Estudo da precipitação de fases na resistência à corrosão do aço Inoxidável Duplex UNS S31803 após tratamento térmico de envelhecimento a 400ºC

Abstract

The UNS S31803 duplex stainless steel has a two-phase microstructure with equal or similar volumetric fractions of ferrite and austenite, in which the distribution and arrangement of the phases confers mechanical and corrosion resistance properties superior to those of other stainless steels. However, when exposed to temperatures above 300ºC, in thermal manufacturing processes, or due to the working temperature, it can promote the precipitation of intermetallic phases, which cause embrittlement and decrease in corrosion resistance, impairing the use of this steel. Thus, this work aimed to characterize and evaluate the transformations of precipitated phases in the heat treatments of aging at 400ºC at 12, 53, 100, 306, 485 and 902 hours, and to evaluate the effects on microstructure, hardness and corrosion resistance, in order to contribute to the sector of maintenance and installation of equipment for the industry. Microstructural characterization was performed by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and quantitative stereology. Hardness and microhardness measurements were also carried out, as well as corrosion tests using the electrochemical techniques of cyclic potentiodynamic polarization, in a 3,5% NaCl solution and of double cycle potentiodynamic electrochemical reactivation, in a solution of 0,5M H2SO4 + 0,01M KSCN. The solubilization promoted the partial dissolution of secondary phases and a homogeneous microstructure, of ferritic matrix with elongated islands of austenite. The aging heat treatments promoted phase transformations, reducing the presence of secondary phases such as sigma, chi, carbides and nitrides. However, due to the exposure of the steel to 400ºC, there was the formation of the alpha line phase, proportionally to the aging time, promoting an increase in hardness. Through the cyclic potentiodynamic polarization tests, it was observed that there was no significant variation in the electrochemical parameters, indicating that the heat treatments did not significantly influence the corrosion resistance. And the double cycle electrochemical potentiodynamic reactivation tests showed that the sensitization of the steel did not occur.