Influence of combined preliminary loading on the brittle strength of refractory steel
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INFLUENCE OF COMBINED PRELIMINARY LOADING ON THE BRITTLE STRENGTH OF REFRACTORY STEEL P. V. Yasnii, P. V. Pshonyak, and I. B. Okipnyi
UDC 620.192.46
We study the influence of combined preliminary loading (tensile straining combined with a lowamplitude cyclic component) on the tensile strength, crack-tip opening displacement, and brittlefracture resistance of 15Kh2MFA refractory steel after thermal treatment simulating the process of embrittlement of the reactor vessel at the end of its service life. The tensile strength is determined under the conditions of uniaxial tension for cylindrical specimens with diameters of the working part of 5 and 8 mm in liquid nitrogen. The influence of combined thermomechanical loading on the brittle strength and kinetics of crack-tip opening displacement is investigated under the conditions of eccentric tension for compact specimens 19 mm in thickness. The basic regularities of the influence of temperature and the level and amplitude of the cyclic component of overloading on the tensile strength and critical stress intensity factor of steel are established. For a temperature of preloading of 623°K, which is much higher than the temperature of brittleness of steel ( Tbr = 390°K ), the combined preliminary thermomechanical loading increases the critical stress intensity factor Kf of steel by up to 30% as compared with its value under static loading. At the same time, for a temperature of preloading of 423°K (close to the temperature of brittleness), the procedure of combined preliminary thermomechanical loading decreases the value of Kf for the analyzed steel as compared with the case of static loading.
The resistance of ferritic-pearlitic steels and, in particular, of the materials of VVÉR-type reactor vessels to brittle fracture can be increased as a result of preliminary thermomechanical loading (PTL) including the procedure of static loading of structures at a certain temperature (exceeding the temperature of ductile-brittle transition of the material) followed by the complete or partial unloading [1–5]. The stability of the effect of PTL is determined by the processes of ductile crack growth, holding at high temperatures, and cyclic loading after PTL [6–11]. However, the applicability of this method to actual structures is limited because it requires loads higher than admissible. It is known that, generally speaking, the increase in the brittle-fracture resistance of materials after PTL is caused by the action of compression residual stresses, crack-tip blunting, and strain-hardening of the material in front of the crack tip [12]. On the basis of the available data of investigations, it was established that the major factors responsible for the elevation of the brittle-fracture resistance of 15Kh2MFA-type steels and their 10KhMFT-steel welds after PTL are the phenomenon of crack-tip blunting and the action of compression residual stresses [8, 10, 11]. Moreover, after the procedure of thermal treatment simulating the radiation-induced embrittlement of the material at the end of
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