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620.187.3 and I. B. OkipnyiThe paper addresses the influence of plastic prestraining in tension and combined tension at 150 and 350°C on the rupture stress in 15Kh2MFA steel upon heat treatment that simulates irradiation embrittlement of materials in a WWER-440 type reactor towards the end of its lifetime. The dependence of rupture stress on the dislocation density in the material upon plastic prestraining is studied. It is found out that as the plastic prestraining in tension and combined tension grows, the dislocation density within small-angle boundaries increases and thus results in a larger rupture stress. An increase in rupture stress in the case of prestraining at 350°C is more intensive than that at 150°C. We analyze the microcrack nucleation and growth micromechanisms in 15Kh2MFA steel during tensile plastic deformation and discuss the effect of the material substructure on the microcrack arrest.

I. Pyndus, V. B. Glad'o, UDC 620.192.46 I. B. Okipnyi, and I. V. Shul'ganWe present a procedure of prediction of the influence of warm prestressing combined with cycling on the brittle strength of steel 15Kh2MFA. Using a finite-element method, the effect of the combined warm prestressing on the stress-strain state at a fatigue crack tip is studied in an elastic-plastic statement. Electron microscopic observations of fracture surfaces have revealed that fracture is initiated at some distance from the fatigue crack front. Based on the pattern of influence of the plastic prestrain level on the cleavage stress of steel 15Kh2MFA and the experimental CID value, a method is put forward for finite-element modeling of the stress-strain state at a crack tip during the specimen fracture. Using the results of the finite-element modeling, the relevant curves have been plotted and an approximating formula has been proposed to represent the influence of the combined warm prestress level on the fracture toughness of steel 15Kh2MFA.Introduction. Experimental studies of the influence of warm prestressing (WPS) on strength of heatresistant steels were started in early 60s [1]. The WPS effect was found to consist in increasing brittle strength in the material, which has a ductile-brittle transition temperature, upon warm prestressing at high temperature (Fig. 1).Based on available publications, there are several known theoretical approaches to assessment of the WPS effect. The main ones are the Chell model [2], which involves the use of the J-integral and takes into account the crack-tip plastic strains upon WPS, and the RKR model [3], whereby the fracture is related to the normal stresses reaching a critical value s c at some characteristic distance r c ahead of the crack tip.Beremin [4] laid the foundation for the statistical approach to assessment of the WPS effect, which was further developed in [5]. In [6, 7] a probabilistic model for the influence of WPS on the fracture toughness was put forward which allows for a combined effect of the WPS temperature and specimen thickness.The main factors that raise the brittle fracture resistance in reactor pressure vessel steels, such as 15Kh2MFA, upon WPS include the combined influence of the crack tip blunting, residual compression stresses, and strain hardening at the crack tip [8][9][10].It has been shown [11] that upon WPS the fracture starts at some distance, called the cleavage initiation distance (CID), from the crack front on the condition that the normal stresses s y should locally exceed some critical value -the failure stress s f . A relation has been established between the fracture toughness K c I , failure stress s f , distance r c , and microstructure of steel. It has been demonstrated that microstructure of the same heat-resistant steel grade has a significant effect on CID, s f , and K c I . Since in a cracked body subjected to WPS there arises a gradient of plastic (residual) strains along the crack extension, it is important to clarify a relationship between the cleava...

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