Sigma phase formation kinetics in hyper duplex stainless steel welding filler metal

This work presents a kinetic study of the sigma phase formation in hyper duplex stainless steel filler metal. Two sigma phase precipitation kinetics models were developed and compared. Initially, experimental sigma phase precipitation was built using isothermal heat treatments with durations from 30 s to 600 s, and temperatures between 600 degrees C and 1100 degrees C performed using a physical simulator. In these experiments, up to 70% of the equilibrium volumetric fraction of the sigma phase was achieved in 600 s. A CALPHAD-based kinetic model was developed using the experimental transformation data. Constant cooling rate conditions were calculated using the CALPHAD-based model revealing a minimum cooling rate of 4 degrees C/s as the threshold for the sigma phase to form. The microstructure evolution of the sigma phase precipitation follows the known eutectoid decomposition mechanism of ferrite transformation to sigma phase and secondary austenite (alpha -> sigma + gamma 2), which evolved at the latter stages of the precipitation times, the lamellar sigma/gamma 2 morphology results from the eutectoid reaction, which is diffusion controlled. Finally, we applied the JMAK kinetic law to model the sigma phase formation on both datasets, the experimental and the CALPHAD-based TTTs. In the JMAK linearized plots, a kinetic mechanism change was found, switching from an eutectoid decomposition stage to a diffusion-controlled growth stage. While the JMAK calculations provided good agreement with the experimental data, the CALPHAD-based data only agreed near the maximum kinetics temperatures between 900 degrees C and 925 degrees C. Nevertheless, the sigma phase transformation kinetics modeled using JMAK equations properly described the experimental data describing its double kinetics behavior and reproduced the CALPHAD-based TTT at the maximum kinetics temperature range.