This increases the machinability of the material by breaking the chips into small pieces, thus avoiding entanglement in the machinery.
[2] There are four main types of free machining steel: leaded, resulfurized, rephosphorized and super.
Sulfur forms the compound manganese sulfide, which is soft and acts as a chip-breaking discontinuity.
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.