Distribution of Bi–MnS Inclusions Existing in 1215MS Steels: Correlation with Thermal Physical Coefficient

This work aims to comprehensively summarize the types and distribution of the isolated Bi particles and MnS inclusions in the Bi-bearing steels, to study the precipitated behavior of sulfide, and to study the effect of Bi particles on grain sizes and the volume thermal expansion coefficients of the steels. Results show that the average percentage of the isolated Bi particles, semi-encapsulation, complete encapsulation, back-bridge, tiny encapsulation, cavity bridge Bi–MnS inclusions in the Bi-bearing steels was 16.89%, 20.49%, 3.44%, 16.89%, 31.63%, 10.66%, respectively. The average equivalent diameters of the grains in the 0 ppm, 130 ppm, 240 ppm, 760 ppm, 1200 ppm and 2300 ppm Bi content steels were 20.88, 20.17, 19.54, 18.13, 13.98 and 13.25 μm, respectively. The precipitated equilibrium mass concentration product of the MnS (w[Mn]·w[S]) was calculated to be 1.07, and the solid fraction fS value was 0.61. At 950 °C, the volume thermal expansion coefficient change (ΔV_E) in 760 ppm Bi content steel was minimum 4 × 10^–10%, while at 461.6 °C the ΔV_E was maximum, 0.09%, which means that Bi affected the volume thermal expansion of the steel strongest in this temperature range.