A grade modified with sulphur, lead or bismuth to break chips and improve finish.
Free-machining additives form microscopic stress raisers in the chip, so it fractures into short pieces instead of forming a long tangle. The result is dramatically better chip control, finish and tool life — AISI 12L14 rates 170% on the machinability scale against 78% for 1018.
The cost is real: the same additives reduce corrosion resistance and toughness and are usually detrimental to welding. Free-machining grades are for parts that are machined and then used as machined, not for welded or highly stressed ones.
Specifying a free-machining grade for a welded assembly. The sulphur that improves chip control causes weld porosity and cracking.
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