PSI - Issue 42

Olha Zvirko et al. / Procedia Structural Integrity 42 (2022) 522–528 Olha Zvirko / Structural Integrity Procedia 00 (2022) 000 – 000

524

3

accumulation in certain trapping sites. Hydrogen can be transported and get trapped at various types of lattice defects in steels, such as: dislocations, grain boundaries, vacancies, voids, phase interfaces, precipitates, cracks, etc. Such circumstances promote the formation, evolution and accumulation of microdefects in pipeline steel (so called in-bulk dissipated damaging), and, consequently, deterioration of mechanical properties, especially, of plasticity and brittle fracture resistance, as demonstrated in numerous issues by Krasovskii et al. (2012), Nykyforchyn et al. (2016), Zvirko et al. (2018, 2019, 2021), Marushchak et al. (2019), Zvirko (2021) and others. Long-term mutual action of corrosion, hydrogenation and working stresses during operation, which intensifies each other, is one the most important reasons of in-service degradation of gas pipeline steels. There are many theories about the mechanism of hydrogen embrittlement of pipeline steels covering different scales, as discussed by Ohaeri et al. (2018) and others. However, hydrogen-enhanced decohesion (HEDE) and hydrogen pressure theory (HPT) are usually dominated in long-term operated pipeline steels, as shown by Nykyforchyn et al. (2016) and Zvirko et al. (2021). 3. Materials and testing methods In this work the susceptibility to hydrogen embrittlement of pipeline steels was investigated. The study objects were the low-carbon low-alloyed 0.17 C steel (17H1S steel, Ukrainian code) and the low-carbon 0.20 C steel (20 steel, Ukrainian code). Both studied steels were equivalent to API 5L X52 strength grade. The investigated steels had a microstructure, which consisted predominantly of ferrite and pearlite. Specimens were manufactured from the pipe of natural gas transit pipeline after 29 years of operation (17H1S steel), and from the pipe elbow of lateral pipeline of natural gas transmission system after 44 years of operation (20 steel). The characteristics of the operated pipeline steels from the point of view of the degradation of their strength, plasticity and hydrogen embrittlement resistance were compared with the properties of the reference material. For this purpose, the 17H1S pipeline steel in as-delivered state (reserved pipe) was used. Concerning the 20 steel, the metal properties of tensioned section of the pipe elbow were compared with the properties of straight pipe material as the reference one, assuming that the degree of degradation of the tensioned section could be higher as it was reported for the similar investigated case by Nykyforchyn et al. (2016). The pipe elbow was part of above-ground lateral pipeline, located behind the compressor station. Tensile tests were carried out to obtain the mechanical properties of the material. Basic mechanical properties of the investigated steels, namely ultimate strength σ UTS , yield strength σ Y , reduction in area RA , elongation δ , and relative displacement Δ were determined using uniaxial tensile tests. The smooth cylindrical specimens with an initial gauge length of 25 mm and a circular cross section with a diameter of 5 mm were cut out from the studied pipe sections made of the 17H1S steel in longitudinal and transversal directions regarding the pipe axis. There were longitudinal and short transversal notched specimens manufactured from the pipe elbow made of the 20 steel. The notched specimens had a circular notch with a 5 mm notch root radius; the diameter of the central section for these specimens (deepest point of the notch root) was also 5 mm; their length was limited by the pipe wall thickness. The strain rate was 3∙10 -3 s -1 . Susceptibility of the investigated steels to hydrogen embrittlement was studied by tension of specimens in air after electrolytical hydrogen pre-charging in an aqueous sulphuric acid solution with pH = 3 at current density і = 10 mА/сm 2 for 150 hours. The plasticity characteristics of the steels without and after hydrogenation from the point of view of their hydrogen embrittlement resistance were compared and susceptibility of the steels to hydrogen embrittlement was evaluated by the change in plasticity characteristics. Considering the higher sensitivity of reduction in air to operational degradation of steels than elongation, as shown by Zvirko et al. (2021), the susceptibility to hydrogen embrittlement is estimated by using an index given by HES :

H RA

1 = −

HES

,

(1)

RA

where RA H – reduction in area, determined in air after preliminary hydrogenation and RA – reduction in area, determined in air without preliminary hydrogenation).