SPA TSNIITMASH (Moscow, Russia)

E. Yu. Kolpishon, Dr. Eng., Prof., Chief Researcher, kolpishon@bk.ru

Peter the Great St. Petersburg Polytechnic University (St. Petersburg, Russia)
A. S. Fedorov, Cand. Eng., Researcher, fedorov_as@spbstu.ru
S. V. Ryaboshuk, Senior Lecturer, ryaboshuk.sv@xmail.ru
P. V. Kovalev, Cand. Eng., Associate Prof., kovalev_pv@spbstu.ru
G. E. Kodzhaspirov, Dr. Eng., Prof.

In the present work, a rational level of niobium alloying for high-chromium steels is substantiated from the standpoint of technical and economic efficiency. Based on the performed calculations and thermodynamic modeling, it is shown that effective dispersion strengthening and grain growth inhibition are ensured only by fine carbonitrides smaller than 100 nm, which are formed at temperatures below the solidus. It has been established that when the niobium content exceeds 0.08 wt.%, the temperature at which carbonitride formation begins rises above the solidification completion temperature, leading to the formation of coarse eutectic particles already during crystallization. Such non-metallic inclusions do not contribute to strengthening, binding nitrogen, and reducing the effectiveness of alloying. It has been experimentally confirmed that, despite the total carbonitride content corresponding to the calculated levels, a significant portion of them is represented by coarse inclusions that reduce the impact toughness of the steel, while the strength properties meet and even exceed the standard requirements. A physicochemical criterion for selecting the upper limit of niobium content is proposed – the transition from carbonitride formation in the solid state to their formation during crystallization. It is shown that the choice of niobium content should take into account not only the required volume fraction of carbonitrides but also the conditions of their formation, as well as the distribution of nitrogen between phases. The results of this work can be used in the design of high-chromium steel compositions with minimal consumption of expensive niobium while maintaining the required set of properties.

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