PSI - Issue 36

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 36 (2022) 306–312 Hryhoriy Nykyforchyn, Leonid Unigovskyi, Olha Zvirko et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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be explained by using transversal specimens for which plasticity characteristics are typically lower than for longitudinal ones. In general, a slight decrease of plasticity due to operation is accompanied by strength loss, which corresponds to the main regularities of the operational degradation specified by Nykyforchyn (2021) – a simultaneous decrease of both strength and plasticity indicates the development of operational microdamages in the material. Table 2. Tensile mechanical properties of steels in the as-received and operated states and the parameter λ, characterizing resistance of steels to hydrogen assisted cracking. Steel state  Y [MPa]  UTS [MPa] Elongation [%] RA [%] λ (Elongation) [%] λ ( RA) [%] As-received steel 344 468 17 59 0 2 After operation 302 404 16 54 26 35 Measurements of hydrogen concentration in the tested steels revealed that the content of residual hydrogen in the operated metal is more than 4 times higher than that in the as-received steel: averaged values were 0.11 ppm and 0.45 ppm for the as-received and operated states, respectively. Evidently, the operational hydrogenation of the pipe wall was caused by corrosion processes on the pipe inner surface due to moisture condensation, as shown by Tsyrulnyk et al. (2008), Hredil and Tsyrulnyk (2010). The results of tensile tests of hydrogenated specimens were compared with the corresponding characteristics obtained for the as-received steel (Fig. 2). It is derived from the Figure 2 that preliminary hydrogenation did not have a significant effect on steel strength in both as-received and hydrogenated states; however, its influence on plasticity is more noticeable. Changes in the plasticity characteristics were presented by the parameter λ as the indicator of the hydrogen effect on steel, namely, its susceptibility to hydrogen assisted cracking (Table 2).

200 250 300 350 400 450 500



 y

UTS

 y ;  UTS [MPa]

Operated Operated As-received As-received

a

10 15 20 25 30

0 10 20 30 40 50 60

Elongation

RA

RA [%]

Elongation [%]

0 5

Operated As-received

Operated As-received b

Fig. 2. Strength (a) and plasticity (b) characteristics of the investigated carbon steel of hydrogenated specimens (black bars) and specimens without hydrogenation (white bars).