Influence Mechanisms of Mg on Solidification Microstructure and Primary Carbides of H13 Steel

The mechanisms of Mg in refining the solidification microstructure and primary carbides in H13 steel and the optimal addition range are of great significance for improving the high temperature performance and mechanical properties of H13 steel. Therefore, H13 steel samples with different Mg contents (0 to 0.0041 wt pct) were first smelted using a vacuum induction furnace. The solidification microstructure, segregation behavior of elements, and primary carbides of H13 steel with different Mg contents were characterized using optical microscope (OM), scanning electron microscope (SEM), electron probe micro-analyzer (EPMA), and X-ray diffraction (XRD). Subsequently, the mechanism of Mg in refining solidification microstructure was analyzed based on the thermodynamics and lattice mismatch. The mechanism of Mg in refining primary carbides was further analyzed, and the optimal addition range of Mg was determined. The results show that with the addition of Mg, the proportion of interdendritic gap and the secondary dendrite arm spacing of samples decrease, and the carbides are refined. It is because that the addition of Mg causes the formation of MgAl₂O₄ and MgO inclusions, which promote the precipitation of austenite and ferrite phases and the heterogenous nucleation of carbides in the solidification. In the Mg-0 and Mg-2 (Mg: 0.0019 wt pct) samples, the number densities of carbides are 183.67 and 149.92 mm⁻², respectively. However, more Mg induces more MgAl₂O₄ inclusions, so more carbides precipitate. In the Mg-4 (Mg: 0.0041 wt pct) sample, the number density of carbides increases to 161.17 mm⁻². In addition, with the addition of Mg, the precipitation temperature of VC carbides is reduced, so the precipitation and growth of the carbide are inhibited.