Annealing twin development in austenite in steels after hot deformation

Twinning development in the annealing of hot-deformed austenite in steels has often been suggested to play a relevant role in e.g. the evolution of grain size and texture across the process. Nevertheless, the phenomenon has not been systematically studied. In this view, a detailed assessment of annealing twin boundary evolution in austenite after hot deformation is carried out for the first time. Particularly, three materials are examined via electron backscatter diffraction (EBSD): a stainless steel, a carbon steel, and a Ni-30Fe alloy. Results demonstrate that twin boundaries form via recrystallization, and disappear by grain growth. However, unlike previously reported for lower annealing temperature in nickel, the number of twins per recrystallized grain does not increase throughout recrystallization. On the contrary, it stagnates before its end, upon activation of concomitant grain growth. Additionally, twin density increases with lower deformation/annealing temperature, higher strain rate, and higher applied strain. This has been rationalized via the higher resultant stored energy, which increases the density of microstructural discontinuities inside the deformed matrix (and, thereby, the rate of growth accidents). By contrast, no correlation has been observed between the measured boundary tortuosity and twin density. While Σ3 and Σ9 boundaries appear at the same rate during recrystallization, Σ9 ones disappear considerably more quickly with grain growth. Finally, the twin density trends examined after EBSD parent austenite reconstruction on the carbon steel have all been consistent. Consequently, that method represents a promising approach to analyze annealing twinning in steels that undergo phase transformations upon cooling.