Lead insolubility dispersion
Utilizes the insoluble properties of lead to create fine chip-breaker particles in free-machining steels to improve machinability.
Lead is practically insoluble in steel. During solidification, it forms fine, punctiform inclusions in the structure. These particles act as predetermined breaking points and break the chip short during machining. That's the whole trick with free-cutting steel. When welding, you have to be careful: the welding heat can melt the lead and cause cracks (hot cracks) in the heat-affected zone. So you buy clean machining with poorer weldability. If you have to weld such steel, keep the heat input low and weld as soft as possible. However, the material is unsuitable for load-bearing structures, so you have to fall back on other free-cutting steels.