Effects of grain size on the microstructures and mechanical properties of 304 austenitic steel processed by torsional deformation.

In this paper, two grain sized 304 austenitic steel rods (coarse-grained steel with an average grain size of ∼20μm and ultra-coarse-grained steel with an average grain size of ∼1mm) were treated by pre-torsional deformation. Gradient microstructures with "dislocations→stacking faults→nanotwins" along the radial direction were formed in the ultra-coarse-grained specimen, and this type of gradient microstructures is beneficial to the mechanical properties. The results also showed that grain size affects the onset of twinning. Under the same strain the onset of twinning in the ultra-coarse-grained specimen was postponed compared to the coarse-grained specimen, and the postponed onset of twinning further affected the martensite transformation. This work showed that applying pre-torsional treatment on large grain sized structural materials is beneficial to the overall mechanical properties.