Ref.: MmeCa03-010
Apresentador: Andre Rocha Pimenta
Autores (Instituição): Tavares, S.S.(Universidade Federal Fluminense); Garcia, P.S.(Universidade do Estado do Rio de Janeiro); Lacerda, F.G.(Universidade do Estado do Rio de Janeiro); Breves, I.M.(Universidade Federal Fluminense); Pimenta, A.R.(Instituto Federal do Rio de Janeiro);
Resumo:
The oil and gas industries have severe operational conditions, which cause many failures in offshore equipment. The Flare System is a pipe designed to safely dispose of relieving hydrocarbon gases and liquids during start-up, operational upsets, emergency shutdown, and maintenance activities. This work reported the failure investigation in a pipe of a high-pressure flare system. The pipe has a diameter of 220 mm and 4 mm of thickness, and according to the equipment datasheet, it was made in AISI 316L stainless steel. A pipe with these dimensions has probably been bended and welded longitudinally with an electric resistance process (ERW-electric resistance welding). Two cracks were detected during the previous non-destructive inspection. Those failures were repaired, but another crack was detected after some months. Chemical composition was investigated using optical emission spectroscopy and elemental analysis. The results are compatible with AISI 316L steel. The metallography investigation shows some defects associated with the welding procedure made to repair the first cracks. However, these defects are not related to the appearance of the third crack. Microstructural investigation using optical and scanning electron microscopes, X-ray diffraction, and microhardness shows the presence of a martensite phase throughout the pipe, not only in the fracture region. Tensile tests were conducted, and the mechanical properties were in accordance with what was expected for the AISI 316L. Electrochemical corrosion tests were also used to investigate the material. A double-loop electrochemical potentiodynamic tests were performed in two different solutions, and the results showed that the steel was not sensitized. The polarization curve obtained also presents a typical behavior of AISI 316L. The fracture surface observation was conducted using optical and scanning electron microscopes. Beach marks were observed in a macroscale, and fatigue striations were observed in the scanning electron microscope. The small size of the striations (~1 µm or less) is related to small stress amplitudes and high cycle fatigue. Austenitic stainless steels, such as 316L, have high resistance to corrosion in various environments. However, it is suggested that duplex stainless steel be used for applications where mechanical strength and fatigue resistance are essential. The biphase microstructure of the duplex stainless steel results in good corrosion resistance and high mechanical properties.