Carburization is a phenomenon where carbon is incorporated into the material. Carbon bonds readily with chromium and chromium carbides will be formed, especially at the grain boundaries. This leads to embrittlement of the material. The bonding of chromium to carbon results in chromium depletion in the matrix, reducing the resistance of the metal to oxidation and sulphidation.
Problems with carburization can occur in processes where gas mixtures containing CO or hydrocarbons, such as methane, ethane, propane and naphtha, come into contact with the material at high temperature.
The extent of carburization will depend on the carbon and oxygen contents of the gas and on the chemical composition of the material.
Carburization is a problem, for instance, in steam reforming plants and cracking furnaces used in ethylene production.
Green rot and catastrophic carburization are two special phenomena connected with carburizing atmospheres. Problems with green rot and catastrophic carburization involve a very rapid disintegration of the material.
Good resistance to carburization tends to go hand in hand with oxidation resistance. Increased contents of chromium (Cr), silicon (Si), nickel (Ni) and aluminium (Al) increase resistance. Just as with oxidation, the addition of rare earth metals and reactive elements such as titanium (Ti), zirconium (Zr) and yttrium (Y) has a highly positive resistance effect.
The following grades have very high or excellent resistance to both carburization and oxidation:
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