Mirzayev D.A., Mirzoev A.A., Buldashev I.V., Okishev K.Yy. Metastable equilibrium of tetragonal bainitic ferrite and austenite in carbide-free bainitic steels

Abstract

Problem Statement: It is a known fact that steels alloyed with aluminium and silicon may see carbide-free bainite form during the process of supercooled austenite decomposition. In recent years silicon steels with “carbide-free bainite” have been used in pipe production. This followed Bhadeshia’s discovery of such steels’ potential for increased structural strength, which was reflected in a number of his papers. The main feature of such steels is the transformation of austenite into ferrite which has a tetragonal rather than a cubic lattice. With the help of first-principles modelling, Bhadeshia came up with a diagramme of tetragonal ferrite (α’) and austenite (γ) equilibrium. It turned out that the equilibrium concentration of carbon in ferrite is times higher than in cubic lattice ferrite, which would explain higher strength. However, the above calculation has some major drawbacks and is rather of qualitative nature. Objectives, Originality: This paper provides the first thermodynamical analysis of the tetragonal ferrite and austenite equilibrium in Fe-C alloys with no carbides allowed during the bainitic transformation stage. Methods Applied: The analysis is based on Chipman’s model of regular interstitial solid solution. The chemical potentials were calculated by adding the corresponding expressions for α-phase with cubic lattice and “tetragonal” additives under the Zener-Khachaturian theory. With the help of the condition of equal chemical potentials of the components of the two phases the authors were able to calculate the borderline concentrations of carbon in α- and γ-phases. Findings: It was established that the calculated concentration of carbon in tetragonal α-phase is 40 to 60 times higher than in the case of common cubic lattice ferrite which is in equilibrium with the γ-phase. Practical Relevance: These findings support Bhadeshia’s qualitative conclusions and open up possibilities for designing high-strength carbide-free bainitic steels.

Keywords

Tetragonality, bainitic ferrite, chemical equilibrium, the theory of Zener-Khachaturian.

Dzhalal A. Mirzayev – D.Sc. (Physics and Mathematics), Professor

South Ural State University, Chelyabinsk, Russian Federation. Phone: +7-312-65-47-13. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.. ORCID: http://orcid.org/0000-0002-4696-8258

Aleksandr A. Mirzoev – D.Sc. (Physics and Mathematics), Professor

Ivan V. Buldashev – Postgraduate Student

South Ural State University, Chelyabinsk, Russian Federation. Phone: +7-312-65-47-13. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Konstantin Yy. Okishev – D.Sc. (Physics and Mathematics), Professor

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