Microstructural evolution of super duplex stainless steel during final annealing: The importance of the grain structure

Abstract

Flat duplex stainless steel acquires its characteristic banded microstructure during its processing sequence, which ends with a final annealing step aimed at fine tuning the phase ratio to equal parts ferrite and austenite. Besides phase fraction, the grain structures, shapes and morphologies of both phases evolve too during this final step. In the present, it was shown that recrystallization is mostly complete at the end of heating and phase equilibration shortly after. As a consequence, chemical and elastic driving forces are rapidly depleted while the sizes and shapes of phases keep evolving, driven solely by capillarity. Using quantitative measurements that enable decorrelation of coarsening and globularization, it is shown therein that the grain structure of the material as well as the annealing temperature play a crucial role in the intensity of those two phenomena. A high grain boundary density enhances the kinetics of both. A higher annealing temperature, which leads to faster mass transport, favors the increase of the characteristic sizes of the microstructure over its morphological evolution.