What Does Graphitization Mean?
Graphitization is the formation of graphite (free carbon) in iron or low-alloy steel, which occurs when their components are exposed to elevated temperatures over a long period.
The formation of graphite is due to the nucleation and growth process that occurs when the steel is exposed to temperatures above 800°F (426°C). Graphite is a weak material with very low ductility and low resistance to thermal or mechanical fatigue or shock. Graphitization causes localized and weak areas on the affected steel component.
Corrosionpedia Explains Graphitization
When steel is exposed to elevated temperatures for long periods, metallurgical degradation of the steel’s microstructure matrix occurs to form free graphite (carbon) and iron (ferrite). At elevated temperatures, carbon tends to migrate to the grain boundaries, leading to the formation of graphite nodules, which have an embrittling effect on metal.
Primary graphitization occurs during the solidification process, and this results in a stable iron-graphite structure. Examples of primary graphitization in cast iron include:
- Gray iron (flake graphite)
- Ductile iron (spheroidal graphite)
- Compacted graphite iron
Secondary graphitization is graphite formation that occurs by the transformation of metastable metallic carbides following solidification. A common process is when elevated temperatures result in microstructure deterioration, such as the decomposition of pearlite into the equilibrium structure of iron and graphite. Examples include age-related graphitization of carbon and low-alloy steel used in elevated-temperature fossil power plants.
Graphitization may be intended, as in the case of cast irons and the manufacturing of graphite electrodes for electric arc furnaces. It can also be unintentional, as in the case of in-service phenomenon in carbon and low-alloy steels. Unintended graphitization results in the degradation of strength and toughness of the affected materials, leading to low tensile strength, creep resistance and ductility. The changes in the properties of the affected materials cause several undesirable effects, such as:
- Premature failure of pressure boundary components such as high-energy piping and boiler tubes
- In steam-based power generation plants, brittle failures in welded carbon-molybdenum steel pipes
- Graphitization corrosion or skeleton corrosion in gray cast iron pipes
- Ductwork failure in coal plants