Graphitization Thermal Aging

Carbon steel vessels that have been in operation at temperatures above approximately 800°F are subject to graphitization. Graphitization can reduce the strength of carbon steel plate below the minimum required by the ASME Code for the maximum allowable design stress, but this effect by itself is usually not a very serious concern. The heat affected zones of welds in the carbon steel plate can be susceptible to a much more damaging type of graphitization that can lead directly to cracking.

Graphitization can be detected by nondestructive examination only after cracks have developed. Visual examination (VT) of the I.D. and O.D. surfaces can reveal these cracks before they cause failure, but magnetic particle (MT) or dye-penetrant (PT) examinations could reveal the cracks at an earlier stage of development. Ultrasonic examination (UT) using shear wave techniques can be used to detect and determine the depth of surface cracks, and to detect and size internal cracks that have not yet propagated to the surface.

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