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. 7. Microfractograms for the API X60 steel in as-received state (a, b) and after accelerated degradation (c-f), obtained in centre parts of specimens fracture surface (b, e, f) and near the side surface (a, c, d) after SCC testing.

The fracture mechanism of the API X60 steel specimen, subjected to accelerated degradation and subsequent SCC testing, was somewhat different (Fig. 7). In addition to the typical parabolic dimples in the conical part of specimen fracture surface of the as-received API X60 steel formed by the shear mechanism, the deep round pits were observed in the same zone of fracture surface of the degraded steel (Fig. 7c). At higher resolution the fragments of decohesion along the interfaces between ferrite and pearlite grains with elements of ductile failure of bridges between them with formation of equiaxial dimples were clearly identified on the bottom of these pits (Fig. 7d). No similar signs were revealed on the non-degraded steel tested in air (Fig. 7a). On this basis, the identified fragments were considered as defects arisen under the influence of hydrogen in the process of accelerated degradation of the steel. The fracture mechanism in the central zone of fracture surface of the degraded API X60 steel was generally similar to that of the 17H1S steel (Fig. 7e, f). However, the fracture mechanism of the bridges between damages along ferrite and pearlite interphase was different. In the degraded API X60 steel these bridges were ductile fractured with formation of equiaxial dimples. Transgranular fracture inside pearlite grains due to stretching of ferrite lamellae up to fracture was also observed (Fig. 7f). This demonstrates that interlamellae decohesion inside favourably oriented pearlite grains under the influence of hydrogen at accelerated degradation of this steel could also occur. Moreover, this fracture features explain a more ductile behavior of the API X60 degraded steel than that of the 17H1S steel. 4. Conclusions Analysis of mechanical behaviour of in-service degraded gas pipeline steels with different strength revealed that the 17H1S steel with the lowest strength among the studied steels exhibited the highest degradation degree caused by the long-term operation; it was associated with rolling texture and delaminations at the micro level. No sensitivity of the as-received 17H1S and API X60 steels to SCC in the NS4 solution was revealed; resistance to SCC of both steels was significantly decreased after the accelerated in-laboratory degradation simulating in-service degradation. Common fractographic feature on the micro scale namely, microdelamination, for the operated and in-laboratory degraded pipeline steels was observed, which indicated the similar mechanism of degradation in both cases and important role of hydrogen in degradation. The significant decrease of plasticity of both steels at SCC testing was observed after degradation in laboratory conditions only. It is attributed to cracking along the boundaries of ferrite and pearlite grains, formation of deep secondary intergranular cracks and delaminations between ferrite and cementite lamellae inside pearlite grains as well. The comparison of fractographic features observed within the conical zones and in central parts of the specimen fracture surfaces of the in-laboratory degraded 17H1S and API