σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500-800 degrees C to total strains of ~4, followed by annealing at 800-1,000 degrees C for 30min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28-0.85 mu m depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of sigma 3(n) CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D-0). The fraction of sigma 3(n) CSL boundaries sharply rose to approximately 0.5 in the range of D/D-0 from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of sigma 3(n) CSL boundaries slowed down significantly in the range of D/D-0>5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% sigma 3(n) CSL boundaries in the recrystallized microstructure.