Microstructural study of softening processes in a duplex stainless steel deformed in hot torsion

The aim of the present study was to undertake a detailed microstructural investigation of softening processes within both ferrite and austenite in a 21Cr-10Ni-3Mo duplex stainless steel, containing about 60% austenite, subjected to hot torsion at 1200 degrees C at a strain rate of 0.7 s(-1) to various strain levels. Quantitative optical microscopy and transmission electron microscopy together with convergent beam electron diffraction were used in the study. Detailed quantitative information about both morphological and crystallographic characteristics of the hot deformation microstructure was obtained, which established unambiguously the nature of softening processes within ferrite and elucidated various mechanisms of nucleation of dynamically recrystallised grains within austenite. It was concluded that ferrite softened under the experimental conditions used via "extended" dynamic recovery characterised by gradual increase in misorientations between neighbouring subgrains with strain. Conversely, dynamic recrystallisation involving nucleation and growth of new recrystallised grains gradually developed within austenite concurrently with dynamic recovery. Dynamically recrystallised grains were observed to nucleate through a range of mechanisms including strain induced migration of the pre-existing austenite/austenite boundaries, creation and movement of large-angle boundaries in the vicinity of the austenite/ferrite interface, large-scale subgrain coalescence and formation of annealing twins.