When added to steel, lead does not go into
solution but exists in a very finely divided state
along the grain boundaries. It greatly assists
machinability as it acts as a lubricant between the
steel and the tool face. Lead is normally added in
amounts between 0.15-0.35% and when combined
with similar amounts of sulphur, optimum
machinability is attained as in such steel as BS 970
230M07 Pb.