Revealing grain subdivision initiated nanocrystalline white layer evolution in AISI 52100 steel via hard turning using transmission Kikuchi diffraction pattern matching

During hard turning, tempered martensitic AISI 52100 bearing steel experiences intense thermo-mechanical interactions. These interactions results in the formation of white layers (WLs), characterized by nanocrystalline grains, on the machined surface. The specific cutting conditions, such as feed rate, cutting speed, and tool wear, influence the formation of either mechanically induced (M-WL) or thermally induced white layer (T-WL). The formation of T-WL is primarily driven by continuous dynamic recrystallization associated with the phase transformation mechanisms. However, the underlying grain refinement mechanism for the M-WL resulting in nanocrystalline grains is not fully understood. Nanoscale analysis involving scanning transmission electron microscopy in scanning electron microscopy (STEM-in-SEM) and transmission Kikuchi diffraction (TKD) was used to characterize the microstructural gradients and orientation relationships. In addition, pattern matching analysis combined with TKD has been successfully employed to investigate the evolution of severely deformed nanocrystalline grains. The results reveal that the formation of a M-WL is characterized by nanocrystalline grains on the machined surface, followed by elongated lamellar grains in the underlying material drag region. The presence of these lamellar grains strongly supports the grain subdivision as an initiation mechanism, driven by predominantly severe plastic deformation. The grain subdivision is characterized by the formation of high-angle geometrically necessary boundaries (GNBs) and low-angle incidental dislocation boundaries (IDBs). Furthermore, we attribute the formation of nanocrystalline grains to mechanically assisted triple junction motion, a known dynamic recovery mechanism. These new findings provide crucial insights into understanding the gradient nature of the microstructural evolution of M-WL. • Revealed the M-WL evolution mechanism in hard turned tempered martensitic steel. • Grain subdivision process initiated the M-WL evolution, forming GNBs and IDBs. • Dynamic recovery by triple junction motion resulted in the nanocrystalline grains. • Compared M-WL with established T-WL formed via dynamic recrystallization process. • TKD pattern matching enhanced the indexing rate of deformed nanocrystalline grains.