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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fracture features on the micro scale: near the side surface and near the central part. The first zone was associated with influence of “external” hydrogen action due to SCC process while the second one – with “internal” hydrogen, absorbed by metal during its preliminary hydrogenation. The dominating role of shear processes during SCC test on the fracture surfaces near the side surfaces of the specimens from both steels in the as-received state (Fig. 6a, Fig. 7a) was confirmed. The ductile mechanism with formation of equiaxial dimples relief formed by microvoids coalescence was also observed in the central part of specimens (Fig. 6b, Fig. 7b). In general, significant corrosive environment effect on the fracture mechanism for the as-received state steels after SCC testing was not found. This is consistent with the mechanical tests results, when the effect of the corrosive environment on the mechanical characteristics did not exceed a few percent (Fig. 5).

Fig. 6. Microfractograms for the 17H1S steel in as-received state (a, b) and after accelerated degradation (c-f), obtained in centre part of specimen fracture surface (b, e, f) and near the side surface (a, c, d) after SCC testing.

The fractographic analysis of fracture surface of the 17H1S steel specimen subjected to the accelerated degradation with following of SSRT test in NS4 solution revealed the differences of the fracture mechanism within both zones of fracture surface. Thus, these differences were observed in the zone beside the specimen lateral surface due to SCC initiation (Fig. 6c, d), and in the middle of the fracture surface of specimen (Fig. 6e, f). It was confirmed that the fracture from the external surface of the specimens occurred by corrosion cracking along the boundaries of ferrite and pearlite grains with secondary deep intergranular cracks. In the case of favourable orientation of the ferrite and cementite lamellae inside pearlite grains, delamination along the lamellae interfaces was also observed. At the macro scale within the central part of the fracture surface of the degraded 17H1S steel after SCC test, the ductile relief with big round areas on the background of the relief with small dimples was observed (Fig. 6e). The brittle fracture features on the bottom of these round fragments were revealed at higher magnification. There were clearly identified facets of intergranular cracking with secondary deep cracking along interfaces of the ferrite and pearlite grains (Fig. 6f) at higher resolution of SEM photograph. The interfaces between the ferrite and cementite lamellae inside pearlitic grains were distinguished on the fracture surface, which could be due to cracking along their interfaces. The similar brittle transgranular elements on the background of the dominated ductile fracture were earlier observed in the central section of the fracture surfaces of pre-hydrogenated smooth specimens from the heat resistant steels of steam pipelines after more than 20 years exploitation tested in air, as it was demonstrated by Student (2006), Nykyforchyn et al. (2007). These brittle elements were linked with local facilitation of brittle fracture of steel under the influence of hydrogen accumulated inside damages during long-term exploitation. The mechanisms of hydrogen influence on the heat resistance steels were analysed by Taylor et al. (2009) and Krechkovs’ka (2016). In the studied case the hydrogen promoted the damages formation during the accelerated degradation of the pipeline steel. Grain boundaries are favourable trapping sites for the accumulation of hydrogen, so intergranular cracking within the brittle elements may be associated with this phenomenon.