Forms of Bi-sulphide in 1215MS steel related to machining and mechanical performance

This work continued to discuss the forms of Bi particles accompanying sulphide, and analysed the effect of Bi on tensile and machining properties in 1215MS free-cutting steel. The machining and tensile tests were carried out based on the as-cast steels. The effect of Bi on the size and hardness of sulphide was investigated. Results show that at 200 ppm Bi or more, the normalised frequency of the large-sized sulphide inclusions with area of above 50 μm2 was maximum, 0.35. In Bi-containing steels, the forms of sulphide inclusions were summarized that: clustering, rodlike, long strip, spindle, ellipsoidal, and spherical shape. Most Bi particles adhering to sulphide inclusions mainly existed in steels. The average hardness of steel matrix and the inclusions in 240 ppm Bi steel were 103 and 116 kgf mm−1, respectively. Bi added to steel increased the ductility and improved the surface finish of the machining steels.