| Abstract Scope |
Tempered martensitic AISI 52100 bearing steel, when subjected to the hard turning process, undergoes severe thermo-mechanical loads that lead to the formation of white layers (WL) on the machined surface. The specific cutting conditions, such as feed rate, cutting speed, and tool wear, influence the evolution of mechanically-induced (M-WL) and thermally-induced white layers (T-WL). Understanding this microstructural evolution on the machined surface is crucial for assessing surface integrity. In this study, hard turning with a cutting speed of 60 m/min and a feed rate of 0.05 mm/rev using a fresh coated BNC 2125 insert resulted in the formation of a M-WL. Tool wear of ~0.2 mm further increased the thickness of the M-WL, characterized by an elongated, refined layer on the machined surface, followed by material drag underneath. Conversely, machining at a cutting speed of 110 m/min and a feed rate of 0.2 mm/rev with a fresh insert produced a discontinuous T-WL. Tool wear of ~0.2 mm further increased the thickness of the T-WL, featuring a cellular refined layer on the machined surface with a dark layer underneath. This study aims to investigate the microstructural variations of M-WL and T-WL, including features such as material drag and dark layers, along with the refined WL using fresh and worn inserts. Nano-scale techniques using transmission electron microscopy (TEM) and transmission kikuchi diffraction (TKD) were employed to characterize the microstructural variations, crystal structure, dislocation distribution, and orientation behaviour. The findings of this research are beneficial for understanding the initiation of M-WL by characterizing the material drag region underneath. |