Effect of microstructure and hardness on the machinability of medium-carbon chrome-molybdenum steel

In this report, part of the cooperative work done by the machinability committee of the Japan Society of Precision Engineering regarding the machinability of a low-alloy structural steel for automobiles is explained. Bar products from a commercial batch of medium-carbon chrome-molybdenum steel, JIS SCM440, were processed in three different ways in order to produce the specimens of different microstructures but of an almost identical hardness. The specimens were obtained by normalizing, quenching followed by tempering treatment, and cold drawing following annealing. As-rolled steel bars were used in the tests as a comparison material. Work-pieces were longitudinally turned on an NC lathe with cemented carbide and cermet tools. Surface roughness, cutting force and chip hardness were measured. The effects of sample hardness on built-up edge formation and the surface finish of the work-pieces were analyzed and discussed. The results show that the formation of a built-up edge when machining the steel depends on the microstructure of the work material even if it has the same hardness and the annealed and subsequently cold-drawn work material produced the best surface finish.