What is machinability? 
Machinability is a property of a material that is usually defined by four factors. These factors include (1) surface finish and integrity of the machined part, (2) obtained tool life or tool wear, (3) the magnitude of force and power required, and (4) ease of chip (excess material) production. Commonly this is achieved by alloying the material with sulfur or lead.
Leaded steels 
Free machining steels are commonly produced by adding lead to molten steel in order to serve as a solid lubricant during the machining process. The process of free machining increases the machinability of the steels and consequently reduces the costs incurred by steel producers. The lubricating effect of lead is due to the lead particles shearing (breaking) off and building up on the chip-tool interface (contact point). As the tool machines the steel, the built up lead flows under the tool and along the leaded steel. Since lead has a low shear stress, it reduces the friction between the tool and the metal. As a result, the tool slides along the lead particles more efficiently. It is believed that the magnitudes of forces and power required to machine the steel is further reduced by the presence of lead in steel. This occurs due to the lead reducing the shear stress in the primary shear zone (focused area of machining). Leaded steels are less commonly being used today due to its toxicity and environmental concerns. Materials such as bismuth and tin are currently being favored over lead to increase machinability.
Resulfurized and rephosphorized steels 
Adding sulfur to steels results in raised stresses within the primary shear zone of the steel. During machining, the chips that are produced are small and easily break up, increasing tool life and also increasing the ease of chip production. From this, the machinability of the material increases in the same manner as leaded steels; the cutting force and power required to complete the process are reduced.
- Serope Kalpakjian, Steven R. Schmid (2008). Manufacturing Processes For Engineering Materials. Fifth Edition. Pearson Education, pp. 450-451. ISBN 0-13-227271-7.
- Groover, Mikell P. (2007). Fundamentals of Modern Manufacturing. Third Edition. John Wiley & Sons, Inc, pp. 574-576. ISBN 978-0-471-74485-6.