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During operation, the railway axle is exposed to static, cyclic and dynamic loading that is accompanied by influence of corrosive environments. This causes the accumulation of structural and mechanical damages in the material, formation of corrosion and mechanical micro-defects that are potential sites of fatigue crack nucleation and growth. This requires the development of failure analysis methods that allow determining main mechanisms of ferrite-pearlite steels fracture on the micro scale level. The railway axle's material -the OSL steel -main regularities of temperature influence on fracture mechanisms in ferrite-pearlite steel type are shown. Qualitative and quantitative analysis of fracture surface of Charpy specimens tested at 20°C and -40°C was performed on micro scale level using SEM investigation.

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We study the influence of the depth of a surface defect and its shape factor on the probability of fracture of a superheater collector made of 12Kh1MF steel after operation for 1.785 ⋅10 5 h . We plot the dependences of the probability of fracture for a model of the collector weakened by a partially circular crack on its inner surface on the depth of the defect under the conditions of static loading, stress corrosion cracking, and cyclic loading. It is shown that the probability of fracture of the superheater collector increases with the crack depth and the value of the shape factor.Keywords: superheater collector, probability of fracture, diagram of fracture estimation, partially circular crack, 12Kh1MF steel.The high-temperature power-generating equipment and, in particular, reactor vessels, superheater collectors, and steam lines working for long periods of time suffer the action of mechanical (caused by internal pressure) and thermomechanical stresses, as well as the action of corrosive (hydrogen-containing) media. This leads to microstructural changes in the material [1-3], initiation and growth of microcracks, their merging followed by the development of the main crack [4], and degradation of the mechanical properties [5][6][7][8]. Numerous multiple macrocracks whose lengths, as a rule, do not exceed 2 mm were detected in the vicinities of holes in the perforated part of the superheater collector of a TP-100-type boiler, and the main macrocrack crossing all holes of the collector in the circumferential direction was discovered [5].Based on the two-criterion approach, the diagrams of fracture estimation (DFE) were constructed for a smooth domain of the collector pipe of the TP-100-type superheater [9]. These diagrams take into account the mechanical characteristics and the crack resistance of 12Kh1MF steel after operation for 1.785 ⋅10 5 h under static and cyclic loading, as well as the safety margins for the mechanism of brittle fracture and plastic collapse. Moreover, the critical sizes of an inner-surface defect perpendicular to the axis of the cylinder were estimated with regard for the fracture toughness of 12Kh1MF steel after long-term operation. It was found that the minimal critical depth of the defect decreases as the ratio of the crack length to its depth l/a increases. Thus, if l /a increases from 2 to 16, then the critical depth decreases from 25.40 mm to 12.77 mm with regard for the safety margins for K mat = K fc .The aim of the present work is to study the influence of the depth of a defect on the probability of fracture of the perforated domain of the collector pipe of a thermal power plant with regard for the statistical spread of the mechanical characteristics of the material after long-term operation.