Tham khảo tài liệu 'modeling and simulation for material selection and mechanical design part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Figure 41 The presentation of C-curve on simulating TTT diagrams. Scheme. Parameters U and S correspond to Table 7. 1. Phase Transformations of Undercooling Austenite At present many computer models of evolution of structure and phase composition of steels during quenching have been developed. Most of them are based on physical models of phase transformations 64-66 . But physical models cannot describe adequately all kinetic features of undercooled austenite transformations. The computer models based on regression analysis of experimental data can best predict steel phase composition changes during steel cooling. It was introduced directly by Davenport and Bain 67 and the timetemperature-transformation TTT diagram was the predominant tool to describe the isothermal decomposition kinetics of supercooled austenite. In most TTT diagrams general S- or C-curves are used to represent the kinetics of a number of isothermal transformation products ferrite pearlite upper bainite lower bainite and martensite. Conversely many experimental results demonstrate that each type of transformation product has a separate C-curve. To build a mathematical model all TTT diagrams published in Refs. 68-71 were analyzed. The rationalization of the kinetics of isothermal decomposition of austenite permitted the establishment of a metastable product phase diagram of a number of steels of different compositions with 6 of total content of all alloying elements. Copyright 2004 by Marcel Dekker Inc. All Rights Reserved. Most isothermal transformations take place by nucleation at the austenite grain boundaries so the original austenite grain size will affect the isothermal decomposition kinetics of austenite. From the total number of factors characterizing austenite matrix the present day experimental knowledge allows only an approximate examination of the statistically recrystallized proportion and estimation of the size of deformed austenite grains. The grain growth kinetics satisfy the law 73
