PSI - Issue 26

Myroslava Hredil et al. / Procedia Structural Integrity 26 (2020) 409–416 Hredil et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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Fig. 7. Effect of hydrogenation (red dots) on fatigue crack growth rate of the as-received 17H1S steel (grey dots – fatigue crack growth curve in air; blue dots – in NS4 solution without hydrogen charging).

A clear evidence of intergranular fracture predominance was revealed on the fracture surface of the specimen corresponded to crack growth in the range of the observed plateau (Fig. 8). These fractographic features were suggested as proof of embrittlement of grain boundaries as a result of contactless hydrogenation of the prefracture zone of 17H1S steel. Since in-service hydrogenation of pipes is quite possible under favorable conditions, this effect should be taken into account.

Fig. 8. Fracture surface of 17H1S steel specimen after the test by the proposed method, which is correspondent to crack growth at the plateau (red dots in Fig. 7).

3.2. Verification of the procedure of in-laboratory accelerated degradation

In-laboratory accelerated degradation of the tested steels was performed using the method elaborated recently by Nykyforchyn 1 et al. (2019). The method is based on the well-known procedure of artificial ageing (GOST 7268), however, it additionally involves a preliminary hydrogen charging and application of tensile stresses to a steel specimen. This method hereby simulates the late stage of operational degradation discussed comprehensively by Nykyforchyn et al. (2010) which characterizes the development of dissipated microdamaging in pipeline steels after their long term operation considered by Hredil (2011). Results of fatigue testing confirmed that trends in the change