Machinability of carbon and alloy steels is a shear process.
Working the metal (Shearing to create chip) provides heat. The
subsequent sliding of the produced chip on the face of the
cutting tool provides heat as well. Three ways to improve
machinability include Optimizing the chemistry to provide for a
minimum shear strength Adding internally contained lubricants
Adjusting cold work The steels that we are talking about are in
large part composed of the ferrite phase. This is advantageous
to us as machinists, because it has a relatively low shear
strength. Because ferrite is also ductile, it does not cut
cleanly and tends to tear. Grade 1008 or 1010 are prime examples
of how pure ferrite machines. Long stringy, unbroken chips, torn
surface finishes and lots of machine down time to clear “birds
nests” are typical results. Adding carbon up to a point improves
machinability by adding a second harder phase (pearlite) into
the ferrite. The good news is that up to a point, the chip
formation is greatly improved, an