PSI - Issue 28

1208 Myroslava Hredil et al. / Procedia Structural Integrity 28 (2020) 1204–1211 Myroslava Hredil , Halyna Krechkovska, Oleksandr Tsyrulnyk, Oleksandra Student / Structural Integrity Procedia 00 (2019) 000–000 5 lower in the transversal direction relative to ferrite-pearlite texture than that along it. This is consistent with the fact that the boundaries between pearlite and ferrite or between adjacent pearlite grains are effective traps for hydrogen, as shown by Chan and Charles (1986). It should be noted that pearlite strips in the steel X70 is exceptionally thin comparing with other pipe steels, and individual lamellae inside the pearlite colonies is difficult to identify. Due to this feature, the steel X70 tends to insignificant hydrogen trapping which prevents damage accumulation along texture interlayers and ensures excellent mechanical characteristics of this steel even after its longest operation time among the considered steels (37 years). 3.3. Fractographic features of operational degradation of pipe steels Fractographic signs of the steel degradation revealed after SCC tests. Described structural features of the operational degradation of gas pipeline steels were manifested clearer on the fracture surfaces of the specimens tested by SSRT in NS4 solution simulating soil environment. Besides, for the operated steel, the effect of degradation is clearer revealed after the testing in the corrosive environment. Delaminations after the tensile testing in air are surrounded by a ductile relief consisted of dimples (Fig. 4a), whereas in the case of the test in NS4 solution, the delaminations become the origins of brittle fracture in the form of transgranular cleavage (Fig. 4b). This trend maintained not only near the external surface of the specimen contacted with the corrosive environment, but also in the central part of its cross section (Fig. 4c). Nykyforchyn et al. (2019) suggested that this indicates a possible occurrence of these delaminations in the structure of the operated steel before the testing, during steel operation, because of cohesion weakening between adjacent layers of ferrite and pearlite in the steel textured microstructure. In contrast, under the same testing conditions of the as-received steel, such fractografic peculiarities were not observed at all.

a

b

c Fig. 4. Fracture surfaces of the operated 17H1S steel after the tensile test in air (a) and SSRT test in NS4 solution (b, c).

Fractographic signs of steel degradation after the impact testing . The analysis of mechanical properties of the tested pipe steels showed that the impact toughness is the most sensitive characteristic to operational changes in the metal. Therefore, fractographical signs of steel degradation should be manifested on the fracture surfaces of impact specimens. Fractographic analysis revealed that a typical ductile relief formed by practically equiaxed voids prevails in all tested steels in the initial state (Fig. 5). Fracture surfaces of the operated steels had also a typical ductile relief formed by microvoid coalescence, similar as in their initial state. However, some fragments on these fracture surfaces were also identified which can be considered