Avaliação microestrutural e eletroquímica da zona termicamente afetada simuladas termicamente de um aço inoxidável super duplex

Abstract

Super duplex stainless steels (SDSS) are steels with a high content of alloying elements. These steels were introduced in the oil and gas industry due to their high resistance to pitting corrosion. These steels are currently used in a wide range of applications due to their excellent corrosion resistance combined with high mechanical strength. They are often selected for applications in chloride-containing environments, such as seawater, where stress and pitting corrosion can occur. In the main applications they are used in equipment for oil and gas exploration and production, in oil and gas pipelines. However, the higher the concentration of alloying elements, the greater the susceptibility to the formation of embrittled intermetallic phases, and therefore, greater care must be taken during the processing of these steels. The arc welding process is widely used in the manufacture of equipment. This process can generate enough heat to cause an unbalance of the ferrite/austenite microstructure and, consequently, the formation of undesirable (deleterious) phases. During arc welding, each point of the joint will undergo different thermal cycles and this can result in undesirable microstructures, such as the formation of the Sigma phase (σ), Chi phase (χ), chromium nitrides, carbides and others. In general, these phases are rich in chromium and molybdenum, which consequently unbalances the levels of these chemical elements around them, producing a favorable environment for the formation of corrosion, affecting the corrosion resistance and its mechanical properties. The purpose of this work is to contribute to the scientific literature regarding the behavior of the AISD UNS S32750 high temperature heat affected zone (HAZ), 10 mm thickness, thermally simulated in a Gleeble 540: Welding Simulator® equipment and finally to evaluate a methodology of analyze of the corrosion resistance of thermal simulated samples. After the thermal simulation of the samples, the microstructures were characterized using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) with Energy Dispersion Spectroscopy (EDS) and Electron Back Scatter Diffraction (EBSD) techniques. The pitting corrosion resistance was investigated by polarization techniques in sodium chloride solution and the critical pitting temperature (CPT) for each evaluated microstructure. The partial results of the simulated samples showed microstructures with HAZ grain morphologies and an increase in the % ferrite, which was expected, since the samples were submitted to thermal cycles that could unbalance the austenite/ferrite content. Furthermore, corrosion pits were identified after the electrochemical tests and the CPT was determined from the inflection of the potential curves as a function of temperature. The methodology used in the evaluation of the susceptibility to corrosion showed great potential when compared to the conventional method, especially when related to the fabrication time of the specimen and the execution time of the test.