Macro and Micro Segregations and Prediction of Carbide Equivalent Size in Vacuum Arc Remelting of M50 Steel via Simulations and Experiments

Macrosegregation and coarse primary carbides caused by microsegregation are typical solidification defects during Vacuum Arc Remelting (VAR) which greatly limit the mechanical properties of M50-bearing steel components. A solid-liquid Eulerian two-phase model that describes solidification behaviors at both macro and micro scales is developed to predict macrosegregation of M50 ingots under a complex multi-physics condition. A good match is observed between the simulated results and experimental data. The segregation pattern of carbon is predicted with and without natural convection. Compared with thermal-solutal buoyancy, the Lorentz force is stronger and regarded as the main driving force for macrosegregation formation. The flows generated by thermal-solutal buoyancy and Lorentz force are in opposite directions. The natural convection flowing downward along solidification front leads to more serious positive segregation at the center of ingot. The uniformity of concentration distribution is improved with the increase of ingot diameter. The main reason is that the increase of diameter leads to the decrease of current density, and further decreases the Lorentz force, thus weakening the fluid flow. For the first time, an analytical formula of the equivalent size of primary carbides in M50 steel is established. It is found that the size of carbides in large-diameter industrial VAR-ingot is larger. The present work provides new insights for co-operative controlling the macro and micro segregations and the size of carbides in VAR ingots. The optimal ingot size with a diameter of 280 mm is found.