Sahith Kokkirala, Material och tillverkning

The advancement in machining processes demands that components exhibit superior surface integrity and functional performance. Hard turning is an efficient machining process that requires precise process control, especially with hardened steels, to achieve the desired surface integrity. However, the intense thermo-mechanical interactions between the workpiece and the cutting tool often lead to the formation of a white layer (WL) on the hard-turned surface, which typically extends from a few hundred nanometers to few micrometers below the surface. WLs are microstructural alterations characterized by nanocrystalline (NC) grains, appearing featureless and white in the light optical microscopy. Based on established literature, often WLs are detrimental because they consist of brittle, untempered martensite with tensile residual stresses, leading to premature component failure. These typically refers to thermally induced WLs (T-WLs), which form through continuous dynamic recrystallization (CDRX) and reverse martensitic transformation and are often accompanied by the formation of a softer dark layer. However, if the process parameters are carefully controlled, a fundamentally different type of WL is formed below the austenitization temperature, known as mechanically induced white layer (M-WL), which exhibits beneficial compressive residual stresses without the presence of a dark layer. These properties make M-WLs a promising process-induced NC surface for demanding engineering applications. Despite its potential, a detailed understanding of the influence of process parameters, tool geometry, initial microstructure on the formation of M-WL with improved surface integrity is currently lacking. In particular, the underlying mechanisms that control microstructure development in M-WL remain unclear.  

This thesis investigates the formation and properties of M-WLs and compares them with T-WLs in AISI 52100 and Hybrid 60 steels after hard turning using a multi-scale characterization approach. The results show that the microstructure development of the M-WL in AISI 52100 steel is primarily initiated by grain subdivision process that lead to lamellar grain formation from the initial lath martensite. This is followed by a mechanically assisted triple junction motion, a dynamic recovery mechanism that leads to the formation of NC grains. Furthermore, compared to T-WL, the M-WL exhibited higher compressive residual stresses, lower surface roughness, and improved nanohardness. A similar M-WL mechanism was observed in Hybrid 60 steel, but in this case it was associated with the dissolution of nanoprecipitates. Nevertheless, this led to an increase in nanohardness due to enhanced grain boundary, dislocation, and solid solution strengthening. Hence, the inherent ability of hard turning to generate severe plastic strain below the phase transformation temperature enables the formation of a tailored microstructure. Achieving this within a single, cost-efficient manufacturing step offers a significant advantage for the production of high-performance surfaces.