CNIITMASH, Moscow, Russia

E. Yu. Kolpishon, Dr. Eng., Prof., Chief Researcher, e-mail: kolpishon@bk.ru

Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russia

P. V. Kovalev, Cand. Eng., Associate Prof., Deputy Director for Educational Activities, Institute of Mechanical Engineering, Materials, and Transport, e-mail: kovalev_pv@spbstu.ru
S. V. Ryaboshuk, Senior Lecturer

The article presents a comprehensive study of the mechanisms of niobium carbonitride formation in steels and alloys for various applications. Based on thermodynamic analysis and experimental data, three main types of inclusions are characterized in detail: primary (blocky, formed in the liquid metal and at the beginning of crystallization), secondary (eutectic, forming within the liquidus-solidus range during the final stages of solidification), and tertiary (solid-phase, precipitating below the solidus temperature). It is shown that the morphology, size distribution, and phase composition of the carbonitrides are determined by the alloy’s chemical composition, modification technology, and crystallization conditions. It has been established that for structural steels, the optimal niobium content does not exceed 0.4-0.5 %, while large primary and secondary inclusions (>1 μm) have a negative impact on plastic and toughness properties. In contrast, in heat-resistant alloys, eutectic carbonitrides, whose sizes are comparable to the cast grain size, form a structure that effectively impedes grain boundary sliding during creep. The highest efficiency in restricting grain growth is demonstrated by nanometric tertiary carbonitrides, which form during the decomposition of supersaturated solid solutions. A critical parameter determining the influence of dispersed particles on the structure is the ratio of their sizes and interparticle spaces to the grain size, where meeting the condition 5dinc < dgrain ensures effective inhibition of boundary migration. The results of the work allow for the optimization of niobium alloying technologies for the targeted formation of the structure in accordance with the performance requirements for the steel or alloy.

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