Effect of Bismuth on Sulfide in High Sulfur Free-cutting Steel

In order to investigate the effect of bismuth content on the morphology of sulfide in free-cutting steel, different pure bismuth particles were added to SAE1215 high sulfur free-cutting steel to modify the sulfide. The samples were observed and analyzed by metallographic microscope and scanning electron microscope. It was found that the size of sulfide decreased with the increase of bismuth content. Bismuth exists in steel in three forms: 1) Bismuth exists alone in steel matrix; 2) Bismuth adheres to sulfide; 3) Bismuth encapsulates sulfide. Sulfide exists in the form of MnS in steel with two morphologies: spherical MnS and rod/dendritic MnS. The sulfide morphology changes from rod or dendritic to spherical or ellipsoid with the increase of bismuth content. (paper)