Influência da energia de soldagem multipasse na resistência a corrosão de juntas de aço inoxidável superduplex UNS S32760 soldadas pelo processo MIG/MAG

Abstract

The aim of this study was to evaluate the effect of heat imput in multipass welding on microstructural transformations and corrosion resistance of UNS S32760 superduplex stainless steel welded pipe joints by GMAW process. For this purpose, three levels of heat imput (0.5, 1.0 and 2.0 kJ/mm) were used. The heat imput values were based on records of welding procedure qualification (RWPQ) drawn up and applied in PETROBRAS units, except for the condition of 2.0 kJ/mm, which was an extrapolation. Weldings had been carried out for pipes with 18 mm thick, with joint geometry of J-groove. All welds were performed using a robotic workbench. The microstructural characterization of the weld regions like Fusion Zone (FZ) and Heat-Affected Zone (HAZ) were the performed by Ligth microscopy (LM) and scanning electron microscopy (SEM). The average ferrite content was determined by image analysis and was considerated as a characteristic response. The corrosion resistance was evaluated by electrochemical potentiodynamic polarization in sodium chloride solutions with different concentrations (60 g/L, 120 g/L and 240 g/L) aiming to simulate the concentration of chlorides in water production of oil reservoir of the pre-salt region. The to evaluate the electrochemical potentiokinetic reactivation cyclic of double loop (EPR-DL) corrosion was done to evaluate of susceptibility to corrosion. Electrochemical tests critical temperature for pitting according to ASTM G150, immersion tests in a solution of ferric chloride following the ASTM G48. Immersion tests in oil emulsions prepared with sodium chloride solutions at different concentrations (60 g/L 120 g/L and 240g/L), heated at 60ºC and different ratio oil/water (10%-90%, 30%-70% and 50%-50%) were conduced. The results indicated that the welding heat imput was the factor that exerted the greatest influence on the average ferrite content in the fusion zone. There were precipitation of chromium nitrides at all heat imputs and sigma phase for the 1.0 kJ/mm and 2.0 kJ/mm. The EPR tests results as well as TCP tests showed that the fusion zone energy of 1.0 kJ/mm was more susceptible to corrosion. The immersion tests according to ASTM G48 showed that the temperature where the formation of pits occurred with considerable mass loss when the samples came in contact with the solution of ferric chloride was 50ºC for all heat imputs. And finally, the immersion test in oil emulsions, showed again that the 1.0 kJ/mm test sample showed the greater amounts of pitting.