About 12L14|The best-machining carbon steel, period. Lead and sulfur additives make chips break into short, clean fragments instead of the rat's nests you get with
1018. Machinability is rated around 190% on the standard scale versus 78% for 1018. You can run it fast, tools last forever, and surface finish is beautiful with minimal effort. Screw machine shops love this stuff. The tradeoffs are real though: it can't be welded (the lead ruins the weld), it rusts significantly faster than 1018, and some automotive OEMs have pulled it from specs due to lead content concerns. Also slightly weaker than 1018. If the part gets machined and doesn't need welding, 12L14 saves serious cycle time and tooling cost. Compare to 1018 (weldable but much harder to machine),
1215 (lead-free alternative, almost as good), and
1144 (stronger with decent machinability).