Creep Embrittlement Thermal Aging

Low-alloy 1¼ Cr – ½ Mo steel is susceptible to creep embrittlement when in service at temperatures above approximately 850°F. Weld heat affected zones usually exhibit the greatest degree of embrittlement. The material becomes highly notch sensitive, and stress-rupture cracks can develop at locations of relatively high stress concentration. The toes of nozzle welds and fillet welds for attachments appear to be the most likely locations for these cracks to occur.

In-service inspection can detect cracks attributable to creep embrittlement in time to permit safely scheduling repairs without unnecessarily interrupting production. Visual examination (VT) of the O.D. and I.D. surfaces of a pressure vessel is usually adequate for detecting and sizing cracks attributable to creep embrittlement, because these cracks most often originate at the surface. Magnetic particle (MT) or dye-penetrant (PT) examinations would provide greater sensitivity for detecting the cracks. Ultrasonic examination (UT) using shear wave procedures can be used to determine the depth of cracks originating at the surface and would also be invaluable for detecting and sizing cracks that may have originated at a stress concentration associated with an internal flaw.

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