The AISI/SAE 12xx series represents resulfurized and rephosphorized, plain carbon steels that are also free-machining steels, with both sulfur and phosphorus additions. The phosphorus addition increases the strength of the steel and promotes chip breaking during machining operations. In order to limit the strength of the steel, the carbon content is restricted to a level under 0.15%.
The figure given below shows the microstructure of a typical resulfurized, rephosphorized steel containing manganese sulfides (the gray, oblong particles marked by arrows). The remaining microstructure is ferrite (white etching constituent) and pearlite (dark etching constituent).