31 Jan Free Cutting Steels
Free cutting steels are non alloyed steels with relatively high Manganese (Mn) and Sulphur (S) content. MnS which originates in the steel structure and is consisted by Manganese (Mn) and Sulphur (S) supplies the formation of small and brittle chips during machining and allows convenience at mass production by high speed milling machines (with one or multiple shafts). The improvement of machining is achived by both the addition of Manganese (Mn), Sulphur (S) and other alloying elements in the steel. Lead is one of these elements which does not dissolve in Iron (Fe) phase and generally exists around MnS inclusions. The cutting speed can be increased by adding 0.15-0.35 % Lead(Pb) in the structure of this steel group. This feature provides a slippery structure by means of sliding of the chips on the tool and consequently extending the tool life. Sometimes Tellurium(Te), Bismuth(Bi) and Selenium(Se) can be used for the same purpose instead of Lead(Pb). Due to higher content of Phosphorus(P) according to other steel grades provides a better surface quality and brittleness of the chips.
Free cutting steels are classified as follows according to the heat treatment applied ;
- Free Cutting Steels without heat treatment
- Heat treated Free Cutting Steels
- Quenched and tempered Free Cutting Steels
- Case Hardening Free Cutting Steels
- Free Cutting Steels are generally used in bright condition due to required tolerances on the diameter.
- The steels called semi-free cutting steels are containing 0,12 % Sulphur(S) and low carbon display good processing properties and used in hot and cold forging and punching parts (example:nuts).
- 9SMn28 involving high Sulphur is successfully being used in hot forging applications.
ADVANTAGES OF FREE CUTTING STEELS
- Higher cutting speed can be applied. This concept is described as shorter operation time.
- Tool life increases according to the increase in wear life of the tool. Due to this effect tool cost and time spent for setting new tools decreases.
- As a result of inrease in surface quality extra operations like grinding can be easily eliminated.
- Required presicion and changability of parts at mass production gets better due to the possibility of working with closer tolerances.
- Lower cutting forces are used and required energy decreases.
- Chip flow over the material surface decreases and no need to maintain superior precautions and stocking of the chips.