The effects of vanadium and aluminum grain refining treatment on the machinability of hot rolled engineering plain carbon and low alloy 4140 steels were investigated using the critical tool wear method during turning of several bars. The flank wear of the machining tool was accurately measured after each machined bar using an SEM. The microstructure and non-metallic inclusions in steel bars and machining chips were characterized using micro-hardness measurements, optical microscopy, and automated SEM/EDX inclusion analysis. High resolution transmission electron microscopy (TEM) was used to characterize MnS/metal matrix boundary in severely deformed chips. Worn machining tools were SEM/EDX analyzed to characterize deposits which were located on the tool surface. The investigation showed that aluminum treated steels exhibit better machinability than vanadium treated steels in both plain carbon and 4140 steel grades. The combined effects of hardness, microstructure and precipitates on the steel machinability are presented and discussed.