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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of fatigue properties of the two steel types (after degradation under operational and laboratory conditions) are practically the same, which indicates the unified nature and the key role of hydrogen in both cases of degradation. In-laboratory degradation of 17H1S steel was manifested on the fracture surfaces after the fatigue testing in air by fragmentation of festoons, appearance of a small amount of intergranular fragments, which were considered as a sign of cohesion weakening between adjacent grains, and by secondary microcracking (Fig. 9a). During the tests in NS4 solution, intergranular fracture elements became to dominate at the fracture surfaces of the degraded steel (Fig. 9b). Intergranular facets were decorated with secondary cracks, which were suggested to be a manifestation of the hydrogenating effect of the testing environment. Hence, it was concluded that the acceleration of fatigue crack growth observed in the Paris region of fatigue crack growth curve in a corrosive environment is caused by weakening of grain boundaries during the accelerated in-laboratory degradation of steel and the hydrogenating effect of the corrosive solution.

a

b

c

d

Fig. 9. Fracture surfaces of steels 17H1S (a, b) and X60 (c, d) degraded in laboratory conditions after fatigue tests in air (a, c) and in NS4 solution (b, d). All features of degradation noted for the steel 17H1S are also revealed on the steel X60 however they are less pronounced (Fig. 9c, d). This concerns primarily the tests in the environment (Fig. 9d), where, as a rule, intergranular cracking is observed (in particular, on the fracture surface of the 17H1S steel after artificial degradation), but its role was not dominative in the case of an artificially degraded sample of X60 steel. This indicate less susceptibility of X60 steel after its degradation in laboratory conditions to the hydrogenating effect of the simulated soil solution. In general, this is consistent with the results of mechanical tests of degraded steels, presented by Nykyforchyn 1 et al. (2019).

Conclusions

Long term operation leads to a decrease in the threshold values of SIF range for pipe steels tested in air. The maximum effect of degradation under operational conditions on the threshold values of SIF was found in the 17H1S steel, and the minimum in X60 steel. The relief of the fatigue fracture surfaces is higher for operated steels due to