Lead is added to steels for improving machinability. Lead is only slightly soluble in liquid or solid steel and, when added, some will be present as a dispersion of discrete but preferably submicroscopic metallic lead inclusions.
In the cut, lead acts as a lubricant between the cutting tool and the workpiece, resulting in longer tool life. This lubricity reduces friction in the secondary shear zone as the chip slides over the face of the cutter. It also helps create more manageable chip configurations by helping the chip to curl and break rather than string out into birds' nests. Lead has a melting point around 621º F—well below steel. As heat builds in the cut, the lead is able to leech out of the steel and lubricate the cutter/part interface. This reduces the heat generated in all three shear zones and leads to longer tool life and the possibility of increased speeds and feeds.
The concentration of lead, sufficient to produce the desired free machining properties, is in the range of 0.15-0.35%. Lead is used alone or in combination with bismuth, tellurium or selenium depending upon the application requirement. Lead can be used in the form of round shots or cored wire. The recovery of lead with shots addition ranges between 60-75% whereas in case of cored wire the recovery is higher i.e. between 75-90%.
Although lead’s boiling point is above normal temperature the metal does have a high vapor pressure and addition will be accomplished by considerable fuming. These vapors are toxic and sufficient venting should be provided.