Study On Forming Mechanism And Morphological Characteristics Of Carbides During The Solidification Of High Chromium Cast Irons

By means of altering chemical compositions and technology effects of nucleating and growing thermodynamic and kinetic conditions on morphology and distribution of carbides in high chromium white cast irons have been studied, expecting to discover the freezing mechanism and morphological characteristics of carbides, aiming to establish the theory foundations for the high chromium white cast irons with better mechanical performance. The results have been shown that:Chemical compositions have significant effects on the carbide morphology. The effects of silicon on carbides morphology have duality: on the one hand, silicon enhances the activity of carbon and accelerate its diffusion. Thus, the primary carbides morphology deteriorates with the increase of the silicon content; on the other hand, the carbon atoms enrich in micro-zone due to the exclusion between silicon and carbon atoms, which causes the increase of the nucleation rate of eutectic carbides. However, with the temperature reduction and the increase of silicon amount around eutectic carbides, the diffusion of carbon and silicon to the growing interface of carbides is restricted, and the growth of carbides is hindered correspondingly, thereby refining eutectic carbides; vanadium and titanium Micro-alloying can increase the heterogeneous nucleation cores, and the solvent titanium holds down the carbon diffusion, thereby hindering the growth of carbides, which is favorable to mechanical properties. When the amount of chromium (carbon) keeps constant, as the amount of carbon (chromium) increases, the amount of carbides increases whereas the impact toughness decreases, especially the increase of carbon amount.During the solidification of hypereutectic high chromium white irons, the nucleation and growth mechanism of carbides are changed by varying the melting and cooling conditions, aiming to improve its morphology. The superheating to 1600℃ can purify the molten metal and promote thermodynamic supercooling, leading to the refinement of carbides; When the cooling rate become larger, the nucleation rate increases, however, it is difficult for Fe, C and Cr atoms to diffuse to the growing interface of carbides, thereby restraining the growth of carbides and at the same time the whole microstructure becomes more uniform.Modification can effectively improve the carbide morphology of hypereutectic high chromium cast irons. The addition of magnesium and aluminum compound modification can promote to form a rich magnesium adsorbed film at the carbides growing interface. The film can effectively prevent the Fe, C and Cr atoms diffusing to the interface, which restrains the growth of carbides, and the carbides become fine, spheroidized and dispersed. The macrohardness can reach HRC 61 and α_k increases to 14.6 J/cm~2.