Free machining steel is steel that forms small chips when machined. This increases the machinability of the material by breaking the chips into small pieces, thus avoiding entanglement in the machinery. This enables automatic equipment to run without human interaction. Free machining steel with

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

also allow for higher machining rates. Free machining steel costs 15 to 20% more than standard steel, but this higher cost is offset by increased machining speeds, larger cuts, and longer tool life.Degarmo, p. 117. The disadvantages of free machining steel are:

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

is decreased; impact resistance is reduced;

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...

-based brazed joints suffer from embrittlement with

bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...

free machining grades; shrink fits are not as strong.Degarmo, p. 118.

Types

There are four main types of free machining steel: ''leaded'', ''resulfurized'', ''rephosphorized'' and ''super''. Super free-machining steels are alloyed with

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fo ...

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and telluriu ...

, and bismuth.

Mechanics

Free machining steels are

carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, coba ...

s that have

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

, lead, bismuth,

, or

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

added. Sulfur forms the compound

manganese sulfide Manganese(II) sulfide is a chemical compound of manganese and sulfur. It occurs in nature as the mineral alabandite (isometric), rambergite (hexagonal), and recently found browneite (isometric, with sphalerite-type structure, extremely rare, known ...

, which is soft and acts as a chip-breaking discontinuity. It also acts as a

dry lubricant Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium. The two main dry lubricants are graphit ...

to prevent a built up edge on the cutting tool. Lead works in a similar way to sulfur. Bismuth achieves a free machining steel by melting into a thin film of liquid for a fraction of a microsecond to lubricate the cut. Other advantages to bismuth include: more uniformly distributed because of its similar density to iron; more environmentally friendly, as compared to lead; still

weldable The weldability, also known as joinability,. of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others (see Rheological weldability). A material's weldability is used to ...

References

Bibliography

*{{Citation , last = Degarmo , first = E. Paul , last2 = Black , first2 = J T. , last3 = Kohser , first3 = Ronald A. , title = Materials and Processes in Manufacturing , publisher = Wiley , year = 2003 , edition = 9th , isbn = 0-471-65653-4. Steels