Corrosion Fatigue Failure Analysis and Life Prediction

A metallurgical failure analysis and life prediction was performed for an economizer tube. The tube failed after approximately nine years of service. The failure was a pin hole leak, elliptically shaped with the long axis in the circumferential direction. On the inside surface of the tube, there were several circumferential cracks and numerous oxygen pits on the top half. The cracks were transgranular and initiated at pits. The failure mechanism is corrosion fatigue and is believed to be due to a cyclic applied bending moment. Life predictions were performed using two crack geometries and zero-tension loading cycle. The geometries were a thumbnail shaped ID crack with an a/c of 0.2 and a 360° ID cracked cylinder. A parametric approach was used with two initial crack depths based on the measured pit depths and three remote stresses centered around the minimum yield strength of the SA-178-A tube material. The failure criteria was the reference stress equal to the flow stress. For the thumbnail crack geometry at a remote load of 179 MPa (equal to minimum specified yield strength), the calculated lives were 15,960 cycles for an initial crack depth of 5% and 3,450 cycles for an initial crack depth of 10%. The cyclic lives of the 360° crack geometry were approximately half of those for the thumbnail crack geometry. The slope of the log life-log stress curve was approximately −5.8.