PSI - Issue 5

S.V. Panin et al. / Procedia Structural Integrity 5 (2017) 401–408 S.V. Panin/ Structural Integrity Procedia 00 (2017) 000 – 000

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The roughness of the fracture surface at various distances from the notch tip has been measured (Fig. 12). In the reserve stock steel the R a value varies smoothly in the range of 2.8 – 4.2 μm (Fig. 12, c , d , curve 1) which can be considered as a constant. In the steel specimen after the operation a noticeable increase of the roughness from 2.2 to ~ 8 μm is observed as the distance from the not ch is increased (Fig. 12, c , d , curve 2). In our opinion, this is related to the presence of highly localized deformation regions being located along the front of the fatigue crack growth for the steel after the operation. Relatively smooth surface for the reserve stock material with the sufficiently high cracking resistance is governed by a uniform redistribution of fracture regions over the plane of the fatigue crack growth. Within the entire testing temperature range, a noticeable fracture toughness decrease was revealed for the specimens after the long-term operation as compared with ones of the reserve stock material (Fig. 4, Table 2). The onset of a brittle-ductile transition for the steel after the operation is established as being equal to T = -20 °C while for the specimens from the reserve it starts at the lower temperature T = -40 °C (Fig. 4, d ). The reason for latter is suggested to be induced by the structure degradation accompanied with decarburization and deformation aging processes described above. The work of crack initiation and propagation for the specimens of both types was estimated (Fig. 4, e , f ). At the crack initiation stage, the greatest values of the work are registered for the emergency reserve steel while in the range of the brittle-ductile transition onset, a sharp drop of the curve occurs that does not takes place for the steel after long term use. The work of crack propagation for the emergency reserve specimens is also higher. However, on the contrary to the above case, there is a sharp decrease of the analyzed parameter when the temperature of the brittle-ductile transition is reached even for the specimens after a long term operation. Note that for the specimens from the emergency reserve when the maximum load of the impact bending P max is reached no instantaneous drop (even at negative temperatures) takes place (Fig. 4, a-c ). This pattern of fracture ensures retaining the larger value of fracture work during the crack propagation stage with decreasing testing temperature as compared with the steel after the exploitation. Also the structure degradation in the latter results the lowering P max values that is responsible for reduction both the crack initiation and propagation work. All these being combined with the lower ductility of the steel at the stage of the macrodefect propagation give rise to lower values of impact toughness. 3.6. Impact toughness

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Fig. 4. Dependences of the impact load versus the specimen deflection: а ) T =20 ºC; b ) T =-20 ºC; c ) T =-70 ºC; d ) temperature dependence of toughness; e ) dependence of the crack initiation work on temperature; f ) dependence of the crack propagation work on temperature; 1) emergency reserve; 2) after long-term operation