Analysis of the failure of a cast iron pipe during its pressure test

This paper presents the failure analysis of a cast iron pipe during the pressure test performed once it was already installed and before entering in service. The pipe had previously passed an in-plant pressure test. In order to determine the causes of the failure, visual inspection together with mechanical, chemical and microstructural analysis of the material has been performed. It has been derived that the pipe section that failed during the pressure test had previously suffered mechanical damage during its transportation or installation. Moreover, it has been demonstrated that the material has a very brittle behaviour, with excessive hardness and poor tensile strength and elongation to rupture, and also presents a deficient microstructure in the outer layer of the pipe thickness. This, together with the presence of pre-existing defects caused by the mechanical damage, has produced the final failure of the pipe. Finally, a fracture assessment has been performed in order to estimate the size of the initial pre-existing defects.