Steels with good machinability are required for the ever-increasing demand for machining efficiency in industry. The manganese sulfide (MnS) inclusions in lead-free resulfurized free-cutting steels act as stress raisers, lowering the shear strength of steel such that the cutting resistance is reduced. Because the morphology of MnS critically determines the machinability of steels, the reactions involving the formation of MnS during solidification need to be carefully assessed, especially with regard to whether they are eutectic or monotectic reactions. In this paper, we established the relationships between alloying elements and solidified microstructures by utilizing both the CALPHAD-type thermodynamic modeling and high-temperature experiments at 1600 °C. The effects of sulfur content on the MnS microstructure were evaluated based on the solidification paths of the ideal iron-manganese-sulfur ternary system as well as the realistic iron-manganese-sulfur-silicon-carbon quinary system. Moreover, we systematically evaluated the effects of various alloying elements, namely hydrogen, boron, carbon, nitrogen, oxygen, aluminum, silicon, phosphor, argon, vanadium, chromium, cobalt, nickel, copper, arsenic, zirconium, niobium, molybdenum, tin, tantalum, tungsten, on the microstructure of MnS. Among those, oxygen is identified as a super-strong monotectic-stabilizer, and the addition of oxygen can drastically enhance the monotectic-type MnS, which is desirable for free-cutting steels. The thermodynamic predictions agree closely with experiments. With the combined efforts of thermodynamic calculations and high-temperature experiments, the morphology, size, and uniformity of MnS inclusions can be optimized for the development of better free-cutting steels.
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