Machinability advantages come from percentages carbon content and additives in the steel. The Grades of plain carbon steel is identified by a 4-digit number. The first two digits identify whether sulfur or phosphorus has been added to improve machinability. They can also represent other elements that may have been added to improve heat treat response or other properties. the second two digits are carbon content multiplied 100%. As an example 1010 Steel is plain carbon steel and has 0.10% carbon content in the steel. A1212 steel is grade that has been resulfurized and rephosphorized to improve machinability and has 0.12% carbon content in the material. A 4140 steel contains Ni, Cr and Mo and has 0.40% carbon in the material. When lead (Sn) is added to steel, the grade designated has an "L" letter between the first and second set of digit that will show us that it is lead-alloy steel. As an example; 12L14 steel has been resulfurized and rephosphorized and additions added lead. The carbon content in the steel is 0.14%. Manganese, sulfides and lead make steel easily machine which is called free machining.

WORK-PIECE MATERIAL CONDITIONS

The physical properties and the conditions of the work-piece material have direct effect on its machinability. The various characteristics and conditions described as conditions of the work-piece material, individually and in combinations, directly effect and determine machinability. The following factors determine the condition of the work-piece material: microstructure, grain size, heat treatment, chemical composition, fabrication, hardness, yield strength and tensile strength.

Microstructure: The microstructure of a metal refers to its crystal or grain structure as shown through examination of etched and polished surfaces under a microscope. Metals whose microstructures are similar have like machining properties. But there can be variations in the microstructure of the same work-piece that will affect machinability.

Grain Size: Grain size and structure of a metal serve as general indicators of its machinability. A metal with small undistorted grains tends to cut easily and finish easily. Such a metal is ductile, but it is also "gummy." Metals of an intermediate grain size represent a compromise that permits both cutting and finishing machinability. Hardness of a metal must be correlated with grain size and it is generally used as an indicator of machinability.

Heat Treatment: To provide desired properties in metals, they are sometimes put through a series of heating and cooling operations when in the solid state. A material may be treated to reduce brittleness, remove stress, to obtain ductility or toughness, to increase strength, to obtain a definite microstructure, to change hardness or to make other changes that affect machinability.