Kinetic Prediction for Isothermal Transformation of Inclusions in a Bearing Steel

Abstract

The mechanism of inclusions transformation during the isothermal heating at 1498 K was elucidated in the current study, that was, the CaO in inclusions diffused to the inclusions surface, reacting with the sulfur to form CaS, and the Al2O3 and MgO receded to the inclusions core, resulting in inclusions of MgO–Al2O3 core with a CaS shell. A comprehensive kinetic model coupling thermodynamics, mass transfer, and changes in inclusions diameters was established. Two diffusion layers for steel elements, named steel layer and CaS layer, and one diffusion region for inclusion components were presumed in the kinetic model. Four model parameters which determine the diffusion process were optimized using the sensitive analysis by comparing simulated results and experimental data. Employing the kinetic model, Time-Diameter-Transformation (TDT) contours were proposed, which revealed the effect of diameter and heating time on the transformation. Calculated results also show a little increase in inclusions diameter during the isothermal heating. Inclusions with an initial diameter less than 5 μm increased by 1.5 μm after the heating. An extreme thickness of the CaS layer in inclusions existed during the heating, which was 0.35 μm for inclusions with a diameter of 10 μm.