PSI - Issue 13

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 13 (2018) 1215–1220 Hryhoriy Nykyforchyn / Structural Integrity Procedia 00 (2018) 000 – 000

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metal caused by its long-term operation should be expected for both intergranular and transgranular fracture modes under such mechanism of ferrite-pearlite pipeline steel embrittlement during operation. Moreover, using the dependence E pol = f (C C ) (Fig. 2), it is possible to predict carbon content on the fracture surface of the operated steel. In particular, based on the research results, carbon content C C for the X52 pipeline steel after 30 years operation was approximately estimated as 1.8% and 2.3% for Х52 - 10 and Х52 -12 steels, respectively (Fig. 2). Significant differences in the carbon content on the fracture surface of operated steels compared with the carbon content on the polished steel surface (approximately an order of magnitude) were predicted by the analysis using the dependence E pol = f (C C ). These results are confirmed by data of impact toughness testing (in several times decrease in impact toughness of the operated steels compared with the as-received steel). The obtained results revealed that potential of fracture surface of operated steels of transit gas pipelines is enough sensitive to their in-service degradation. Further research will be focused on a seeking an acceptable correlation between changes in mechanical (impact toughness) and electrochemical (open-circuit potential) characteristics of steels caused by long-term operation. A new scientific and methodical approach to diagnostics of in-service degradation of long-term operated pipeline steels by the electrochemical method has been developed. It is based on determination of electrochemical properties of fracture surface of steel specimen obtained under brittle fracture testing as informative features of operated metal condition. In-service degradation of ferrite-pearlite pipeline steels was accompanied by a sharp shift in open-circuit potential of the fracture surface (brittle fracture) of specimens after impact toughness tests compared with that of polished steel surfaces. A significant difference between potentials of the fracture surface and the polished steel surface (over 60 mV in 0.3% NaCl solution) of specimens made of ferrite-pearlite pipeline steels observed after their long-term operation was evidently due to the increased content of carbon compounds on the fracture surface. Mechanism of ferrite-pearlite pipeline steels embrittlement under operation consisted in carbides enrichment not only grain boundaries, but also intragranular defects, has been revealed, as it is indicated by an increase of carbon content on transgranular fracture surfaces determined electrochemically. 4. Summary

Acknowledgements

The research has been supported by the NATO in the Science for Peace and Security Programme under the Project G5055.

References

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