Failure Analysis of Shift Gas Tubes

In a scheduled maintenance inspection of a fertilizer power station, a series of circumferential cracks were found in shift gas heat transfer tubes. In order to get an understanding of the failure mechanism, a failure analysis was performed on the leak-off shift gas tubes in this paper. The tubes were observed and analyzed by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The fractography and metallurgical examination results showed that the crack initiated along grain boundaries and propagated through grain boundaries and the crack tips were oxide filled and not branched. Typical fatigue striations were observed on the fracture surfaces. The surface intergranular corrosion cracks caused by sensitization resulted in stress concentration. Fatigue cracks propagated along slip plane of grains under varying load. Moreover, the oxides on the surface of slip planes reduced the cohesive strength of steel substrate on both sides of slip planes. The synergistic effect of corrosion and fatigue, herein referred to as corrosion-fatigue, was the dominant mechanism of the failure. The vibration analysis of the tubes was carried out, and the results indicated that the vibration was induced by Karman vortex shedding and wobbling turbulent flow. In addition, prevention methods have been proposed to avoid corrosion-fatigue of the tubes.© 2014 ASME