National Research Center "Kurchatov Institute" – Central Research Institute of Structural Materials "Prometey", St. Petersburg, Russia:

A. S. Kudryavtsev, Cand. Eng., Head of the Laboratory – Deputy Head of the Research and Production Complex for Research, e-mail: kudryavtsev@yandex.ru
Yu. M. Markova, Leading Engineer, e-mail: yulia.markova@inbox.ru
D. A. Artemyeva, Leading Engineer, e-mail: stoma_87@mail.ru

Steel grade 10Cr2Mo is used for equipment operating at elevated temperatures at creep, the service life of which is ensured by the stability of the material properties. The most accurate assessment of the effect of operation on the properties of steel is obtained as a result of studying the metal fragments of equipment that has exposed at high-temperature for long time. In this paper the results of the study of mechanical properties, creep rupture strength, toughness and microstructure of steel grade 10Cr2Mo after operation at temperatures of 315 and 450 °C during 120 thousand hours as part of the evaporator of the steam generator of the BN-600 reactor plant are presented. The data obtained are compared with the properties of steel in the state of delivery, and in the initial (before operation) state. The initial state of the steel was obtained by heat treatment corresponding to the state of delivery with an additional technological tempering simulating post-welding heat treatment. It is established that the passport data of the properties of steel in its initial state don’t contain information of changes in mechanical characteristics as a result of post-welding heat treatment. The decrease in the mechanical properties and impact strength of steel after operation as a result of changes in the structural and phase state of the material has been established. These changes are associated with the process of thermal aging of steel and are reversible after heat treatment. The data obtained indicate the inexpediency of using steel grade 10Cr2Mo as a structural material for steam generator of a high-power reactor with sodium coolant, however, it can be used to assess the possibility of increasing the specified service life of the evaporator module of the steam generator reactor BN-600.
Experimental studies were carried out on the equipment of the Center for the Collective Use of Scientific Equipment "Composition, Structure and Properties of Structural and Functional Materials" of the National Research Center "Kurchatov Institute" - Central Research Institute of Structural Materials "Prometey" with the financial support of the Ministry of Science and Higher Education – Agreement No. 13.TsKP.21.0014 (075 -11-2021-068). The unique identification number is RF----2296.61321X0014.

1. Karzov G. P., Kudryavtsev А. S., Markov V. G., Grishmanovskaya R. N., Trapeznikov Yu. М., Ananyeva М. А. Development of structural materials for fast neutron nuclear power plants with sodium coolant. Voprosy materialovedeniya. 2015. No. 2. pp. 23–33.
2. Chaudhuri S., Ghosh R. N. Creep behavior of 2.25Cr1Mo steel – Effects of thermal ageing and pre-strain. Materials Science and Engineering: A. 2009. Vol. 510–511. pp. 136–141.
3. Kudryavtsev А. S., Okhapkin К. А. The effect of long-term thermal aging as being a part of heatexchange equipment of a fast neutron reactor on the structure and properties of austenitic chromium-nickel steel. Izvestiya vuzov. Chernaya metallurgiya. 2018. Vol. 61. No. 11. pp. 907–913.
4. GOST 1497–84. Metals. Methods of tension test. Introduced: 01.01.1986.
5. GOST 9651–84. Metals. Methods of tension tests at elevated temperatures. Introduced: 01.01.1986.
6. GOST 9454–78. Metals. Method for testing the impact testing at low, room and high temperature. Introduced: 01.01.1979.
7. Norms for calculating the strength of equipment and pipelines of nuclear power plants (PNAE G-7-002–86). Moscow : Energoatomizdat, 1989. 525 p.
8. Ruisi Xing, Dunji Yu, Guofu Xie, Zhihai Yang, Xuxin Wang, Xu Chen. Effect of thermal aging on mechanical properties of a bainitic forging steel for reactor pressure vessel. Materials Science & Engineering: A. 2018. Vol. 720. pp. 169–175.
9. Tenneti Sharma, Sunil Kumar Bonagani, Naveen Kumar N., Harish Donthula et al. Effect of thermal aging on embrittlement of Cr–Mo–V pressure vessel steel. Journal of Nuclear Materials. 2019. Vol. 527. pp. 136–141.
10. Rui Si Xing, Xu Chen, Dun Ji Yu. Evolution of impact properties of 16MND5 forging for nuclear reactor pressure vessel during thermal aging at 500 °C. Key engineering materials: A. 2019. Vol. 795. pp. 54–59.
11. GOST 5639–82. Steels and alloys. Methods of reveal and determination of grain size. Introduced: 01.01.1983.
12. Margolin B. Z., Gulenko А. G., Buchatskiy А. А., Nesterova Е. V., Kashtanov А. D. Study of the effect of thermal aging on the long-term strength and ductility of Kh18N9 steel. Voprosy materialovedeniya. 2010. No. 4. pp. 118–127.
13. Ivanı´ de Souza Bott, Luı´s Felipe Guimara˜es de Souza, Jorge Carlos Ferreira Jorge et al. Comparison between real and simulated degradation in a 1.25% Cr–0.5% Mo steel for high temperature service. Materials Characterization. 2005. Vol. 54. pp. 206–215.
14. Gurovich B. А., Erak D. Yu., Zhurko D. А., Kuleshova Е. А., Maltsev D. А., Sandler V. Yu., Frolov А. S. Restorative annealing to extend the service life of the vessels and internals of operating VVER-1000 reactors. International Scientific and Practical Conference on Nuclear Energy "Safety, Efficiency, Resource". Sevastopol, October 3–6, 2017. pp. 74, 75.
15. Changheui Jang, Hun Jang, Sunghoon Hong, Jae Gon Lee. Evaluation of the recovery of thermal aging embrittlement of CF8M cast stainless steels after reversion heat treatments. International Journal of Pressure Vessels and Piping. 2015. Vol. 131. pp. 67–74.
16. Goldshteyn М. I., Grachev S. V., Veksler Yu. G. Special steels. Moscow : Metallurgiya, 1985. — 408 p.
17. Senthilkumar T., Ajiboye T. K. Effect of heat treatment processes on the mechanical propereties of Medium Carbon Steel. Journal of Minerals and Materials Characterization and Engineering. 2012. Vol. 11. No. 2. pp. 143–152.