Efeito do tratamento térmico de envelhecimento na microestrutura e na resistência à corrosão do aço inoxidável duplex

Abstract

Duplex stainless steels are widely employed in the chemical and petrochemical industry, where optimal mechanical properties, weldability and high resistance to corrosion in highly aggressive environments are required. The purpose of this work was to analyse the effect of temperature and thermal aging treatment time on the microstructure of DSS SAF 2205, in addition to verifying the influence of the alpha prime phase precipitation in the resistance to corrosion. The DSS SAF 2205 was solubilized at 1100°C for 30 minutes and cooled in water. Afterwards, thermal aging treatment was undertaken at the temperatures of 450, 475, 500, 600, 800 and 850°C for 1, 3 and 12 hours, with cooling in water. The microstructural characterization was undertaken by optical microscopy, scanning electron microscope , energy-dispersive spectrometry, X-ray diffraction, quantitative stereology, ferritescope, hardness and microhardness of the phases. In the thermal analysis tests, ferromagnetic-paramagnetic transformations of the ferrite were observed at around 520°C, in addition to the consumption of the ferrite by intermetallic phases around 860°C. Solubilization fostered a homogenous microstructure, with a ferritic matrix and elongated austenite islands. The thermal aging treatments fostered the dissolution of the ferrite phase with an increase in the time and temperature. In the aging treatments at 450 and 500°C for 1 hour, and 600°C for 12 hours, precipitation of the alpha prime phase occurred through the nucleation and growth mechanism, leading to an increase in the microhardness of the ferrite phase. In the aging at 475°C for 12 hours, the forming of the alpha prime phase also occurred, but through spinodal decomposition, however. In the aging at 800 and 850°C, for 1, 3 and 12 hours, precipitation of the sigma phase occurred, increasing the hardness of the DSS SAF 2205. In tests for cyclic potentiodynamic polarization corrosion, it was observed that aging at 450°C for 1 hour presented better performance among the conditions tested. In the cyclic potentiodynamic reactivation tests, it was observed that the DSS SAF 2205, in the conditions studied, did not exhibit the sensitization phenomenon.