Investigation Of Precipitation And Elimination Of Primary Carbide In H13 Steel

H13 steel is the most popular hot work die steel worldwide.Primary carbides with a size of tens of micrometers will precipitate in the interdendritic region during solidification because of the alloying elements segregation.Primary carbide is detrimental for the properties of the materials for the reasons that it consumes alloying elements in the matrix and act as crack source when the mould is working.It is of great significance to investigate the precipitation and elimination mechanism of primary carbide for the purpose of optimizing production process of H13 steel.In this paper,the type and precipitation mechanism of the primary carbides were determined by various experimental methods as well as theoretical analysis.The effect of solidification cooling rate on the precipitation of primary carbide was analyzed experimentally.The confocal laser scanning microscope and automatic particle analysis system Feature were utilized to analyze the evolution of primary carbide during homogenization treatment.The effect of alloying element content on the characteristics of primary carbide and the properties of H13 steel were investigated as well.There exist two kinds of primary carbides in as-cast H13 steel,which is vanadium rich MC type primary carbide and molybdenum rich M2C type primary carbide.Some of the MC type primary carbides contain titanium and nitrogen as well,and the shape of M2C could be stripy or eutectic morphology.Primary carbides are precipitate in the interdendritic region because of alloying elements segregation.The segregation of alloying elements in H13 ingot increase with the increase of solidification cooling rate,which lead to the solid fraction that primary carbide precipitate decrease with the increase of solidification cooling rate.However,solid fraction that the vanadium rich MC type primary carbide precipitate is not less than 0.87,and solid fraction that the molybdenum rich primary carbide precipitate is not less than 0.97.The length and number of the primary carbide are significantly reduced as the solidification cooling rate increases,but the shape of the primary carbide is insensitive to the solidification cooling rate.The average area of M2C type primary carbide decreases remarkably with the increase of solidification cooling rate,while the average area of MC type primary carbide is insensitive to solidification cooling rate.Under the conditions of conventional ingot production process,it is not a reasonable method to avoid the precipitation of primary carbides in H13 steel by adjusting the solidification cooling rate.The in-situ investigation indicates that partial liquid phase appears around primary carbide as long as the H13 steel ingot was heated to temperature above 1150 ? for 1 minute.The position where the liquid phase first appear is on the interface between the M2C type primary carbide and the matrix.After holding at temperature above 1200 ? for 1 minute,the primary carbide in H13 steel ingot will be encompassed entirely by the generated liquid phase.The alloying elements segregation in the interdendritic region as well as the decomposition of primary carbides are responsible for the premature of the liquid phase,because the solidus temperature around primary carbide decrease to a range between 1150 and 1224?.The appearance of the liquid phase is beneficial for the decomposition of primary carbide,for the reason that the diffusion of the alloying element that generated from the decomposition of primary carbide is much more sooner in liquid phase.However,the thermal deformation of the material is unstable because of the appearance of liquid phase,and cavities will be generated in the locations where the liquid phase was located because of the shrinkage of the liquid phase during quenching.The M2C type primary carbide in H13 steel starts to decompose rapidly after holding at 1150 ? for 1 minute.However the MC type primary carbide decompose slowly at temperature above 1200 ?,and it take a long holding time for the completely elimination of MC type primary carbide.Reducing the content of silicon and vanadium in H13 steel is an effective method to avoid the precipitation of MC type primary carbide.The composition optimization of H13 steel is beneficial for the hardness and toughness of the material,but is indesirable for the thermal fatigue resistance of the material.