Corrosion Fatigue Failures of Economizer Outlet Header Stub Tubes

Abstract

Nondestructive and metallurgical examinations were performed following the failure of a stub tube for an economizer outlet header. The leak in the header stub tube was at circumferentially oriented cracks that initiated at the inside surface. The cracked regions were approximately 180° apart circumferentially and located on the sides of the tube inline with the header axis. This cracking pattern is consistent with an applied bending moment that is out of the plane of the tubing assemblies. The ultrasonic inspections used the failed tube as a calibration standard. The deepest cracks (reflectors as a percent of calibration standard) were found primarily at the ends of the header, the first sixteen rows from the north end and the last twenty-two from the south end. The cracking at the ends of the header is believed to be due to thermal loads and the restraint to expansion of the economizer tubing at the crown seals. Both cracked and failed tubes were metallurgically examined using visual inspection, fractography, and optical metallography. The fracture surface has beach marks and the crack shape is thumbnail or semi-elliptical, characteristic of fatigue cracking. There are multiple cracks and many of the shorter crack segments have joined or linked up to form the final crack length. The cracks are transgranular and oxide-filled. The failure mechanism is corrosion fatigue.