ESSC & DUPLEX 2019

The aim of the present work is to perform computational simulations of the equilibrium and phase transformation kinetics of a superduplex stainless steel UNS S32750 in two different heat treatment cycles: i) during heating to 1250°C from its initial duplex condition, or ii) during heating at temperatures for duplex structure from the equilibrium microstructure obtained at 1250°C solution treatment. Thermo-Calc® and DICTRA® simulations were executed, as well as the experimental validation in the laboratory of the results of these simulations. The experimental validation showed, by quantitative stereology, that equilibrium phase fractions stabilize after 30 min of heat treatment in both heat treatment cycles, reaching 71% of ferrite in cycle “1” and 51% of austenite in cycle “2”, validating the kinetic simulations concerning phase fractions. However, simulations showed that during heat treatment cycle “i”, although ferrite fraction stabilizes between 100s and 1000s (16 min), chemical elements took 2160s (36 min) to come into equilibrium in ferrite, and 36000s (10h) in austenite, indicating that equilibrium is only fully attained long after the phases’ fraction stabilizes, a fact which the experiments are not able to detect. The geometric model that simulates planar grains and considers only the main chemical elements of interest is the one that best describes the experimental results.