Effect of PMO on Mechanism of Carbide Precipitation in GCr15 Bearing Steel

The Pulse Magneto Oscillation (PMO) solidification homogenization technique has been successfully applied in the continuous casting production of GCr15 bearing steel. To further investigate the effects of PMO technology on the precipitation mechanisms of primary carbides in GCr15 bearing steel, a series of directional solidification experiments were conducted under the influence of PMO. Characterization and analysis of the area proportion, number density, particle-size distribution, elemental distribution, and types of primary carbides in the as-cast bearing steel under various PMO parameters were conducted using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA), and Electron Backscatter Diffraction (EBSD). The findings reveal that the predominant carbide species precipitated in GCr15 bearing steel is M3C type, characterized by a higher Cr content. As the PMO peak current and pulse frequency increase, there is a significant reduction in both the area proportion and number density of primary carbides. Compared to non-PMO conditions, the application of PMO results in a maximum decrease in carbide area proportion by up to 75.6% and a reduction in number density by up to 58.4%, leading to a more dispersed and uniform carbide distribution. Moreover, under the influence of PMO, the local solidification duration of the solution shortens, leading to an increase in the quantity of inclusions such as MnS, which undergo refinement. This facilitates the refinement of primary carbides that utilize inclusions as heterogeneous nucleation sites. Additionally, the reduction in dendritic arm spacing within the solidification structure and the enhancement of solute distribution near the solid–liquid interface, induced by PMO, also create favorable conditions for the reduction in size and quantity of primary carbides. Based on the distinctive characteristics of primary carbides under varying PMO parameters, an innovative dynamic model for the formation of primary carbides during the solidification process of GCr15 bearing steel has been proposed.