АНАЛІЗ ЕФЕКТУ МОДИФІКУВАННЯ ІТРІЄМ, ГАФНІЄМ ТА НІОБІЄМ СПЛАВУ ЖС3ДК-ВІ ЗІ ЗНИЖЕНИМ ВМІСТОМ ВУГЛЕЦЮ

Purpose. Theoretical prediction of carbide formation and possibilities of compensating for the lack of carbide strengthening in the ЖС3ДК-ВІ alloy with a reduced carbon content through additional modification with  yttrium, hafnium, and niobium. Research methods. By analyzing the corresponding stoichiometric reaction equations, the minimum conditions for obtaining yttrium and hafnium carbides were determined. Using known computational and analytical techniques, the predicted 100- and 1000- hour long-term strength of the ЖС3ДК-ВІ alloy at different alloying levels without modification and a conditional complexly modified with yttrium, hafnium and niobium variant were determined. Scientific novelty. Previous studies have established that in the structure of samples with a reduced carbon content (0.015...0.020 % C) carbides are practically absent, and the grain boundaries are very thin. When niobium, yttrium and hafnium are introduced, carbides are formed in the form of globular particles, mainly evenly distributed in the volume of the metal. Calculations have established that 0.02014 % carbon is sufficient for the carbide formation reaction of 0.015% yttrium and 0.25 % hafnium. As was previously established, when 0.15...0.25 % hafnium is introduced into the ЖС3ДК-ВІ alloy, all carbides are isolated in the form of globular particles, and are located along the grain boundaries and interdendritic spaces. Thus, since it is known that yttrium carbides are released from the melt and are a substrate for the growth of carbides of the MС type in the form of globular particles, we can conclude that the amount of yttrium and hafnium obtained by calculations may be sufficient for the formation of rounded carbides in the metal ЖС3ДК-ВІ with a carbon content of 0.06…0.11% (according to OST 1.90.126-85). Thus, it can be concluded that with a low carbon content: - yttrium does not participate in carbide reactions, but is spent on local alloying of the phase interface, which slows down diffusion processes, increases the structural stability and heat resistance of the alloy, reduces grain size, and refines the dendritic structure; - hafnium prevents the destruction of grain boundaries by suppressing the release of secondary carbides, and strengthens the γ´- phase. To compensate for the lack of carbide strengthening of the alloy with a carbon content of 0.015…0.020 %, the possibilities of strengthening the γ´- phase by alloying the alloy with hafnium and niobium were studied using computational and analytical methods. Practical value. The calculated increase in the long-term strength of a conventional alloy complexly modified with hafnium, niobium and yttrium up to ~12 % relative to the serial alloy ЖС3ДК-ВІ according to OST 1.90.126-85 has been established. Based on the data obtained, it can be concluded that the addition of hafnium in an amount of 0.25% and niobium – 0.50 % compensates for the lack of carbide strengthening of the ЖС3ДК-ВІ alloy at reduced carbon contents.