Bismuth Existing in 1215MS Steel Affecting Grain Size and Thermal Expansion

In the free-cutting field, Bi-bearing steel is widely concerned because it supplies the parts for instruments. This work aims to explore the forms of Bi particles existing in 1215MS steel and to detect the effect of Bi on the grain size and thermal expansion performance of steel by heat treatment and thermal expansion experiments with a thermal expansion instrument and a box type electric resistance furnace. Results show that both co-existence of the Bi-MnS inclusions and the isolated Bi particles merely existed in the austenite grain boundary of steel. The higher Bi content in steel is helpful to refine the austenite grain size of as-cast steel. When the austenite grain sizes were in the range of 0~5 μm, the normalized frequency of grains in 0, 130, 240 and 760 ppm Bi content steels was 0.014~0.042, while the normalized frequency in 1200 and 2300 ppm Bi content steels was 0.45 and 0.23, respectively. With adding Bi to steel, the net elongation curves of the Bi-bearing steels showed the upward convex peaks in cooling process. Liquid Bi inhibited the volume shrinkage of heated steel, showing the abnormal expansion phenomenon of Bi.