PSI - Issue 17

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 17 (2019) 568–575 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000

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strain equals 5% and 10% (two series of steel samples); then they were expose d to artificial aging at 250ºС for 1 hour. Applied hydrogen charging procedure enabled providing a homogeneous hydrogen distribution by electrochemical hydrogen charging for a reasonable charging time of several days and avoiding development of damages in the metal during hydrogenation. The key idea of the accelerated degradation procedure applied in the study was to simulate, on a laboratory scale, degradation of pipeline steel during long-term operation under simultaneous action of hydrogenation and working loading. The specimens for tensile and impact toughness tests were machined from the steel samples after abovementioned procedures of artificial deformation aging and in-laboratory accelerated degradation. The cylindrical smooth tensile specimens had a gage length of 25 mm and a diameter of 5 mm. For impact toughness testing the standard-size (10 mm × 10 mm × 55 mm, working cross-section 10 mm × 8 mm) V-notch specimens were used. Tensile tests were carried out at the strain rate  = 3  10 -3 s -1 at ambient temperature using tensile specimens to obtain the basic mechanical properties of the material: ultimate strength σ UTS , yield strength σ Y , reduction in area RA and elongation δ . Impact toughness KCV was evaluated by impact tests using Charpy V-notch specimens. The values of the average diffusible hydrogen concentration V H in the studied steels measured by desorption at different temperatures are presented in Table 1. It is seen that the X52-10 steel in the post-operated state, especially the steel from inner bottom pipe section, is characterised by a higher amount of hydrogen in comparison to that for the X52 steel in the as-received state. It can be caused due to internal corrosion of the pipe during long-term operation; it means interaction between moisture condensed from transported hydrocarbons and the inner surface of the metal pipe as it was shown by Tsyrul'nyk et al. (2008). It should be noted that measurement of hydrogen concentration in the studied steels was carried out after more than one year later than pipes were put out of operation. 3. The role of hydrogen in degradation of gas pipeline steels

Table 1. Concentration of diffusible hydrogen V H in API X52 5L pipeline steel measured by desorption at different temperatures T Pipeline steel Steel state Pipe section T, 200  C T, 400  C T, 600  C Total V H , ppm X52 As-received - 1.40 0.07 0.04 1.5 X52-12 Operated 30 years Top, outer 0.10 0.50 0.60 1.2 X52-12 Operated 30 years Bottom, inner 0.01 1.00 0.40 1.4 X52-10 Operated 30 years Top, outer 0.30 0.60 0.80 1.7 X52-10 Operated 30 years Bottom, inner 0.15 0.80 4.15 5.1

Therefore, it was assumed that the mobile hydrogen was diffused from the steels and hydrogen in traps was only remained before the hydrogen concentrations measurements. It is evidenced from data of Table 1 that residual hydrogen in the operated steels was mostly trapped at the high-energy traps in comparison with the as-received state steel; it was desorbed at higher temperatures (400 and 600  С). The observed difference in hydrogen behaviour in the investigated steels depending on the steel state indicated increasing of multiple damaging (hydrogen traps) in steel during operation. In spite of the observed difference in hydrogen concentration between the as-received and operated steels it is still no proof of acceleration of in-service degradation of the steel by hydrogen. Increasing of defectiveness in the metal can be also caused by mechanical loading, and an increase in concentration of residual hydrogen in the operated metal could be the consequence of damages accumulation but not the cause. Therefore, it is important to compare the mechanical properties of operated steels not only with the properties of pipeline steels in the as received state, but also the properties of the operated metal from different sections of structure, only a part of which could contact with corrosion environment during operation, should be compared between themselves. Thus, data on concentration of hydrogen C H measured by hot extraction and impact toughness KCV of the 10HS (0.1C-Mn-Si) and X52 pipeline steels, presented in Table 2 according to research carried out by Tsyrul'nyk et al. (2004),