Decarburization is a process that reduces the amount of carbon constituents in a material (usually steels). Steels are generally classified as low-carbon, medium-carbon, and high-carbon steels. A general rule is that as the amount of carbon content decreases, the strength and the hardness of the steel decreases.
Decarburization may have desirable or adverse effects. Some of the effects include reduced ductility, reduced strength, and increased susceptibility of crack initiation. These effects are concentrated at the surface of the metal. It’s worth noting that equipment failure is rarely a direct result of decarburization. However, most personnel in industry view the process as a serious issue that has detrimental effects on the fatigue life of steel components and may be indicative of a more severe underlying problem.
Decarburization occurs at high temperatures (typically at or above 700℃). At this temperature carbon diffuses out of the metal and the metal becomes more susceptible to absorbing hydrogen and oxygen gases. The rate of carbon diffusion increases with increasing temperature. This can lead to more serious problems such as high temperature hydrogen attack and hydrogen embrittlement.
One way to test for decarburization is to perform a hardness test on the material. This will provide a general idea of how much carbon diffused from the surface of the metal because the metal will be softer than its original state. Again, this is because there is less carbon and the lower the carbon content the less hard the material will be.
Other forms of decarburization testing exist that estimate the depth of decarburization along the surface of steel (e.g. ASTM E1077). The outcome of the test reveals if the surface has undergone complete decarburization (100% loss of carbon), if more than 50% has undergone decarburization, or if less than 50% has undergone decarburization. These forms of decarburization are classified as Type 1, Type 2, and Type 3, respectively.
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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. |
