Analysis of Tool Wear Mechanisms and Surface Roughness in High Speed Hard Turning of AISI 4340 Steel Using PVD Coated Carbide Inserts

This study investigates surface roughness and tool wear progression during dry hard turning of heat-treated AISI 4340 steel using PVD AlTiN-coated carbide inserts. Experiments were conducted at cutting speeds of 60, 95, 180, and 250 m/min, with a constant feed of 0.1 mm/rev and depth of cut of 0.5 mm over a fixed cutting length of 360 mm. Surface roughness (Ra) was measured after three successive cuts at each speed and analyzed using a general linear model (GLM). The results reveal a non-monotonic response of surface roughness, with Ra decreasing as cutting speed increased up to 180 m/min and then rising at 250 m/min. In contrast, flank wear increased with cutting speed, reaching 63 µm at 60 m/min, 152 µm at 95 m/min, 455 µm at 180 m/min, and 690 µm at 250 m/min. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analyses indicated a transition in wear mechanisms from mild adhesion and oxidation at lower speeds to severe flaking, edge chipping, crater formation, and coating delamination at higher speeds. Although the smoothest surface was obtained at 180 m/min, the pronounced flank wear at this speed highlights a trade-off between surface quality and tool condition, suggesting potentially unstable cutting despite low roughness. In comparison, a moderate cutting speed of 95 m/min provides the best overall balance between acceptable surface finish and controlled tool wear. These findings underscore the importance of understanding non-monotonic roughness behaviour, progressive wear, and coating degradation for selecting optimal cutting parameters and improving machining stability in hard turning applications.