Revestimento por soldagem MIG/MAG empregando ligas de níquel para apliacações em componentes do setor de petróleo e gás natural

Abstract

The weld overlay of nickel-based alloys is an alternative often used in oil and gas industry to prevent corrosion in pipes and equipments operating under severe conditions. In many situations the requirements of weld overlays are based in a maximum of 5% iron content in the weld metal to ensure corrosion resistance. The aim of this study was to determine the main welding parameters used in GMAW process to obtain coatings with iron content less than 5%, as well as, to analyze the mechanical properties, metallurgical behavior and corrosion resistance of the weld overlays. To evaluate the welding parameters was used as an analysis tool, the Taguchi method. Initially, the welds with nickel alloys were carried out in simple deposition on ASTM 516 Gr 60 steel plates in the flat position. Based on these results some parameters were selected to deposition of the coatings. In this work were used as filler metal three nickel-based alloys: AWS ERNiCrMo-3, AWS ERNiCrMo-4 and AWS ERNiCrMo-14. The microstructure analysis consisted of optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), energy dispersive of X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Cyclic polarization tests (ASTM G 61-09) and immersion (Method C of ASTM G 48-09) were carried out to evaluate the corrosion resistance of the welds. Microhardness tests and shear test were used to evaluate the mechanical properties. The results showed that the optimal welding conditions were obtained when pulsed current mode was employed, resulting in coatings with a good surface appearance, without defects, and with low dilution level, beyond reinforcement/width (R/W) ratios and reinforcements satisfactory. For coatings with thickness around 5 mm was required to deposit at least two layers. It was found that low dilution level in the first layer is essential to achieve a low iron content (<5%), considering the metal removal by machining to improvement the surface finish. The best cost-benefit for the combination of welding parameters between the layers was found to be the low dilution in the first layer and a high productivity in the second The microstructure of coatings deposited with the alloy AWS ERNiCrMo-3 consisted of a γ matrix with secondary phases rich in Nb like Laves phase and a complex of titanium nitride/niobium carbide. The microstructure of coatings deposited with the alloys AWS ERNiCrMo-4 and AWS ERNiCrMo-14 consisted of a γ matrix and secondary phases rich in Mo (σ, P e μ). The results of shear test showed that the shear interface between the coating/substrate is about three times the half the stress required by ASTM A 265-09. The electrochemical polarization test was not able to evaluate the performance of coatings for corrosion pitting, while the immersion test according to Method C of ASTM G 48, was capable to distinguish the corrosion resistance through the critical pitting temperature (CPT). Based on the results of this test it was possible to verify that the alloy AWS ERNiCrMo-14 resist up to the maximum temperature established by standard (>85 oC). The alloy AWS ERNiCrMo-4 was corroded between 75 oC and 80 oC. Finally, the alloy AWS ERNiCrMo-3 presented the worst resulted compared to the other alloys, has been corroded in the 50 oC, showing a different behavior in terms of corrosion resistance of the alloys deposited.