Metallography of telluride on free cutting steel

The results of investigation and analysis of telluride and sulfides in free cutting steel containing tellurium and sulfur can be summerized as follows.No tellurium is found in steel matrix. All the tellurium exists in solid solution in manganese sulfide. The extra amount of tellurium in excess of the solubility in manganese sulfide exists as MnTe in steel. The tellurium content in manganese sulfide is 2.5 wt% on average, and the sulfur content in MnTe is about 3 wt %. In annealed steel, the content of iron solutionized in MnS (steel containing 1–2 % Mn) and in MnTe are both about 2 wt %. MnTe usually forms complex inclusions with MnS. The greater the Te/S, the higher the MnTe content in the complex inclusions. There is no single phase MnTe in steel with Te/S as high as 0.84. When steel is hot worked, MnTe deforms more easily than MnS.MnTe appears light grey in a bright field, and opaque in a dark field. In cast steel it is anisotropic and in forged steel it is isotropic under polarized light. In cold drawn steel, most of the MnS is anisotropic and remained so after annealing at 1100°C. If Te/S in steel is too high, the MnTe formed is prone to stretch in the working direction during press work. Hence, in so far as controlling the shape of the sulfide with the aid of tellurium is concerned, a Te/S of about 0.1 is appropriate. When the MnTe and MnS in steel are separated electrolytely in the four different solutions used in this work, the equilibrium potential of MnS is relatively positive, and that of MnTe is relatively negative in comparison with iron. When an organic solution (No. 3) is used, the MnS in steel can be almost completely and MnTe partly separated.