Decarburization | Materials And Corrosion Control

·         Time, temperature and carbon activity of the process stream are the critical factors.

·         The material must be exposed to a gas phase that has a low carbon activity so that carbon in the steel will diffuse to the surface to react with gas phase constituents.

·         The extent and depth of decarburization is a function of the temperature and exposure time. Typically, shallow decarburization can decrease the strength of the material, but has no detrimental effect on the overall performance. However, it is indicative that the steel may have been overheated and suggests other effects may be present (e.g., decarburization associated with High Temperature Hydrogen Attack in hydrogen services).

·         Potential loss in room temperature tensile strength and creep strength may occur.

Carbon steel and low alloy steels.

Equipment

·         Almost any equipment that is exposed to elevated temperatures, heat treated or exposed to a fire.

·         Piping and equipment in hot hydrogen service in hydroprocessing units or catalytic reforming units as well as fired heater tubes.

·         Pressure vessel components that are hot formed during fabrication.

·         Alloy steels with chromium and molybdenum form more stable carbides and are more resistant to decarburization.

·         Steels operating in high temperature hydrogen environments should be selected in accordance with API RP 941.

·         Hardness testing as decarburization results in softening

·         Damage occurs on the surface exposed to the gas environment but in extreme cases may be through wall.

·         The decarburized layer will be free of carbide phases.
Carbon steel will be pure iron.