From bands to blobs: how final annealing changes super duplex stainless steel
Résumé
Super duplex stainless steel grades are modern stainless steel grades combining high specific mechanical properties with excellent corrosion resistance, especially to pitting corrosion. Their very specific microstructure consists in a mix of ferrite and austenite, usually at a 50/50 ratio, arranged in layers. This ratio is finely adjusted by a final annealing step during which, this fine, strongly banded microstructure evolves towards a coarser, globularized morphology in a matter of seconds. To determine the effect of annealing time and temperature on the final morphology, the kinetics of this morphological evolution have been quantitatively assessed in commercial and model alloys. In particular, stacks of austenite and ferrite with compositions found in the super duplex microstructure were assembled using HIP to produce an ideal super duplex microstructure with infinite, smooth bands. The obtained results suggest that grain boundaries and phase interfaces play a key role in the morphological changes. A phase field model of the polycrystalline two-phase microstructure was developed to simulate the observed phenomenon and rationalize the influence of the different parameters. Furthermore, the change in morphology of the phases was quantitatively analyzed against the change in mechanical properties. TEM and EBSD observations allow to relate these variations to the arrangement of dislocations in the microstructure. The present study affords a better understanding of the effect of final annealing on the microstructure of super duplex and opens new possibilities to tailor their properties.