In-situ Observation of Engulfment and Pushing of Small Particl in Molten Metal by Advancing Melt/Solid Interface

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Engulfment or pushing of inclusion particle by advancing melt/solid interface of steel has been investigated in-situ with a confocal scanning laser microscope. When the velocity of advancing interface exceeds a critical value for a particle, solid alumina cluster particle or liquidsilicate/aluminate globular particle in steel melt is found engulfed by the interface which extends in a convex way to form a bump toward the particle. At lower velocities or for smaller radii, the particles in both classes are pushed by the interface which does not form the bump. The shape of the bump is reasonably reproduced by a two-dimensional FEM analysis of heat transfer from the melt across the particle to the interface. Reduced heat transfer caused by the intervening particle is considered responsible for the engulfment.We also show that. the smaller alumina inclusions are not entrapped in the dendritic solid during solidification, based on estimates of the interdendritic solidification rates and empi … More rical data on the engulfment of alumina inclusions by solidifying steel. Engulfment by the interdendritic liquid however is enhanced by the gradients of the concentrations of oxygen and sulfur in the interdendritic liquid during solidification. These elements are surface active on the liquid-steel/alumina interface and m turn introduce very strong capillary forces on the interdendritic alumina inclusions.Phase transformation of delta-ferrite/gamma-austenite in low carbon steels was observed. Dihedral angles at triple points of one high energy delta/delta grain boundary and two incoherent delta/gamma interphase boundaries were measured. From the average value of observed dihedral angles, free energy of gamma/delta incoherent. boundary for a steel with low sulfur content on the dihedral angle and free energy of incoherent, d/g boundary were discussed. At. the same time. the dendritic growth of gamma-phase was observed and analyzed by the constitutional supercooling and instability theories. which show good agreement between the observations and theories. Less