*Article* **Formation of Complex Inclusions in Gear Steels for Modification of Manganese Sulphide**

**Haseeb Ahmad <sup>1</sup> , Baojun Zhao 2,\*, Sha Lyu 3,\*, Zongze Huang <sup>4</sup> , Yingtie Xu <sup>4</sup> , Sixin Zhao <sup>4</sup> and Xiaodong Ma 1,\***

<sup>1</sup> Sustainable Minerals Institute, The University of Queensland, Brisbane 4072, Australia; h.ahmad@uq.edu.au


**Abstract:** Suitable MnS inclusions in gear steel can significantly improve the steel machinability and reduce the manufacturing costs. Two gear steel samples with different sulphur contents were prepared via aluminium deoxidation followed by calcium treatment. The shape, size, composition and percentage distribution of the inclusions present in the steel samples were analyzed using an electron probe micro-analysis (EPMA) technique. The average diameter of MnS precipitated on an oxide inclusion is less than 5 µm. It was found that the steel with high sulphur content contains a greater number of elongated MnS precipitates than low sulphur steel. Moreover, there are more oxide inclusions such as calcium-aluminates and spinels with a small amount of solid solution of (Ca,Mn)S in low content sulphur steel after calcium treatment, which indicates the modification of solid alumina inclusions into liquid aluminates. The typical inclusions generated in high sulphur steel are sulphide encapsulating oxide inclusions and some core oxides were observed as spinel. The formation mechanisms of complex inclusions with different sulphur and calcium contents are discussed. The results are in good agreement with thermodynamic calculations.

**Keywords:** gear steel; inclusions characteristics; EPMA; sulphide modification
