Кафедра "Зварювання"

Formation of creep pores and their transformation into creep cracks in the metal of welded joints of steam pipelines made of heat-resistant pearlite steels (12Kh1MF and 15Kh1M1F), with the long-term operation (over 270,000 h) under creep and low-cycle fatigue conditions (via repeated start-stop modes), largely depends on the structural and phase state of the welded joints. Welded joints are characterized by structural heterogeneity caused by welding heat. The heterogeneity increases with their further service life. This heterogeneity in the metal of welded joints is much greater than the heterogeneity of the base metal of the steam line, which has not been subjected to welding heat. In long-term operation, the structure of steam lines and their welded joints is transformed into a ferritic-carbide mixture at different rates. The rate of this transformation in the metal of welded joints is much higher than in the base metal of steam lines. It was found that the intensity of damage to the metal of welded joints by pores and creep cracks largely depends on the level of transformation of the initial structure into a ferritic-carbide mixture. It was found that creep pores are mainly formed along the boundaries of ߙ-phase grains and, to a lesser extent, along the grain body. We studied the peculiarities of physical and chemical processes occurring in the metal of welded joints of steam pipelines. We established a link between the self-diffusion of alloying elements (chromium, molybdenum, and vanadium) and the movement of dislocations. It was found that pore nucleation in the metal of welded joints occurs mainly at the boundaries of ferrite and bainite grains, where the coagulating carbides M23C6 and M7C3 are located rather than along the grain body. In the process of diffusion movement, vacancies, by merging, are transformed into microcontinuities that move in the direction of vacancy movement. It was found that the formation of creep pores in the metal of welded joints largely depends on its structural and phase state. Creep pores are most intensively formed in areas where the structure consists of tempering bainite, ferrite, and coagulating globularized pearlite. It was found that with an increase in the metal deformation of the zone of thermal influence of welded joints, the formation of creep pores is accelerated.

The recrystallization process in the structure of welded joints of steam pipelines from heat-resistant pearlitic steels, operated for a long time under creep conditions (Te = 545…585 °С, Рe = 20…25 MPa), reduces their performance. Studying the peculiarities of the recrystallization process will allow slowing down its passage, which is important for increasing the service life of welded joints of steam pipelines, which are characterized by the presence of a certain structural heterogeneity.

Кафедра "Зварювання"

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