PSI - Issue 47

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 47 (2023) 190–194 Hryhoriy Nykyforchyn, Olha Zvirko, Oleksandr Oliynyk et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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pronounced in operationally degraded material. Similar effect of operational degradation on anisotropy of resistance to hydrogen embrittlement was reported by Zvirko (2022) for carbon and low-alloyed steels.

Table. Impact toughness KCV [ J/сm 2 ] experimentally observed for pipeline steels at ambient temperature Steel Steel state Specimen type Longitudinal Transversal

Short transversal

152

129 113 221 106 326 214

-

17H1S (API 5L X52 strength grade) As-delivered

-

19

Operated 30 years

283 169 342 326

- -

API 5L X52 strength grade

As-delivered

Operated 30 years

58 37

API 5L X60 strength grade

As-delivered

Operated 25 years

The lowest impact toughness values were obtained for short transversal specimens (Table), in which fracture propagates parallel to the rolling direction. For the post-operated steels, the impact toughness values are extremely low indicated low resistance to brittle fracture in the plane parallel to the rolling direction. It is associated with the operational weakening of the cohesion between the fibres of the rolled steels, as well as between the matrix and non metallic inclusions, which are stretched in the direction of rolling and form chains, which, accordingly, contributes to the development of micro-damages oriented in this direction. Absorbed hydrogen during long-term operation can negatively effect on a steel by the creating high hydrogen pressure in defects as a result of molecular hydrogen recombination causing dissipated damaging and delaminations between fibres along the rolling direction as demonstrated in Nykyforchyn et al. (2019, 2020). The influence of hydrogen charging on weakening of the banded microstructure and formation of delaminations at Charpy impact testing of API 5L X70 pipeline steel was also demonstrated by Cauwels et al. (2022). Analysing components of the fracture energy for the crack initiation and the crack propagation, as presented in Figure 1 for longitudinal specimens made of the X52 pipeline steel at ambient temperature, it was revealed that decrease in impact toughness caused by operational degradation is mainly associated with reducing the crack propagation energy. That means that crack growth is easier in operationally degraded material and it is obviously a result of operational damaging.

Fig. 1. Total fracture energy and its components of crack initiation and crack propagation during Charpy tests of longitudinal specimens made of the X52 pipeline steel at ambient temperature. 3. Peculiarities of the experimental studies of in-service degradation of rolled steels under cyclic loading For rolled steels operated under cyclic loading, two stages of degradation were also distinguished (Figure 2). Cyclic loading during long-term operation of carbon and low-alloyed rolled steels commonly resulted in increasing their