PSI - Issue 42

Halyna Krechkovska et al. / Procedia Structural Integrity 42 (2022) 1398–1405 Halyna Krechkovska / Structural Integrity Procedia 00 (2022) 000 – 000

1404

7

100

I II III

80

60

40

S, %

20

0

DS 1 DS 2

DS 1 DS 2

DS 2

IS DS 1

IS

IS

Fig. 5. The part of intergranular fragments on the unit area of the fracture surfaces of the steel specimens in the initial (IS) and degraded states subjected to 501 (DS 1 ) and 576 (DS 2 ) shutdowns of the power units of TPP after fatigue tests (I), fracture toughness (II) and impact (III) tests. These intergranular fragments were considered the main signs of the steel degradation. They indicated in-service structural changes with precipitation and coagulation of complexly alloyed carbides along the grain boundaries, their separation from the ferrite matrix with the formation of micro pores and further stretching of partitions between them until the formation of intergranular damages. Their amount and distribution in the steel led to the formation of the 3D surface, following the path with the minimum fracture resistance and thus involving the maximum number of in-service intergranular damages in the zone whose width corresponded to the width of the pre-fracture zone under corresponding types of tests. The established features of the fractographic relief transformations of the steel surface depending on the operational conditions are important for understanding the nature of the degradation processes and quantifying its structural and mechanical state. Conclusions Based on the results of brittle fracture resistance evaluation of the steel 15H1M1F in the initial state and after about 2·10 5 h of operation on the steam pipelines of TPP with 501 and 576 shutdowns of power units, the impact of operational conditions on the steel degradation has been estimated. The sensitivity of various types of mechanical tests for estimating the degree of in-service damages has been shown. The signs of in-service damages were visualized in the form of intergranular fragments clearly distinguishing on the background of the the transgranular areas of spontaneous fracture or near-threshold fatigue crack growth on the fracture surfaces of the specimens after impact tests, fracture toughness and fatigue crack growth resistance evaluations. These intergranular fragments were considered the key feature of the steel degradation caused by structural changes with precipitation and coagulation of carbides along the grain boundaries, their separation from the matrix with the formation of pores and stretching of partitions between them until their fracture with the formation of intergranular cracks. It is supposed that fragments of intergranular fracture found in the near-threshold region of fatigue crack growth are associated with intergranular damages (complete separation of adjacent grains) formed as a result of creep during long-term operation of the steel. Besides, the part of the intergranular fragments observed on the fracture surfaces of specimens after impact and fracture toughness tests was also associated with boundaries weakened by formed pores along them, which reduced cohesion between adjacent grains and facilitated fracture along these boundaries. The obtained results make it possible to recommend the area of intergranular relief on the unit fracture surface of the operated heat-resistant steel as an effective quantitative indicator of its structural-mechanical state. Acknowledgements This research has been partly supported by the National Academy of Sciences of Ukraine under the Project 12/01-2022(3) provided to research groups of young scientists.