Tool Wear Effect on Machinability and Surface Integrity in MQL and Cryogenic Hard Turning of AISI 4340

Hard turning has emerged as a cost-effective and flexible alternative to conventional grinding for machining hardened steels such as AISI 4340. However, its performance is significantly influenced by the choice of cooling and lubrication strategies, as well as the condition of the cutting tool. Inadequate thermal management and tool wear can lead to elevated cutting forces, high interface temperatures, degraded surface quality, and an altered microstructure. This study investigates the machinability performance of AISI 4340 alloy steel (50 HRC) using CVD-coated carbide tools under two distinct cooling/lubrication environments: minimum quantity lubrication (MQL) and cryogenic cooling (LN₂). Experiments were conducted at the beginning and end of tool life with both environments to capture the influence of tool wear on key performance indicators, including cutting force, chip temperature, surface roughness, and microstructural integrity. Results indicate that LN₂ cooling outperformed MQL in mitigating thermal loads and maintaining surface quality, particularly under worn tool conditions. LN₂ reduced cutting forces by up to 37.10%, chip temperature by 56.68%, and surface roughness by 36.95% compared to MQL. Microstructural analysis revealed significantly thinner deformation and white layers under LN₂, suggesting improved subsurface integrity. These findings highlight the potential of LN₂ cooling for enhancing the machinability of hard turning operation and improving overall performance in industrial applications.