AISI/SAE 1108 is a resulfurized, low-carbon free-machining steel: carbon stays low at 0.08–0.13%, while sulfur is deliberately raised to 0.08–0.13% — several times what a standard carbon steel carries. That sulfur combines with manganese during solidification to form manganese sulfide (MnS) inclusions distributed through the microstructure. These inclusions act as internal chip-breakers and stress concentrators, so chips fracture short instead of tearing and dragging, and they also provide a degree of internal lubrication at the tool-chip interface. The result is a noticeably easier-cutting steel than a plain low-carbon grade with the same carbon content.

Because its base carbon content is low, 1108 isn't a high-strength grade — it's chosen specifically for its machinability on high-volume screw machine and CNC lathe work, where cycle time, tool life, and surface finish matter more than mechanical properties. Typical uses include fasteners, fittings, small shafts, and other turned or milled parts produced in large quantities where the free-cutting behavior directly translates into lower machining cost. The tradeoff is reduced transverse ductility and weldability compared with a non-resulfurized steel, since the MnS inclusions that help machining also act as planes of weakness across the rolling direction.

Plain low-carbon steel is notoriously gummy to machine: it's soft and ductile, so instead of shearing cleanly it tends to smear across the tool face, build up on the cutting edge, and throw long, stringy chips that tangle in the machine and degrade surface finish. 1108's added sulfur addresses that problem directly. The manganese sulfide inclusions it forms break up the shear plane as the chip forms, so material separates more predictably and chips curl and snap off in short segments rather than stringing out.

That same inclusion structure reduces the tendency for material to weld onto the cutting edge, which is one of the biggest contributors to poor finish and edge buildup on soft, low-carbon steels. The practical result is a steel that cuts noticeably faster and cleaner than its plain-carbon equivalent, tolerates higher feed rates, and leaves a better surface finish with less babying of tool geometry.

Because the base material is still soft, cutting forces stay low and tool wear is generally light — the main things to watch for are chip control at very light depths of cut and making sure feed rates stay high enough to keep the tool cutting into fresh material rather than rubbing, since rubbing on any low-carbon steel can still cause local smearing even with sulfur's help.