The effects of heat treatment on the machinability of mild steels

In this study, the effects of microstructure and mechanical properties on the machinability of hot rolled SAE 1050 steel that was annealed and normalised before machining, have been investigated. The machinability has been characterised by measuring the tool life, chip root morphology, cutting forces, surface finish, and tool/chip interface temperature. Here, the optimum machinability, especially from the stand point of tool life, has been determined for hot rolled steel which had minimum impact energy and minimum ductility. By annealing this material, a coarse pearlitic microstructure and a microstructure having 10% spherical cementite was obtained with an increasing ferrite+pearlite banding. This led to an increase in ductility and impact energy, but the decrease in hardness shortened the tool life and worsened the machinability. With normalising heat treatment, on the other hand, the banding disappeared, hardness, ductility and impact energy increased; but the tool life shortened more and more. The maximum built-up edge (BUE) thickness occurred at lower cutting speeds in machining annealed specimens. The minimum surface roughness was observed on the hot rolled specimen at final cutting speeds. The heat treatment operations applied did not bring about a considerable difference in cutting forces. A significant correlation between the machinability and the hardness of specimens could not be determined.