Free machining steel is
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
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 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 is decreased;
impact resistance is reduced;
copper-based
brazed joints suffer from embrittlement with
bismuth free machining grades;
shrink fit
Shrink may refer to:
Common meanings
* Miniaturization
*Shrink, a slang term for:
** a psychiatrist
** a psychoanalyst
** a psychologist
Arts, entertainment, and media
* ''Shrink'' (album), album by German indie rock/electronica group The Notw ...
s 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,
selenium, 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, cobalt ...
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 formula ...
, lead, bismuth,
selenium,
tellurium, or
phosphorus added. Sulfur forms the compound
manganese sulfide, 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
In single point cutting of metals, a built up edge (BUE) is an accumulation of material against the rake face, that seizes to the tool tip, separating it from the chip.The Open University (UK), 2001. T881 Manufacture Materials Design: Block 2: Cut ...
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