Machinability study of hardened steel under cryo-lubrication using coated tools in high-speed turning process

Hard turning is increasingly recognized as a sustainable alternative to conventional grinding for finishing hardened steel components. However, high thermo-mechanical loads generated during dry turning can leads to accelerated tool wear and surface integrity issues. To address these limitations, sustainable practices such as cryogenic cooling (LN₂) and minimum quantity lubrication (MQL) have been explored. Although both techniques have demonstrated promising results independently, their combined use as cryo-lubrication strategy remains an emerging research area, especially for hard turning using coated carbide tools. In this context, this study investigates the machinability of hardened AISI 4340 steel under cryo-lubrication (LN₂+MQL). Machining parameters including cutting speed (V) = 300–350 m/min, feed rate (f) = 0.05–0.1 mm/rev, and depth of cut (doc) = 0.1–0.2 mm were arranged as per Taguchi L₄ orthogonal array to evaluate machinability based on cutting force, tool life, wear mechanisms and chip morphology. Results showed that depth of cut had the highest influence on cutting force and tool life. The optimal setting V = 300 m/min, f = 0.05 mm/rev, doc = 0.1 yielded the lowest cutting force (90.81N) and longest tool life(54.8 min). While, highest combination of feed rate and depth of cut resulted in highest cutting force (195.50N) and shortest tool life (12.7 min). SEM and EDS analysis identified significant abrasion, noticeable coating delamination, and adhesion as the main wear mechanisms. Furthermore, chip morphological analysis revealed closely spaced saw-tooth segments at lower speeds and highly serrated free ends at higher speeds. This research highlights the potential of cryo-lubrication as an effective machining technique for enhancing the performance of hard turning process using carbide tools.