Chemical Composition
Chemical composition by itself does not have any influence on machinability. However they influence machinability to the extent they affect the hardness and tensile strength of the material. Under certain machining, the chemical constituents of the work material influence the chemical phenomenon taking place at the tool- chip interface and hence affect the tool life.
Macro inclusions occur in steel unavoidably during the process of manufacture. The inclusions could be hard or soft. Hard inclusions like Al2O3 and SiO2 increase the rate of abrasive wear and reduce tool life. They have no influence on cutting force or surface finish. However, their presence can induce self excited vibrations during cutting.
On the other hand soft inclusions like or manganese sulphide in steel (occurring due to the addition of lead or sulphur to render the steel free machining) apparently smear over the rake face and prevent metal to metal contact between the chip and the tool. Thus soft inclusions improve tool life. In addition, the soft inclusions reduce friction between the chip and tool and reduce cutting force and power. They also improve surface finish by reducing the formation of built up edge.