Effect of Cooling Rate on the Solidification Microstructure and Characteristics of Primary Carbides in H13 Steel

The microstructure, alloying elements segregation and characteristics of primary carbides in AISI H13 steel that solidified at different cooling rate were investigated by optical microscope (OM), field emission scanning electron microscope (FE-SEM), electro-probe microanalyzer (EPMA) and automated inclusion analyzer ASPEX. The microstructure of H13 steel samples become more refined with increased cooling rate. The equation of relationship between cooling rate (Rc) and secondary dendrite arm spacing (lambda) for H13 steel could be expressed as lambda = 175.4R(C)(-0.322). Primary carbides are located in interdendritic region, where existed obvious Cr, Mo, V and C segregation. Higher cooling rate promoted higher alloying elements segregation and facilitated earlier precipitation of primary carbides during solidification process. The number, size, amount and mean area of primary carbides decreased significantly with the increased cooling rate, however the shape of the primary carbides were insensitive to cooling rate. Thermodynamic calculation indicated that V-rich primary carbides precipitated at solid fraction larger than 0.94, Mo-rich primary carbides precipitated at solid fraction larger than 0.99 in the cooling rate range investigated. Lower cooling rate suppressed alloying elements segregation, but the precipitation of primary carbides could not be avoided in the cooling rate range.