Effect of the annealing temperature on the mechanical properties, formability and corrosion resistance of hot-rolled duplex stainless steel

The industrial sequence for obtaining duplex stainless steels by rolling includes two annealing steps: one intermediate after the hot rolling, and the final one after the cold rolling. The aim of this paper is to study the consequences of deviations from the current industrial intermediate annealing temperature. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), together with X-ray diffraction (XRD), were used to investigate the microstructural changes of the steel. The only transformation observed was the precipitation of sigma phase for all the considered annealing conditions. A decrease in the heat treating temperature augmented the percentage of sigma phase, which generated an increase in tensile strength and hardness, concurrently with drastic reductions on ductility and impact resistance. In addition, compression testing and cold rolling at laboratory scale were done in order to compare the formability of the steel subjected to the new annealing conditions with the current industrial rolled products. On the other hand, the pitting potential of the thermo-mechanically treated samples was determined by potenciodynamic polarization tests. The electrochemical results showed that sigma phase markedly decreases the pitting potential. SEM observations revealed that pits appear mainly in regions adjacent to sigma phase, where secondary austenite forms.