PSI - Issue 28

1206 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 3 3. Results and discussion 3.1. Brittle fracture resistance of pipeline steels Previous research by Hredil et al. (2019) and Krechkovska 1 et al. (2019) have shown that strength of considered steels changed insignificantly after their operation whereas plasticity essentially decreased, with the exception of the steel X70 for which plasticity did not practically change despite its longest operation time. Meanwhile, the most sensitive characteristic for assessing the degradation of pipeline steels is their brittle fracture resistance, namely impact toughness. This parameter decreased significantly after long-term operation of analyzed steels, as derived from Fig. 1. A slight decrease in impact toughness was observed for the steel X70 (11%). In X60 steel, KCV is decreased by 23% relative to the corresponding value for the as-received pipe. However, impact toughness of 17H1S steel was the most worsened (56% decrease after 36 years of service). Hredil et al. (2019) have found that after 51 years of operation, the reduction in KCV of this steel reached as high as 87%. Such a low resistance to brittle fracture of operated steels was suggested to be a result of the accumulation of defects in the metal during so long period of operation, as shown by Hredil (2011). The values of impact toughness of operated steels also noticeably depend on the specimen orientation, as shown for X60 steel (Fig. 1, on the right). Since the fracture of transverse specimens occurred in the longitudinal direction relative to the pipe axis, the impact toughness of the operated steel testified a decrease in its resistance to brittle fracture due to the presence of operational damages in the rolling direction, such as delaminations along the ferrite-pearlite texture strips. In the case of longitudinal specimens, when the fracture propagates in the radial direction, the fracture plane swallows up the damages along the lines of the texture and, as a result, the crack branches. In this case, the fracture path can deviate along the direction of delaminations, as shown in Fig. 2(b, c). It is obvious that the use of the specimens oriented longitudinally to the pipe axis can lead to incorrect estimation of the steel degradation degree after its long-term operation. This is especially important to take into account, since just this specimen orientation is typically used in most examinations of the current state of operated steels.

Fig. 1. Changes in impact toughness for pipeline steels after long-term operation. Specimen orientation: L – longitudinal; T – transversal.

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Fig. 2. Typical fracture of the steel X60 in the as-received state under impact loading (a) and delaminations along the rolling direction in operated steels (b, c).