Prediction of hot flow curves of construction steels by physically-based constitutive equations
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Prediction of hot flow curves of construction steels by physically-based constitutive equations G.Varela-Castro1, J.M. Cabrera1,2 1 Fundació CTM Centre Tecnològic, Av. de les Bases de Manresa 1, 08242-Manresa, Spain. 2 Departament de Ciència del Materials i Enginyeria Metal·lúrgica, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028-Barcelona, Spain. [email protected]. ABSTRACT The development of accurate constitutive equations is important for the success of computer simulations of high temperature forming operations. Often, these simulations must be made on alloys that have not been completely characterized. For that reason physically-based constitutive equations taking the chemical composition into consideration, involving deformation mechanisms and characteristic properties of the material are necessary. The influence that exerts the solute elements to an alloy on the mechanisms of diffusion on deformation processes at high temperatures is not an easy subject and the available information in literature is scarce. This study examines that influence working on the basis of eight structural plain carbon steels with the chemical composition ranging between 0.15-0.45%C, 0.2-0.4%Si and 0.61.6%Mn produced by Electro-Slag Remelting ESR process and tested by isothermal uniaxial compression technique. The studied deformation conditions include strain rates ranging between 5·10-4 to 1·10-1 s-1 and temperatures between 0.6-0.75Tm, with Tm the melting temperature. A constitutive expression for the hot working behavior is proposed, it includes the variation of the diffusion parameters with the chemical composition. To such aim the effect of the chemical composition of the alloy on the pre-exponential factor D0 of the gamma iron selfdiffusion coefficient Dsd is included. Finally, a comparison of the experimental and predicted results shows the good agreement of the model with experimental flow data. INTRODUCTION Accurate constitutive equations are fundamental in order to success in hot working simulations, and the availability of reliable material data is crucial in designing materials and manufacturing processes [1]. In a given hot deformation process the diffusion kinetics are clearly influenced by the solute elements into an alloy. Excellent references about diffusion and selfdiffusion for different kinds of metals and alloys can be found [2,3]. However it is easy to see that there is relatively little information about the study of the effect of solute elements, in combination, on the self-diffusion coefficient of Fe in austenite (-Fe), in particular on the preexponential factor of diffusion coefficient, D0. This factor can be considered as constant, nevertheless there is experimental evidence that reveals a variation of D0 with chemical composition. Traditionally the diffusion of C into austenite, for example, has received much attention due to its importance in the design and application of many heat treatments of steels. Therefore it is possible to find references showing different types of
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