Relation Between Microstructures and Constitutive Behavior of Advanced Tempered Martensitic Steels
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Relation Between Microstructures and Constitutive Behavior of Advanced Tempered Martensitic Steels Philippe Spätig, Robin Schäublin and Max Victoria Fusion Technology, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, CH-5232 Villigen PSI, SWITZERLAND ABSTRACT In an effort to better understand the plasticity of the tempered martensitic steels, the strainhardening of two 7-9Cr steels is examined in terms of dislocation mechanics. It is shown that, over the temperature range investigated (173K-523K), the strain-hardening as a function of stress can be described by an equation taking account the dislocation storage and annihilation. The model of strain-hardening used in this study is based on the original Kocks description of the dislocation density evolution with plastic strain but the heterogeneous distribution of dislocations resulting from the lath boundaries and prior austenite grain boundaries is taking into account. The effect of stress and temperature on the storage and annihilation is discussed. The relation between the determined mean dislocation path associated to the heterogeneous lath dislocation structure and the transmission electron microscope observations performed on the two different steels is outlined. INTRODUCTION Two steels, the F82H and the EUROFER97, which belong to the 7-9Cr class of tempered martensitic steels, are investigated in this study. Owing to their good thermophysical and mechanical properties as well as low sensitivity to swelling, the tempered martensitic steels are leading material candidates for fusion reactor structures [1]. A detailed description of their constitutive behavior is necessary for safety assessment as well as structural analysis. While a great deal of studies on the tensile properties have been done on the ferritic and martensitic steels, most of them consist in reporting the temperature dependence of the yield stress, of the ultimate tensile strength and of the uniform elongation. The effects of irradiation and irradiation temperature on the previous parameters have also been quite extensively studied, e.g. [2,3]. However, only few studies have been done on this class of steels to establish the overall constitutive equations and strain-hardening laws based on the dislocation theory [4,5]. The goal of this paper is to better describe the strain-hardening as a function of stress along a whole tensile curve by taking into account the initial microstructure. The effect of temperature and stress on the strain-hardening is also considered. EXPERIMENTAL PROCEDURES Two tempered martensitic steels of the 7-9Cr class were used. The first one, designated as F82H, contains 7.65 wt% Cr, 2 wt% W and Mn, Mo, V, Ta, Si and C below 1 wt% in sum total, and Fe for the balance. The F82H was heat-treated by normalizing at 1313K for 0.5 hour and tempering at 1013K for 2 hours. The other steel is the EUROFER97, whose chemical composition is: 8.93 wt% Cr, 1.07 %wt W and Mn, Mo, V, Ta, Si and C below 1 wt% in sum total, and Fe for the balance. The EURO
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