PSI - Issue 26

Olha Zvirko et al. / Procedia Structural Integrity 26 (2020) 219–224 Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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properties, considering that decrease in plasticity is often accompanied by material hardening. Therefore, instrumented indentation testing is reliable method for evaluation of degradation degree of pipeline steels at the stage of deformation aging, when there is an increase in hardness and strength characteristics and a decrease in plasticity and resistance to brittle fracture (Nykyforchyn et al. (2011)). However, a usage of indentation method can be limited if degradation degree of a tested metal is high, when the second degradation stage associated with dissipated damaging is dominated. At this stage a decrease in plasticity and brittle fracture resistance characteristics can also be accompanied by a decrease in hardness and strength. It should also be mentioned that toughness is not clearly reflected by indentation tests. The prospective newly developed method for evaluation of degradation degree of steels is an electrochemical method (Nykyforchyn et al. (2017a), Zvirko et al. (2019b)). It is based on correlation between relative changes in electrochemical and mechanical properties of pipeline steels caused by in-service degradation. Analyzing in-service degradation phenomenon of gas pipeline steels, it was revealed that both mechanical and electrochemical characteristics were deteriorated. A possibility of non-destructive evaluation of impact toughness of steel, being brittle fracture resistance characteristic regulated by normative documents (DSTU EN ISO 3183:2017 / API 5L, and others), based on changes in electrochemical behaviour caused by long-term operation, was analysed. The revealed correlation between changes in impact toughness and polarisation resistance is the basis of non-destructive electrochemical method for prediction of in-service degradation degree of pipeline steels. The method was verified for the low-carbon ferrite-pearlite API 5L strength grade X52 pipeline steels. It would be very prospective study concerning seeking similar electrochemical correlation for other mechanical characteristics, which prediction may be considered more important than impact toughness in engineering calculations, for example, fracture toughness. The main advantages of the non-destructive electrochemical method are that it enables quantitative evaluation of degradation degree of pipeline steels and prediction of brittle fracture resistance, namely impact toughness. The method is reliable for assessing the metal degradation degree at any stage of degradation, including the stage of dissipated damaging. In-service degradation of pipeline steels with different strength (API 5L X52, API 5L X60 and API 5L X70 strength grade) caused a deterioration of both mechanical properties and corrosion resistance. Brittle fracture resistance characteristics (impact strength and fracture toughness) can be considered as degradation indicators for evaluation of degradation degree of pipeline steels under operation. Polarization resistance was one of the most sensitive parameters to in-bulk steel degradation among electrochemical properties. Considering difference in mechanical behaviour of pipeline steels being operated under complicated operational conditions, some peculiarities of evaluation of degradation degree of pipeline steels, using non-destructive methods, were distinguished. Instrumented indentation testing is reliable method for prediction of operational changes in strength characteristics of metal being operated at the stage of deformation aging, when there is an increase in hardness and strength characteristics. However, a usage of indentation method can be limited if degradation degree of a tested metal is high, when the second degradation stage associated with dissipated damaging is dominated. Electrochemical method enables quantitative evaluation of degradation degree of operated pipeline steels and prediction of brittle fracture resistance, namely impact toughness, at any stage of degradation, including the stage of dissipated damaging. 5. Concluding remarks

Acknowledgements

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

References

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