Effect of Initial Microstructures on Grain Refinement by Burnishing

The development of ultrafine grained microstructures under severe plastic deformation by burnishing process was investigated using spherical cementite-ferrite (SA) steel and pearlite (P) steel of AISI 52100. Microstructures were analyzed using FE-SEM, FE-TEM and EBSD observations. In the SA steel, equiaxed ultrafine ferrite grains were formed at the burnished surface where the equivalent strain was about 3.9. These ultrafine grains were formed by continuous dynamic recrystallization because they consisted of high angle grain boundaries. On the other hand, in the P steel, the initial lamellar structure was maintained even at the equivalent strain about 4.3, and ferrite grains with a large aspect ratio were formed. These ferrite grains were considered to be non-recrystallized grains because the KAM value within these grains was high. In addition, many dislocation contrasts in the same direction were observed within a ferrite grain by FE-TEM observation. These results suggested that active dislocation slip system in these ferrite grains is limited by lamellar structure. As the strain increased by repeated burnishing process, these ferrite grains of P steel became coarse and the KAM value within these grains decreased. In addition, several dislocation contrasts in multiple directions were observed within a ferrite grain. It can be concluded that the limitation of active dislocation slip system in these ferrite grains were relaxed, and dynamic recovery was occurred in these ferrite grains.