PSI - Issue 13

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 13 (2018) 1215–1220 Hryhoriy Nykyforchyn / Structural Integrity Procedia 00 (2018) 000 – 000

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the fracture surface of the steel to its in-service degradation compared with properties measured on the polished metal surface. In this case, in particular, sensitivity of potential Е fr of the fracture surface of the X52 pipeline steel to its in-service degradation was higher than that of potential E pol of polished steel surface. Comparison of the results obtained by measuring of potential of the as-received X52 pipeline steel on the polished surface and on the fracture surface showed that the difference between their values is insignificant (Fig. 4). However, value of potential Е fr of the fracture surface was slightly more negative relative to value of potential E pol of polished steel surface (approximately 10 mV only). This difference can be explained by influence of the metal deformation on potential due to the fact that the metal near fracture surface is deformed in contrast to the polished steel. This is an important result because it indicates effect of metal deformation on its potential value. Nevertheless, the difference between potentials E pol and Е fr for post-operated pipeline steels is significant: 67 mV for the X52-12 steel and 88 mV for the X52-10 steel, correspondingly (Fig. 4, Table 1). The investigated steels were operated during 30 years and their degradation degree was characterized by impact toughness KCV values. According to the data presented in Table 1 a higher degradation degree of the X52-10 steel in comparison with that of the X52-12 steel was revealed. It should be noted that the X52-10 steel was characterized by a more negative shift of potential compared with the as-received X52 pipeline steel than it was observed for the X52-12 steel.

Fig. 3. Time dependencies in 0.3% NaCl solution of potential Е fr of the fracture surface of specimens made of the X52 pipeline steel in the as received condition ( 1 ) and after 30 years of operation (Х52 -12 ( 2 ) and Х52 -10 ( 3 )) fractured after impact toughness testing.

Fig. 4. Potential E in 0.3% NaCl solution of the polished steel surface E pol ( 1 ) and that of the fracture surface E fr ( 2 ) of specimens made of the X52 pipeline steel in the as- received state (Х52) and after 30 years of operation (Х52 - 12 and Х52 -10) fractured after impact toughness testing.

Consequently, in-service degradation of ferrite-pearlite pipeline steels was accompanied by a sharp shift in open circuit potential of the fracture surface (brittle fracture) of specimens after impact toughness tests compared with that of the polished steel surface. Taking into account the obtained dependence E pol = f (C C ) (Fig. 2), this difference in potentials was explained by the increase in carbon content (due to precipitation of carbides) directly on the fracture surface, that is fracture surface was enriched by carbon compounds. This fact can serve as a proof of assumption that long-term operation of pipeline steel caused its embrittlement due to precipitation of carbide nanoparticles not only at grain boundaries, but also at intragranular defects. It should be noted that reduction in brittle fracture resistance of