Hot Deformation Characteristics of Functionally Graded Steels Produced by Electroslag Remelting
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INTRODUCTION
SCIENTIFIC research on functionally graded materials (FGMs) considers, in a large sense, functions of gradients in materials comprising thermodynamic, mechanical, chemical, electromagnetic, optical, or biological aspects. In essence, FGMs are characterized by spatially varied microstructures created by nonuniform distributions of the reinforcement phase with different properties, sizes, and shapes, as well as by interchanging the role of reinforcement and matrix materials in a continuous manner.[1] Functionally graded steels (FGSs) are a group of FGMs, introduced by Aghazadeh Mohandesi and Shahossinie,[2] containing graded layers of ferrite, austenite, bainite, and martensite that may be fabricated via diffusion of alloying elements during electroslag remelting. Microstructural studies of the FGSs have shown that it is possible to obtain multilayered composites consisting of graded ferrite, austenite, bainite, and martensite phases. By selecting the appropriate arrangement and thickness of the initial ferritic and austenitic steels used to set up the electrodes, it is possible to control the variety and thickness of the merging phases after remelting. When the primary electrode contains B. NADERI, Master Graduate Student, and J. AGHAZADEH MOHANDESI, Full Professor, are with the Department of Metallurgical Engineering and Materials, Amir Kabir University of Technology, 158754413 Tehran, Iran. Contact e-mail: babaknaderi61@ gmail. com Manuscript submitted April 28, 2010. Article published online February 12, 2011 2250—VOLUME 42A, AUGUST 2011
two slices of ferritic and austenitic layers as a; c ; the morphology of the resultant composite is (abc); hence, o o R a c !ðabcÞcom el
where a is the ferritic layer, c is the austenitic layer, b is the bainitic layer, el represents the electrode, com represents the composite, and R is remelting. Similarly, when the primary electrodes contain three slices of ferritic and austenitic steels, the following phases appear in the final composite after remelting: R a c a !ðabcbaÞcom el
R c a c !ðcMcÞcom el
where M is the martensitic layer. Finally, by using four slices of austenitic and ferritic steels, it follows that R a c a c !ðabcMcÞcom el
Diffusion of chromium, nickel, and carbon atoms taking place at the remelting stage in the liquid phase controls the chromium, nickel, and carbon atom, distribution pattern in the produced composites. As alloying elements diffuse, alternating regions with different transformation characteristics are created. The diffusing atoms individually or together stabilize different phases such as bainite or martensite.[3] Tensile[3] and impact behavior of FGS[4] have already been studied. METALLURGICAL AND MATERIALS TRANSACTIONS A
The understanding of metal and alloy behavior under hot deformation condition is of great importance to designers of metal forming processes because of its effective role on metal flow pattern as well as the kinetics of metallurgical transformation. For this reason, in order to d
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