PSI - Issue 28

Myroslava Hredil et al. / Procedia Structural Integrity 28 (2020) 1204–1211 6 Myroslava Hredil , Halyna Krechkovska, Oleksandra Student, Oleksandr Tsyrulnyk / Structural Integrity Procedia 00 (2019) 000–000

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as the signs of steel embrittlement due to degradation, namely, numerous and various in size delaminations (indicated in red in Fig. 5a–c) with smooth unstructured relief, typically with chains of small pores at the bottom. Microvoid coalescence was observed preferentially in the fracture surface areas separating the nearest delaminations. Besides delaminations, some rounded fragments of cleavage were detected against the background of the ductile relief of the steel 17H1S after 51-year operation (Fig. 5d). Both mentioned brittle fracture elements were considered as features of operational degradation of pipe steels. Hydrogen absorbed by the metal during its operation and accumulated in defects along ferrite-pearlite boundaries of their texture, facilitated the development of delaminations, and also promoted the nucleation of transgranular cleavage in the critically degraded steel (Fig. 5d). It should be noted that similar fractographic features of embrittlement have been found by Kharchenko et al. (2017), Nykyforchyn 1 et al. (2019), Krechkovska (2016) and Hredil et al. (2019) for other ferrite-pearlite pipe steels after their operation.

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Fig. 5. Fracture surfaces of Х70 (а), Х60 (b) and 17H1S (c, d) steels operated for 37 years (a), 25 (b), 30 (c) and 51 (d) years after the Charpy impact testing.

Kosarevych et al. (2013) have elaborated the software which enables processing and quantifying fragments of fractographic images to determine the area of the described brittle fracture elements (delamination and cleavage) S cr on the unit area S of the fracture surface of impact specimens . The ratio S cr / S = α has been introduced as the quantitative fractographic parameter of steel operational degradation. Relationship between mechanical and fractografic indicators of the technical state of operated steels. Comparison of the mechanical properties regarding their sensitivity to operational degradation allows concluding that impact toughness is much more sensitive to in-service changes in material than strength and plasticity, so it was chosen for the following material’s characterization. Thus, a correlation was built between the ratio KCV op / KCV in for the analysed steels and corresponding values of the proposed fractographic parameter α (Fig. 6). The established relationship alpha vs relative changes in KCV parameter allows ranking operated pipeline steels according to their current state (degree of their embrittlement) regardless their strength, time of operation and service conditions. As depicted in Fig. 6, the points attributed to different steels fit quite well into one curve.