CALPHAD-assisted morphology control of manganese sulfide inclusions in free-cutting steels

  • 林 士桂

Student thesis: Master's Thesis


It is necessary to develop new free-cutting steels with good machinability in order to meet the ever-increasing demand for machining efficiency in industry The addition of sulfur (S) can improve the machinability of steel by forming manganese sulfide (MnS) inclusions These inclusions lower the shear strength of steel such that the cutting resistance is reduced with MnS being the stress raiser Since the morphology and the uniformity of the MnS inclusions critically determine the machinability of steels the reactions involving MnS formation during solidification need to be carefully assessed especially with regard to whether they are eutectic or monotectic reactions Globular MnS is formed from the MnS-rich liquid (L2) through a monotectic reaction which provides a greater benefit for machining However the temperature difference (“?T”) between these two reactions is so close that doping elements may alter the solidification path of the liquid steel and may result in a eutectic reaction In this study we focus on establishing the relationships between alloying elements and solidified microstructures by utilizing both the calculation of phase diagram (CALPHAD) method and high-temperature experiments with an atmosphere-controlled high-frequency induction furnace Based on a commercial thermodynamic database TCFE7 we simulated the solidification path of the pure Fe-Mn-S ternary system and some alternative paths with alloying elements carbon (C) and silicon (Si) to further evaluate the effects of S content on the microstructure of MnS Moreover we also systematically evaluated the effects of various alloying elements on the microstructure of MnS based on their effects on changing “?T” These alloying elements can be categorized into three groups: C Si Nb Cr V and Mo are eutectic-stabilizers O Cu and Al are monotectic-stabilizers and Ta Zr Ni N P W H Ar B and Co are inert dopants which do not noticeably change the microstructure of MnS Among these oxygen (O) is identified as a super-strong monotectic-stabilizer and the addition of oxygen addition can drastically enhance the monotectic-type MnS which is desirable for free-cutting steels The thermodynamic predictions agree closely with the results of high-temperature 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
Date of Award2016 Aug 31
Original languageEnglish
SupervisorShih-kang Lin (Supervisor)

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