Study of microhardness variations in hole making processes on 4340 steel

In this research, microhardness variations of subsurface in hole making on a AISI4340 steel workpiece was studied experimentally. For this purpose, four hole making methods were used including; helical milling, profile milling, drilling with and without predrilling. The design of experiments utilized full factorial method in which two main cutting parameters including cutting speed (Vc) and feed rate (fz) were changed in three levels. Nine experiments were performed for each process and Hardness variations of substrate layer along the hole radial and axial distances were investigated (216 hardness measurements points). Results showed that the measured hardness in all of the experiments were higher than bulk material hardness, regardless of cutting conditions and the maximum hardness value was found in the upper levels of cutting parameters of traditional drilling method (729 Vickers). In addition, due to workpiece temperature and work hardening increasing with prolongation of the process time, the maximum hardness value was obtained on the exit surface of hole in all processes. Also, least microhardness variations was found when using traditional drilling with predrill which represents superiority of non-continues, multistage hole making processes and conventional drilling using predrill in creation of holes with more uniform properties.