I. V. Vlasov, S. V. Panin, P. O. Marushchak, A. V. Eremin, A. V. Byakov

INFLUENCE OF LONG-TERM OPERATION ON THE STRUCTURE AND IMPACT TOUGHNESS OF THE 09Mn2Si PIPE STEEL

The microstructure and mechanical properties of the 09Mn2Si steel used for construction
of the Mastakh-Berge-Yakutsk natural gas transportation pipeline are investigated comprehensively after 37 years of operation. The results obtained for the specimens cut out from the pipe are compared to those obtained using emergency stock (reference) specimens. It is demonstrated that long-term operation in harsh climate does change the microstructure appreciably. Strain aging, however, can be identified through precipitation of finely dispersed carbides in the bulk grains, and hydrogen charging may occur in the steel, which gives rise to partial decarburization. The latter causes some embrittlement of the steel, measured under impact loading. The revealed microstructural changes do not practically affect static tension or hardness properties. This can be attributed to the development of two competing processes: i) softening as a result of decarburization and ii) hardening due to pinning of dislocations on carbide precipitates. The microstructure degradation results in sharp reduction of impact toughness, which drops by a factor of two at temperatures below zero Centigrade. The decrease in crack resistance under impact bending is primarily related to service-induced embrittlement accompanied by a pronounced decrease in the maximum load under which the main crack initiates. The reasons and mechanisms of the observed phenomena are discussed.

Acknowledgments: The work was performed in the framework of the fundamental research Program of the Rus-sian State Academies of Sciences for 2013–2020, line of research III.23, with a partial support from RFBR Grants No. 18-08-00516_a, 18-38-00679 and RF President Council Grant for the support of leading research schools NSh-5875.2018.8. Impact bending tests were carried out at the National Research Tomsk Polytechnic University within the framework of the Competitiveness Enhancement Program of Tomsk Polytechnic University.

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