Peter the Great St. Petersburg Polytechnic University (St. Petersburg, Russia):

A. A. Kazakov, Dr. Eng., Prof., Head of “Metallurgical Examination” Lab., E-mail: kazakov@thixomet.ru
A. I. Zhitenev, Engineer, “Metallurgical Examination” Lab., E-mail: zhitenev@thixomet.ru
P. A. Ishpaev, Magister, “Metallurgical Examination” Lab.

National Research Center “Kurchatov Institute” — Central Research Institute of Structural Materials “Prometey” (St. Petersburg, Russia):

O. V. Fomina, Dr. Eng., Head of the Research and Production Complex “Organization and Management of R&D, Computer Science and Computer Technology, Quality Management and Copyright Protection”
P. V. Melnikov, Cand. Eng., Head of Department

The thermodynamic criteria previously proposed for interpretation of the origin of δ-ferrite in high-strength corrosion-resistant austenitic steels were enhanced by results obtained after hot physical simulation. Combination of these approaches enabled understanding of the details of the behavior of δ-ferrite during solidification and cooling of the solid steel, and to ensure its hot-cracking resistance in the presence of the optimum amount of δ-ferrite during hot plastic deformation or welding, as well as subsequent austenitization of the structure to minimize or eliminate δ-ferrite in the finished metal. The compositions of steels for studying the behavior of δ-ferrite were selected based on previously developed thermodynamic criteria. It was shown that the δ-ferrite fraction retained in cast metal was determined not only by these criteria characterizing its thermodynamic stability during solidification of liquid and cooling solid steel, but also by its cooling rates in the temperature ranges of δ-ferrite and austenite existence. The mechanism and conditions for δ-ferrite formation of globular, lacy or vermicular morphology depend on the steel composition and its cooling rate which has been revealed when analyzing experimental results of hot physical simulation. All the results obtained are summarized by a regression equation that adequately describes the effect of the steel composition and its cooling rate on the δ-ferrite fraction retained in the finished metal.

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