Failure Analysis of Stress Corrosion Cracking of a Ball Valve in Service

Abstract

In this work, the failure analysis of a ball valve, with a nominal diameter of 80 mm, is shown. The analysis was performed by: visual inspection, chemical analysis, mechanical tests, metallographic examination and fractographic examination. From the mechanical tests results, it was found that the hardness of the failed bolts “I” and “J” were of 40 HRC, which is larger than the specified for this type of steel (35 HRC). The fractographic analysis of the fracture surface of the failed bolts (I and J), revealed brittle fracture characteristics, where a flat surface and not appreciable plastic deformation were seen. The G and H bolts presented ductile fracture, showing a large amount of plastic deformation associated to the fracture; where a rough surface, an area reduction of 8.6% and the presence of shear lips. Due to a wrong leak mitigation in the seals of the valve, bolts I and J were exposed to a corrosive environment. Hence, the combined effect of corrosion and the stresses to which the bolts were subjected, caused a stress corrosion cracking (SCC) failure. Finally, after the fracture of bolts I and J, the remaining bolts G and H failed by overloading.