Application and Advantages:
Small amounts of bismuth are used to enhance the machinability of metals, especially steel, a use fostered by concern about the toxicity of lead, a traditional free-machining additive for this purpose. Bismuth has been used both as an additive to resulfurized, rephos-phorized, and leaded AISI 1214 steel and as a substitute for lead in AISI 1214 and 1215 steels. As a substitute, it is believed to improve machinability in a manner similar to lead. As an additive, it alloys with lead, one half the lead content, typically 0.1%Bi, and 0.2%Pb. providing more effective internal lubrication than lead alone, further improving machinability.
Bismuth is added individual in amounts to 0.05 - 0.4 wt %, typically 0.2 wt % depending on requirement and presence of impurities. Its advantages over lead also have been attributed to its lower density, which permits more uniform dispersion in the steel matrix. It has no strong effect on mechanical properties but substantial improvements in machinability.
Bismuth like lead thought to act as an internal lubricant, reducing the friction between tool & chip. Bismuth improves machinability because it has the lowest melting point among free-machining additives and the strongest ability to weaken interfacial boundaries. Bismuth exists in the form of particles/ inclusion attached not only to manganese sulfide inclusions, but also to ferrite-pearlite interfaces and grain boundaries, which intensifying its effec-tiveness of machining and provides 20-30% improvement in machinability than lead. Tellurium and selenium can be added with Bismuth, because they also enhance the wetting ability of bismuth in steel.
Bismuth has no effect on transformation or microstructure produced by heat treatment in steels. Bismuth is not, however added to Nickel-Bearing austenitic stainless steels since it seriously impairs hot workability in these materials as shown in figure- 4;