Study of Free Cutting Austenitic Stainless Steel Containing Sulfur, Rare Earths and Bismuth

Austenitic stainless steels are difficult-to-machine materials. Sulfur, rare earths and bismuth were added to conventional austenitic stainless steel. The machinability and mechanical properties were investigated by vacuum melting, cutting testing as well as microstructure and mechanical properties measuring. The results have indicated that the inclusions are almost composed of MnS. Rare earths are enwrapped in MnS inclusions, and bismuth element is attached to double end of MnS inclusions. The shape of MnS inclusions are mainly spherical and spindle, and this is attributed to metamorphose and spheroidization of rare earth. Sulfur, rare earths and bismuth addition decreases the cutting force and tool wear, and improves the chip shape. An excellent machinability can be obtained for the developed alloy. The mechanical properties of free cutting austenitic stainless steel without lead still exhibit a satisfactory combination of tensile strength and total elongation in spite of the presence of the inclusions. This is because the sulfides and bismuth are all soft phases, and they do not induce formation of cracks during deformation. On the other hand, the spheroidized inclusions can improve not only the machinability, but also the mechanical properties for free cutting austenitic stainless steel.