Abstract
In this study, failure analysis of a gas tubing string was investigated. Visual inspection of the tubing string showed that some of the tubings were corroded locally at pin thread ends, which resulted in abrupt replacement. In order to determine the cause of failure, chemical composition, mechanical strength, Charpy impact, and hardness, and microstructures of the corroded and non-corroded pins were investigated. The chemical composition of the tubing material was found to be API 5CT Grade C-75-2. The samples taken from the body of both the corroded and non-corroded tubings showed the same impact energy, yield strength, ultimate tensile strength, and elongation. In addition, the hardness of the body and thread ends of tubings were the same except for corroded tubings, which showed locally more hardness at thread region. Our analyses indicate that cold working the tube strings, during the make-up process in the field, caused localized corrosion of the male pins.
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