PSI - Issue 42

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

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3. Results and discussion Romaniv et al. (1998), Nykyforchyn et al. (2010), Hredil et al. (2009), Student 1 et al. (2021) stated that among mechanical properties of heat-resistant steels, the characteristics of brittle fracture resistance (namely, impact toughness, fracture toughness, fatigue crack growth resistance) are most sensitive to their operational degradation. Structure and phase state of these steels is transformed as a result of a prolonged action of high temperature (up to 540°С) and stresses in a pipe wall during operation. As described Baltu š nikas et al. (2007), Krechkovs'ka (2009), Ostash et al. (2009), Pietryka et al. (2010), such operational conditions facilitate diffusion and redistribution of alloying elements and carbon toward the grain boundaries as thermodynamically favourable sites for the precipitation of complexly alloyed carbides which weaken the cohesion between grains. Dzioba 2 (2010) and Balyts’kyi et al. (2010) noted that enlargement of grains and carbides along their boundaries due to creep additionally reduced the resistance to brittle fracture of degraded steels. Taking into account that structural elements and specimens of operated steels typically break in the section with the minimum resistance to fracture, most weakened by operational damages along the grain boundaries, it is expected that signs of cohesion weakening between adjacent grains will be visualized as intergranular fracture on the fracture surfaces of the tested steel specimens. Concerning the sensitivity to operational degradation, Krechkovska (2021) supposed that fractographic approaches to the assessment of operational damage are more appropriate than metallographic ones since metallographic studies, as a rule, are carried out on a randomly oriented surface of a section, on which the maximum damage density is almost never observed. Moreover, an increase in analyzed sections does not noticeably improve the sensitivity of the method. Assessment of the mechanical state of the steel 15H 1М1 F based on its resistance to brittle fracture. Mechanical testing revealed a clear decrease of all considered characteristics of the steel as a result of the long-term operation (Fig. 1): the more severe operational conditions (more shutdowns), the lower mechanical characteristics of the steel, despite almost the same operation time. Therefore, the distribution of thermal stresses in the cross section of thick walled pipes, calculated by Nykyforchyn et al. (2010), Krechkovska et al. (2019), significantly affects the intensity of steel degradation due to different temperature gradients under different pipeline cooling regimes during scheduled and emergency shutdowns of TPP units.

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Fig. 1. Brittle fracture resistance characteristics (a – KCV; b – K c ; c – Δ K th and Δ K th eff ) for the steel 15H 1М1 F in the initial (IS) and degraded states subjected to 501 (DS 1 ) and 576 (DS 2 ) shutdowns of the power units of TPP.

Fractographic signs of steel degradation on the fracture surfaces of Charpy specimens. There is a zone of ductile fracture with dimples in the vicinity of the notch. As expected, the width of this zone decreases with more intense steel degradation caused by more shutdowns. It is usually used as an indicator of the energy consumption for the specimen’s fracture . However, the main sign of steel degradation is revealed on micro level in the area of spontaneous fracture as intergranular facets clearly distinguished against the background of transgranular relief (Fig. 2 а ). Amount of intergranular facets is higher on the fracture surface of the steel with more shutdowns where intergranular fracture prevail (Fig. 2b). The occurrence of these areas is considered evidence of the cohesion weakening between adjacent grains, which have happened even at the stage of the long-term operation of steel in the steam pipeline, and the fracture of the specimens during impact testing just visualized them. Note that ductile relief dominates on the whole fracture surface of the specimens in the initial state. Thus, for impact specimens, the cohesion weakening between