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.
A laboratory study was carried out to better understand the factors that contribute to the formation of complex inclusions, as inclusions play an important role during steel production; if not properly managed, inclusions can cause nozzle clogging during continuous casting and also damage the steel’s mechanical properties and machineability. To determine the chemical composition of inclusions that are less detrimental to the machineability of Al-deoxidized and Ca-treated gear steels, thermodynamic calculations and automated scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), as well as electron probe micro-X-ray analysis (EPMA) with the wavelength dispersive spectroscopy (WDS) technique, were utilized. The findings demonstrated that the morphology of inclusions changed from irregular to a more spherical type and the composition also changed to dual oxy-sulfides from pure oxides and sulfides as the Ca concentration in the steel was increased up to 36 ppm. The amount of Pure MnS sulfides also reduced significantly after Ca treatment. The ternary phase diagram and stability diagram for the inclusions revealed that 15–25 ppm Ca is the optimal range for the modification of both oxides and sulfides into the desired morphology and composition under the stipulation that the concentration of O in the steel is maintained at or below 50 ppm.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.