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.
Highlights
Gears structure a fundamental part in the running of different machines and cars for transmitting power
The worsening of mechanical properties depends on the amount, size, morphology and dispersion of non-metallic inclusions
Adding a small amount of sulphur is known to cause an outstanding improvement in the machinability of gear steels via manganese sulphide formation, which improves the machinability via chip embrittlement and increases tool life
Summary
Gears structure a fundamental part in the running of different machines and cars for transmitting power. Park et al studied the mechanism of complex inclusions formation by using synthetic slag without adding Ca in the melt and predicted the critical sulphur [S] level required to precipitate CaS in the inclusions. To clearly define the role of Ca in modifying the sulphide and oxide inclusions in Al-killed Ca-treated gear steels, it is necessary to obtain the fundamental knowledge to understand the formation mechanism of inclusions including the composition of each phase from the viewpoint of thermodynamics and evolution during the steelmaking process on an industrial scale. In this study we focused on the characterization and formation of complex oxide-sulphide inclusions in different sulphur-level industrial samples with an Al-deoxidation and Ca-treatment process. The formation mechanisms and thermodynamic prediction are discussed for the optimization of manganese sulphide modification
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