Computer Simulation and Experimental Study of Isothermal Bainitic Transformation in Alloy Steels
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COMPUTER SIMULATION AND EXPERIMENTAL STUDY OF ISOTHERMAL BAINITIC TRANSFORMATION IN ALLOY STEELS Yu. V. Yudin,1 A. A. Kuklina,1 M. V. Maisuradze,1 and P. D. Lebedev1, 2 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 60 – 67, July, 2020.
The main factors responsible for the kinetics of isothermal bainitic transformation in alloy steels are determined using computer simulation of the solid-state phase transformation. The effect of the initial configuration of nuclei of the new phase in the volume of the metal on the parameters of the process of bainitic transformation is determined. Comparative analysis of the computed kinetics of phase transformation at different rates of nucleation of the new phase and of the experimentally observed bainite kinetics in steels 25G2S2N2MA, 300M and 50KhMFA is performed.
Key words: steel, bainite, kinetics, transformation, isothermal hold, computer simulation, KJMA equation, Austin–Rickett equation.
for years to obtain convincing enough evidence of both concepts. The theory of a shear nature of bainitic transformation has been put forward by A. Hultgren in 1926 for explaining to the “troostite needles” observed by him. After the introduction of the term “troostomartensite”, bainite started to be associated with martensitic microstructure. The theory suggested in [3] described bainitic transformation as growth of supersaturated plates of a-phase and subsequent squeezing of carbon into the surrounding untransformed volume of austenite. Later on, this idea was developed and is still applied [4 – 8]. In 1930, J. M. Robertson [9] suggested an idea of closeness of between bainite and pearlite. The difference was in the fact that pearlite starts to form with nucleation of cementite plates, while bainite starts to form with appearance of ferrite plates. This theory found development in the concept of diffusion-controlled bainitic transformation [10 – 14]. The classification of bainite into “upper” and “lower” one in accordance with its morphology has made it possible to combine the two different concepts of bainite formation, i.e., it is assumed that the upper bainite forms chiefly due to a diffusion process, while the lower bainite forms due a shear rearrangement of the crystal lattice [15]. However, other varieties of bainite (for example, granular, acicular, or columnar one) have been reported [16], and the conditions of their formation have not been studied exhaustively.
INTRODUCTION The mechanisms of formation of bainite in steels have been argued about for quite a long time, virtually from the discovery of bainite by Davenport and Bain in 1930 [1]. However, despite the dispute, some distinctive features of bainitic transformation in steels do not cause doubt. This concerns the discrimination of the bainite morphology into “upper” and “lower” one, the temperature of the start of bainitic transformation Bs , and the fact that bainitic transformation does not provide total disappearance of the initial austenite phase [2]. There are two basic
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