Avaliação microestrutural de juntas de união similar e dissimilar de liner utilizados na fabricação de tubos MLP.

Abstract

Due to the discoveries of immense oil reserves in Brazil, the oil and gas industry has been growing and developing, seeking improvements in materials and manufacturing methods in order to make them more resistant at the lowest possible cost. A lower cost alternative for the transport of corrosive fluids are MLP (Mechanically Lined Pipe) tubes, externally made of high strength low alloy steel (HSLA), and internally coated with nobler materials such as nickelbased alloys. The nickel-based alloy Inconel 625 has been commonly used as base and filler metal in MLP pipe liner, a noble material, with high cost and excellent mechanical properties and corrosion resistance. However, there are other materials that have excellent corrosion resistance and that could be used in the manufacture of the liner, such as austenitic and superaustenitic stainless steels with high Mo content. For this, three joints were welded with different base metals: nickel-based alloy Inconel 625, stainless steel AISI 317L and 254SMo. Two welding processes were used: PAW keyhole with cold wire feed and autogenous GTAW in overhead position, all joints had Inconel 625 wire as filler metal. Samples were taken and analyzed in an optical microscope, in order to identify the microstructures, and in SEM, EDS had been made in center of the dendrites of the two processes to calculate the segregation coefficient. The microstructure of the welded zone was mostly columnar dendritic, however in the joint welded with AISI 317L and Inconel 625 eutectic microstructures were found in the PAW bead. In the PAW bead of the joint welded with 254SMo, shrink porosities were found in the central region of the bead and close to the surface, however, none were found in the similar joint of Inconel 625. By calculating the segregation coefficient k, it was obtained that Fe and Ni have a tendency to incorporate into the solid, and Nb, Mn, Cr and Mo segregate.