Abstract
Abstract The 13Cr valve cage of a tubing pump fractured after 191 days service in an oilfield in China. The failure causes were analyzed by direct-reading spectrometer, Rockwell hardness tester, optical microscope, scanning electron microscopy, energy dispersive spectrometer, electrochemical method and finite element method in this paper. The results showed that the failure reason was typical corrosion fatigue fracture initiating at the root of the last thread next to the thread relief groove, where numerous corrosion pits and cracks were observed. In general, co-effect of two factors occurred to cause the valve cage to corrosion fatigue fracture. The downhole medium with high CO2 and Cl− concentrations was the environmental factor of the failure. Alternating stress caused by pumping cyclic motion was the main stress factor of the failure, while the high stress concentration on thread connection further induced cracking.
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