PSI - Issue 42

Halyna Krechkovska et al. / Procedia Structural Integrity 42 (2022) 1398–1405 Halyna Krechkovska / Structural Integrity Procedia 00 (2022) 000 – 000

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growth resistance, have been used to assess the state of long-term operated steel. Student et al. (1997, 2021 2 ), Krechkovska (2021) considered them among the most sensitive indicators of changes in the structural and mechanical state of heat-resistant steels. High locality of the fracture process at the threshold SIF range is usually due to the peculiarities of the steel fracture at the substructural level within individual grains. These features of metal deformation in the vicinity of the fatigue crack tip determine the transgranular mechanism of fatigue crack growth, which is characteristic of the steel in the initial state. However, during long-term operation under high temperatures and stresses, which promote creep, a network of intergranular defects forms in the steel. The difference between the fracture mechanisms due to creep under static (a intergranular mechanism) and cyclic (typically intergranular one) loads can be used to distinguish the grain boundaries damaged as a result of high-temperature creep. For this purpose, we compared the fracture surfaces of steel specimens in the near-threshold area of the fatigue crack growth in the initial state and after the high-temperature operation. Fracture of the specimens made of the steel in the initial state was predominantly transgranular with the formation of narrow and elongated areas of local crack growth in the form of unidirectional festoons, oriented in the main direction of fatigue crack growth, with fatigue striations across them. In addition, pseudo-striations changing their orientation from grain to grain were observed in the near-threshold section of crack growth (Fig. 3 а ), described for the first time by Romaniv at al. (1981). Considering the fracture surfaces of the operated steel volume-convex fragments of intergranular fracture were additionally revealed, which protruded against the background of a generally shallow fatigue relief, typical for the steel's initial state (Fig. 3b). Their occurrence is explained by damage to a certain part of the grain boundaries in the steel even before its fatigue testing, as a result of structural transformations caused by creep during long-term operation. The crack, propagating under the action of even near-threshold SIFs, easily absorbed the damaged grain boundaries in its path, which are visualized as intergranular facets on the fracture surface . Their area was predictably bigger on the fracture surfaces of the operated steel subjected to more shutdowns of the technological process. (Fig. 3c). Fig. 3. Typical fracture surfaces on the near-threshold areas of fatigue crack growth of the tested steel in the initial state ( а ) and after operation with 501 (b) and 576 (c) shutdowns of the power units of TPP. As Student et al. (1999) emphasized, volume-convex intergranular damages in the long-term exploited steel, which occurred along the path of the transgranular fatigue crack, front, merge with it, thereby enhancing the crack closure effect due to an increase in surface roughness. As a result, the difference between the effective and nominal thresholds of SIF for the degraded steel is greater compared to the steel in the initial state. A larger area of intergranular fragments per unit area of the fracture surface in the steel, operated with more shutdowns, indicates its more intense damage and corresponds to a lower threshold level of SIF caused by structural changes as a result of operational degradation. Student et al. (1999) noted that despite a larger part of intergranular fracture on the fracture surfaces of the impact and fracture toughness specimens, the sensitivity of threshold crack growth characteristics to the steel degradation is considerably higher than common characteristics of strength and plasticity due to the crack closure effect caused by an increased roughness of the fracture surface of the operated steel. Thus, the proposed fractographic indicator, determined on the near-threshold area of the SIF range, seems less sensitive to intergranular in-service damages than those obtained in other types of the performed tests. For instance, the fracture surfaces of impact specimens made of the operated steel with more shutdowns were almost completely covered with intergranular facets. The revealed difference in the sensitivity to cohesion weakening between adjacent grains can be explained by a different size of the pre-fracture zone in various types of experiments; it is the largest in 25 µm a 25 µm b 25 µm c