Modelamento computacional das transformações de fase durante os ciclos térmicos de processamento de um aço inoxidável superdúplex

This project aims to perform the computer simulation of the transformation’s kinetics and phase evolution during the thermal processing cycles of a superduplex stainless steel, considering the stages of heating, hot working and cooling, using DICTRA® software. The input data for the simulations were chemical composition and phase size, desired simulation temperature, and heating and cooling rates when necessary to describe the thermal cycle. TCFE9 thermodynamic database and MOBFE4 atomic mobility database were used in order to obtain results for different models, determining the one that best describes the phase transformation kinetics. Different rates were simulated during the heating of the material from 950 ° C, considering the initial microstructural condition of 50% of ferrite [alpha] and 50% austenite [gamma], up to 1250 ° C, typical forming temperature. In heating, a maximum fraction of 66.6% [alpha] was obtained at a rate of 0.30 ° C/s, [alpha] value close to the 70% expected by the equilibrium simulation in Thermocalc®. After 1000 s of plateau at 1250 ° C and cooling to the solubilization temperature, 1090 ° C, at the rate of 0.30 ° C/s, the fraction of [alpha] reduced to values of 58.7%. In the sequence, different cooling rates were also simulated with or without the presence of solubilizations plateau. Considering 3600 s of plateau at 1090 ° C, it was possible to recover the desired duplex condition, reaching 55.5% [alpha], but not reaching the 50% expected by the equilibrium balance, since there is still a composition gradient in [alpha] and [gamma] by DICTRA® simulations. Seeking the maintenance of the duplex microstructure, a cooling was performed from 1090 °C to 790 ° C at a critical rate of 3.0 ° C/s, obtaining volumetric fractions of 56% [alpha], 43% [gamma] and sigma fractions equal to or less than 1%. If the plateau at 1090 ° C was not considered, that is, promoting cooling from 1250 °C, where the condition of 58.7% [alpha] was reached, to 790 ° C at the rate of 3.0 ° C/s volumetric fractions of 59.6% alpha], 39.4% [gamma] and 0.9% [sigma] were obtained. DICTRA® was unable to simulate the precipitation of chromium nitrides (Cr2N) during cooling, either because there was no nitrogen supersaturation (N) in [alpha] or because it was unable to predict this supersaturation. From the results of kinetics and evolution of the phases’ volumetric fraction obtained in the thermal cycle of steel processing UNS S32750, it was possible to obtain the computational model that best describes the real behavior of the studied steel