Effect of tin microalloying on the microstructure of low-carbon free-machining steels

Sn is a well-known grain boundary segregation element that improves the machinability of steel. Sn has been considered as a replacement for Pb in super-free-machining steels. The effect of Sn on the microstructure of the Fe-0.05C-0.03Si-1.28Mn-0.36S-0.05P base composition containing 0.0002 (without addition), 0.062, 0.12, and 0.18 Sn (wt.%) low-carbon free-machining steels was investigated though thermodynamic calculations, optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and high-temperature laser scanning confocal microscopy. The microstructure of the free-machining steels was composed of α-ferrite, pearlite, and manganese sulfide. Sn significantly decreased the pearlite content of the steels. Most of the Sn was dissolved in the matrix, and the remainder was dissolved in manganese sulfide. No FeSn intermetallic compound precipitation was observed through transmission electron microscopy, but a significant strengthening effect was observed in α-ferrite. Sn had little effect on the solidification behavior or sulfide precipitation behavior of the steels when its content was lower than 0.2 wt.%. Similar to Al and Si, Sn is a ferrite-stabilizing element that expands both the δ-ferrite and α-ferrite phase regions, promotes α-ferrite formation, and inhibits carbide precipitation. Sn segregates at the interface, decreasing the interfacial energy and promoting the Widmanstätten ferrite phase transformation. A much lower cooling rate than that of Sn-free free-machining steels should be adopted to restrain the formation of Widmanstätten ferrite after hot rolling. Sn addition greatly improved the machinability of the experimental steels.