PSI - Issue 16

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

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Fig. 4. Microfractograms of the specimen fracture surfaces of the API X70 (a-c) and API X60 (d) steels in as-received state (a) and after long term operation (b-d) after tensile testing in air.

3.2. SCC tests SCC resistance and SCC fractographic features of two pipeline steels, 17H1S and API X60, in the as-received state and after the accelerated degradation were analysed. Comparison of the results obtained by tensile testing in air and by SSRT in the NS4 solution showed that the strength and plasticity characteristics of both steels in the as received state are insignificantly changed under the influence of the test environment (Fig. 5). At the same time, the artificial degraded steels were characterized by a high sensitivity to the environment (the significant increase of σ YS was caused by deformation hardening during the preliminary plastic deformation of specimens in accordance to the procedure of accelerated degradation and was not connected with the environment effect). Moreover, environmental sensitivity of the degraded 17H1S steel was higher in comparison with the degraded API X60 steel. In particular, the plasticity characteristics of the degraded 17H1S steel under the test environment effect was decreased more than that of the degraded API X60 steel.

Fig. 5. The effect of the NS4 solution on the mechanical characteristics P of the 17H1S and API X60 pipeline steels in the as-received state and after the accelerated degradation in comparison with that in air: λ = ( P NS4 -P air )/P air ∙ 100%.

The mode of fracture of the degraded 17H1S and API X60 steel specimens observed at macro scale was predominantly ductile. The fracture of specimens occurred by the classic scenario: fracture by cup and cone type through the necking formation. Two zones on the fracture surfaces of specimens were distinguished for study of the