Effects of shallow cryogenic treatment on surface characteristics and machinability factors in hard turning of AISI 4140 steel

Hardened AISI 4140 steel is widely used in automotive industries due to its high strength, high hardness, and good formability. It can be subjected to cryogenic treatments to increase wear resistance, increase the toughness and decrease the residual stress. Although many studies have been performed related to the machinability of AISI 4140 steel, the number of studies that considered four responses simultaneously are limited. In this study, the effects of cutting speeds, feed rates, and depths of cut on the vibration, motor current, machining noise, and surface roughness were investigated in turning hardened AISI 4140 steel using coated carbide insert under dry cutting conditions. Besides, the attempt was made to predict surface roughness based on machining noise, motor current, and chuck vibration during the turning process. Analysis of variance and response surface methods were carried out to analyze the experimental results and the prediction models were developed. The machining test revealed that the most effective parameter on the surface roughness value was the feed rate with a 96.07% contribution. The depth of cut and the feed rate affected vibration with 57.61% and 26.44% contribution, respectively. The machining noise was dominantly affected by the depth of cut with 76.15% contribution, it was followed by feed rate with 17.67% contribution. However, feed rate and depth of cut significantly affect motor current with 54.53% and 36.64% contribution, respectively. The graphical analysis of the effect of vibration, motor current, and machining noise on the surface roughness was presented. Based on the results, there is a direct relationship between machining noise, vibration, and motor current with surface roughness. As the level of the aforementioned responses increases the quality of the machined surface deteriorates.