Efeito do tratamento de solubilização na microestrutura, propriedades mecânicas e de resistência à corrosão em liga modelo de aço inoxidável superferrítico com adição de boro

Abstract

For the present research, a superferricible stainless steel alloy model with 4.4% wt of nickel (Ni) was added to improve its toughness, with a chromium (Cr) content of 25.7% w to maintain phase stability ferritic, keeping the Mo content constant at 4.6% w, to preserve resistance to corrosion. An important factor for the study of this alloy was the addition of 0.07% p (700ppm) of boron, the creep resistance and stabilize the grain boundaries of this alloy. The present research is part of a study for the development of new alloys from the composition of commercial superferrites, with the objective of improving them so that they can be used in equipment such as heat exchangers and distillation towers operating in extreme conditions of temperature, pressure and in environments containing high concentrations of chlorides, H2S and CO2. The effect of thermal treatment of solubilization on the microstructure, mechanical properties and corrosion resistance of the model alloy containing 700 ppm of boron was studied for solubilization temperatures of 1150 ° C and 1200 ° C at different treatment times. For the selection of the heat treatment temperatures, a thermodynamic study was carried out using the Thermo-Calc® software. The stability temperature of the ferritic phase was determined and the possible precipitated phases and the mass fraction of each were determined with the temperature variation. In this temperature range is the ferritic matrix and is the beginning of the dissolution of the boron precipitates, in order to observe the effect of this precipitate on the mechanical properties and corrosion resistance of the alloy, information scarce in the literature. The microstructure of the alloy was characterized by showing that the ingot has a sigma and austenite nucleated both internally and intragranular in the ferritic matrix, whereas the centrifuged tube shows only the sigma phase in the ferritic matrix. After the thermal treatments, it was verified the dissolution of the precipitates found in the microstructure. The alloy under study presented higher mechanical strength and hardness than commercial alloys with similar composition. The alloy under study presented higher mechanical strength and hardness than commercial alloys with similar composition. After the solubilization treatments, there was an increase in the corrosion resistance and the critical pite temperature of the alloy under study, showing that the solubilization treatments performed were efficient for the alloy to present good properties for its industrial application.