Machining performance of hardened AISI 4340 steel using eco-friendly cutting fluids under small quantity lubrication

Cutting fluids significantly influence machinability by reducing cutting forces, tool wear, and surface roughness during metal cutting. This study investigates the turning performance of hardened AISI 4340 steel (40 HRC) under Small Quantity Lubrication (SQL) using environmentally sustainable cutting fluids. Turning experiments were performed with coated carbide inserts by varying cutting speed (130–240 m/min) and feed rate (0.12–0.16 mm/rev) while maintaining a constant depth of cut of 1.5 mm. Dry machining, a rice bran oil-based emulsion, and a shampoo-based surfactant solution were comparatively evaluated. A central composite design based on response surface methodology was employed for experimental design, modeling, and optimization. Machining performance was assessed through chip morphology, chip reduction coefficient, cutting forces, surface roughness, and tool wear. The results demonstrate that both eco-friendly cutting fluids under SQL outperform dry machining by reducing cutting forces and improving surface finish. The shampoo-based surfactant solution (1:15) exhibited superior lubrication, resulting in lower tool wear, improved machining stability, and longer tool life than the rice bran oil-based coolant. Multi-objective optimization identified the optimum conditions at a cutting speed of 130 m/min and a feed rate of 0.12 mm/rev. The findings confirm that the shampoo-based surfactant solution (1:15) was an effective and sustainable alternative to conventional petroleum-based cutting fluids for machining hardened AISI 4340 steel.