Evaluation of machined surface of the hardened AISI 4340 steel through roughness and residual stress parameters in turning and grinding

The development of new cutting tool materials favors the hardened steel turning process, which has in some cases replaced the grinding process. Thus, this study evaluates the machined surface of hardened AISI 4340 steel via turning and grinding operations. The turning process evaluated the effects of the cutting tools (material, grade, and coating) and the process parameters (cutting speed and cooling condition). In the grinding process, the effects of the plunge rates and volumes of removed material Z_w were evaluated. The machined surface was evaluated by amplitude roughness (Ra, Rt, Rp, and Rz), functional (Kp, Rk, Rpk, and Rvk), and statistical parameters (Rsk and Rku) and by the type and magnitude of induced residual stress (compression and tensile). In general, the turning process displayed the best results in the roughness parameters by promoting a better surface finish, mainly when the parts were machined at a cutting speed of 300 m/min. The increased cutting speed provided better surface integrity. The ceramic tool CC6050 presented the best performance, followed by the polycrystalline boron nitride (PCBN) and ceramic CC650 tools under cooling and dry cutting conditions. With respect to the residual stress, the grinding process induced compressive residual stresses in all tested conditions. On the other hand, turning-induced tensile stresses when using ceramic cutting tools (class CC650 and CC6050) and compressive stresses when using a PCBN cutting tool (class CB7115) were examined. In general, the use of cutting fluid in turning process did not provide a better performance in relation to the dry cutting condition. Based on the obtained results, the turning of hardened materials is a viable alternative to the grinding process.