittle and may exhibit spalling or cracking .2 Notably, carburization is of significant concern in industries such as refining, petrochemicals, and power generation.

The process of carburization typically involves the following:

  1. Elevated Temperatures: Carburization occurs at elevated temperatures, typically above 593°C (1100°F) and often even higher depending on the specific materials and conditions.
  2. Carburizing Environment: The presence of carbon-rich atmospheres, such as hydrocarbons or carbon monoxide, is a key factor.
  3. Diffusion of Carbon: At elevated temperatures, carbon atoms from the carbon-rich atmosphere diffuse into the surface of the metal. This diffusion is a function of temperature and time, and it causes an increase in carbon concentration at the material’s surface.
  4. Carbide Formation: The diffused carbon reacts with alloying elements in the metal to form carbides. Common carbide-forming elements include chromium, tungsten, and molybdenum. These carbides are hard and brittle compounds that can reduce the material’s ductility and toughness.
  5. Microstructural Changes and Reduced Mechanical Properties: The formation and accumulation of carbides alters the microstructure and reduces mechanical properties of the affected material. The surface becomes harder and impact resistant, but more brittle. This change in microstructure and mechanical properties can lead to increased susceptibility to cracking and mechanical failure. Moreover, it can lead to loss of weldability, and reduced corrosion resistance.
  6. Cracking and Failure: As the carburization process progresses, the material becomes more susceptible to cracking and fracture. This is particularly critical in applications where mechanical strength and integrity are essential, such as in components subjected to high temperatures and stress.

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References

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