Characterization of the morphological evolution of MnS inclusions in free-cutting steel during heating

Abstract

The size, morphology and distribution of MnS inclusions in the sulfur bearing free-cutting steel are the key factors to determine the performance of the free-cutting steel. It is thus important to accurately characterize and control the morphology of the MnS inclusions in the steel. The morphology of MnS inclusions are complex during rolling due to their good plasticity. Additionally, the morphology of the MnS inclusions would change during heating due to the decomposition and mass transfer of the Mn and S in the steel matrix. The traditional metallographic for characterizing the MnS inclusions on the cross-sections only reveal the local characteristics of the inclusions and results in misunderstanding of the real morphologies. In this study, the three-dimensional (3D) morphology of the MnS inclusions in the sulfur bearing free-cutting steels is characterized by using the X-ray Micro-CT in the beamline of BL13W1 at Shanghai Synchrotron Radiation Facility (SSRF). The stereology and morphological evolution of the MnS inclusion during heating are characterized and analyzed based the 3D observation. The number of MnS inclusions increased more than twice, the length of the inclusion with sizes less than 9 μm increased from 25.79% to 71.28%. The anisotropy index was reduce from 2.30 to 1.04 due to the spheroidization of the MnS inclusions during heating. The mechanism of the morphological evolution of the MnS inclusions is discussed on basis of the local chemical potential of the MnS caused by the variation of the curvature on the surface of the inclusions.