Failure analysis of a large ball valve for pipe-lines

•Failure analysis of a valve of large dimension.•Microstructure analysis and mechanical test were performed.•An incorrect thermal treatment and a sharp notch effect are responsible of the failure.

In this work the failure of a sub-sea ball valve, used in an oil-piping line, is analysed. The valve was of the same type and material already used for the construction of valves that were worked in service without any problem. The valve failed in the first pressure cycles during the preliminary laboratory tests, although the applied pressure was less than the design value. Metallographic and microstructural analysis of the fracture surfaces performed by means of optical and scanning electron microscope (SEM), residual stress and hardness measurement, tensile, toughness and Charpy tests, were executed in order to identify the causes of the failure. The results allowed assessing that the failure was due to two concomitant factors: a severe notch effect and an incorrect thermal treatment.