PSI - Issue 16

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 16 (2019) 153–160 Hryhoriy Nykyforchyn et al. / StructuralIntegrity Procedia 00 (2019) 000 – 000

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The specimen fracture surfaces of the 17H1S steel in as-received (Fig. 2a) and operated (Fig. 2b) states, inspected after tension tests, indicated a ductile mechanism with a forming of fracture surfaces by cup-cone type in both cases. At the same time the fracture surface of the operated steel was characterized by the clear features associated with texture of steel rolling which was not observed in the case of non-operated steel.

Fig.2. Macrofractograms of the 17H1S steel specimens in as-received state (a) and after 30 years of operation (b) after tensile testing in air.

Examination at a higher microscope resolution showed that the as-received 17H1S steel specimen was fractured by a typical ductile mechanism with a forming of dimples relief on the fracture surface (Fig. 3a). In the operated steel such elements as fairly long delaminations were found on the background relief of equiaxial dimples typical for the non-operated steel (Fig. 3b). An identical orientation and distance between adjacent delaminations confirmes their connection with a texture of metal. Similar delaminations practically did not find out on the fracture surface of the as-received steel, that is why they were considered as fractographic signs of dissipated damages formed in the steel during gas pipeline exploitation.

Fig. 3. Microfractograms of the central part of the specimen fracture surfaces of the 17H1S steel in as-received state (a) and after 30 years operation (b) after tensile testing in air.

The typical ductile relief formed by dimples was also detected by microfractographic investigation of the central parts of the fracture surfaces of both API X60 and API X70 steel specimens after tensile tests in air. The strips on the fracture surfaces with equiaxial dimples of open type were alternated with strips covered by parabolic dimples of shear type with the negligible quantity of micro delaminations (Fig. 4a). The peculiarity of the operated steels fracture consisted in appearance in a centre of fracture surface comparatively large and flat disc- like voids (the type of delaminations) with shallow (up to 1 μm in diameter) pores on their bottom (Fig. 4b). Such defects were considered as places of hydrogen accumulation of high pressure. The subsequent coagulation of the nearest pores under the influence of hydrogen accumulated in these shallow voids promoted to the formation of the large and flat disc-like voids. The numerous deep defects which initiated an appearance of delaminations were also observed (Fig. 4c, d). The surfaces formed around them with characteristic parallel traces formed by an exit of slip bands to their surfaces were considered as evidence of an existence of these defects yet before the tests, it means during operation on the pipeline. Thus, it is established that macrotexture of the fracture surfaces of operated steels with different strength and the deep microdelaminations in their central parts are features of dissipated damages in pipe walls associated with an existence of rolling texture and effect of hydrogen absorbed by metal during long-term operation.