AISI/SAE 1140 is a medium-carbon resulfurized free-machining steel, combining a substantial 0.37–0.44% carbon content with sulfur raised to 0.08–0.13% specifically to improve machinability. That's a different balance than the low-carbon free-cutting grades like 1108 or 1112: 1140 keeps enough carbon to deliver real strength and a meaningful response to heat treatment, while the added sulfur still forms manganese sulfide inclusions that act as internal chip-breakers during cutting. The result is a steel that machines noticeably better than a plain medium-carbon grade like 1045 while retaining comparable strength potential.

That combination of strength and machinability makes 1140 a popular choice for shafts, studs, bolts, gears, and other medium-strength parts that are produced in volume on CNC lathes and screw machines, where the free-cutting behavior meaningfully reduces cycle time and tool wear on long production runs. As with other resulfurized grades, the sulfur addition trades off some transverse ductility and weldability, so 1140 is generally specified for machined components rather than parts requiring extensive welding or fabrication.

1140 gives a good look at how sulfur addition changes the machining picture on a medium-carbon steel. Plain 1040-series steel at this carbon level already cuts reasonably well, but the manganese sulfide inclusions from 1140's sulfur content give it a real edge: chips break more predictably, cutting forces drop slightly, and the risk of built-up edge on the tool is reduced because the inclusions interrupt the sliding contact between chip and rake face.

Because the carbon content is still meaningfully higher than in the low-carbon free-cutting grades, 1140 generates more cutting force and heat than something like 1108 or 1112, and it retains some tendency to work-harden if the tool is allowed to rub rather than cut. The sulfur content here is more modest than in the heavily resulfurized screw-stock grades, so the machinability boost is real but not extreme — think of 1140 as a stronger steel that machines noticeably easier than its plain-carbon equivalent, rather than a dedicated ultra-free-machining grade.

In practice, 1140 responds well to standard coated carbide grades and conventional positive-rake geometries. Keeping feeds high enough to avoid rubbing, using adequate coolant to manage heat, and matching insert grade to the supply condition (as-rolled versus heat-treated) will get consistent tool life and finish across typical production runs.