Additives
There are a variety of chemicals, both metal and non-metal, that can be added to steel to make it easier to cut. These additives may work by lubricating the tool-chip interface, decreasing the shear strength of the material, or increasing the brittleness of the chip. Historically, sulphur and lead have been the most common additives, but bismuth and tin are increasingly popular for environmental reasons.
Lead can improve the machinability of steel because it acts as an internal lubricant in the cutting zone. Since lead has poor shear strength, it allows the chip to slide more freely past the cutting edge. When it is added in small quantities to steel, it can greatly improve its machinability while not significantly affecting the steel's strength.
Sulphur improves the machinability of steel by forming low shear strength inclusions in the cutting zone. These inclusions are stress risers that weaken the steel, allowing it to deform more easily.
Read more about this topic: Machinability, Steels