A 234 steel flange ASTM is widely used in oil and gas. A damage flange from a unit of submersible pump operated at an offshore platform. The leakage was occurred at the flange connection at submerged part of the vertical pump. The external protection of the section was external coating. The leakage had occurred twice at the same vertical pump. The present investigation aims to analyze the main cause of the failure by conducting a standard failure analysis test including chemical composition test, visual examination, microstructural examination by optical microscope and scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS), X-ray diffraction and Brinell hardness measurement. The results of this investigation on the failed flange showed that the microstructure of the flange material is found to be ferrite-pearlite with banded structure on the pearlite. This banded structure …