PSI - Issue 28

1210 Myroslava Hredil et al. / Procedia Structural Integrity 28 (2020) 1204–1211 Myroslava Hredil , Halyna Krechkovska, Oleksandr Tsyrulnyk, Oleksandra Student / Structural Integrity Procedia 00 (2019) 000–000 7

As-received state

1,0

X70, 37 years

0,2 KCV op / KCV 0 0,4 0,6 0,8

X60, 25 years

30 years

36 years

51 years

17H1S

0,0



0,00

0,05

0,10

0,15

0,20

Fig. 6. Dependence between the loss in brittle fracture resistance KCV op / KCV in and the part of brittle fracture elements at the unit area of fracture surfaces of Charpy specimens S cr / S for pipeline steels after their long-term operation on gas mains. Similar approach has been earlier proposed by Student et al. (2019) for long-term operated carbon steels from the profile metal rolling, for which the critical value of KCV op / KCV in was additionally substantiated. According to that approach, operated steels reach the critical state when transgranular cleavage elements occur on the fracture surfaces of impact specimens instead of delaminations. It is important to note that in our case cleavage was found only on the fracture surfaces of a heavily degraded steel (after 51 years of service) for which a drop of impact toughness was the most considerable. Similar as in the mentioned study, occurrence of cleavage can indicate that steel has reached a certain critical state, and its further operation could be concerned with a real risk of brittle fracture. Such a substantiation of the critical state for pipeline steels would be a criterion of their further safe service. Conclusions Operational degradation of pipeline steels is manifested fractographically by the occurrence of microdelaminations and cleavage fragments as evidence of steel embrittlement. Their occurrence is promoted by hydrogen accumulated along the boundaries of ferrite and perlite grains between adjacent layers of texture and caused a weakening of cohesion between the strips of ferrite and perlite. The relative area of brittle fragments on the fracture surface is proposed to use as a fractographic indicator in the assessment of the current state of operated pipeline steels. The critical state of the long term exploited steels is proposed to be determined based on the qualitative change in the dominant sign of their degradation, namely, the change of the main brittle elements on the fracture surfaces from delaminations against the background of a ductile relief with typical dimples to brittle transgranular cleavage. The obtained unified correlation between the fractographic and mechanical indicators of the technical state for operated steels of various strength opens up the prospect of predicting their state at any stage of further operation. Acknowledgements This research has been supported by the NATO in the Science for Peace and Security Programme under the Project G5055. References Chan, S. L. I., Charles, J. A.: Effect of carbon content on hydrogen occlusivity and embrittlement of ferrite–pearlite steels. Journal Mater. Science and Technology 2(9), 956–962 (1986) . Gabetta, G., Nykyforchyn, H., Lunarska, E., Zonta, P. P., Tsyrulnyk, O. T., Nikiforov, K., Hredil, M. I., Petryna, D. Yu., Vuherer T., 2008. In service degradation of gas trunk pipeline X52 steel. Materials Science 44 (1), 88–99. Gredil, M.I., 2008. Operating degradation of gas-main pipeline steels. Metallofizika i Noveishie Tekhnologii 30, 397–406. Hredil, M., Tsyrulnyk, O., 2010. Inner corrosion as a factor of in-bulk steel degradation of transit gas pipelines. Proc. of the 18th European Conference on Fracture (ECF-18), Dresden, Germany. Manuskript No.483.