PSI - Issue 17

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 17 (2019) 568–575 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000

572

5

Nykyforchyn et al. (2008) and Gabetta et al. (2008), enabled comparing metal properties from top and bottom of the operated pipe and as-received metal as well. The observed differences in hydrogen content and corrosion damages on the top and the bottom of the operated pipes indicated corrosion and hydrogenating influence of residual water condensed from transported hydrocarbons on pipeline steel. Consequently, lower value of impact toughness was observed for the operated metal with higher hydrogen concentration compared to the as-received steel. A higher degradation degree of the steel of the bottom pipe section in comparison with that of the top pipe section seems to be caused by its hydrogenation during operation as a result of electrochemical interaction between steel and a condensed medium on internal pipe surface.

Table 2. Concentration of hydrogen, measured by hot extraction, and impact toughness of pipeline steels Pipeline steel Steel state Pipe section Concentration of hydrogen C H (ppm) Impact toughness KCV (J/cm 2 ) 10HS As-received - 1.6 180 10HS Operated 28 years Top 2.6 95 10HS Operated 28 years Bottom 3.4 - X52 As-received - 1.5 196 X52 Operated 30 years Top 2.4 77 X52 Operated 30 years Bottom 4.2 57

It is seen in Table 3 that hardness and impact toughness of the 17H1S pipeline steel near the internal surface of the pipe were degraded more intensively than that near the external one. It is obviously associated with higher hydrogen content in the steel near the inner surface of the pipe in comparison with that in the metal near the external pipe surface due to its hydrogenation during operation. Hydrogen charging of a metal leads to molecular hydrogen accumulation in certain trapping sites as a result of recombination of hydrogen atoms and to creation of high pressures in them. This causes a development of in-bulk damages at nano- and microscales (so called accumulated or dissipated damaging), which was demonstrated by Nykyforchyn et al. (2008, 2016). Moreover, in some cases, reported by Nykyforchyn et al. (2016, 2017), hydrogen assisted macrodelamination between fibres of structure can be formed in long-term operated gas pipeline steels due to steel hydrogenation.

Table 3. Mechanical properties of the 17H1S pipeline steel Steel state Pipe section Hardness HRB

Impact toughness KCV (J/cm 2 )

As-received As-received

Inner Outer Inner Outer Inner Outer

90 95 87 89 78 81

206 194 165 169 115 133

Operated 28 years Operated 28 years Operated 31 years Operated 31 years

Thus, two important factors of in-service degradation of gas pipelines, namely stress and hydrogenation, should be taken in consideration at developing the method for in-laboratory accelerated degradation. 4. Application of in-laboratory method for accelerated degradation of pipeline steels Following the experimental procedure described above, the mechanical properties experimentally determined for the X52 pipeline steel in different states are presented in Figures 2 – 4. To compare the influence of 30 years operation and applied procedures of artificial deformation aging and in laboratory accelerated degradation on the mechanical properties of the X52 steel, the changes in properties relative to the values for the as-received state steel are presented in Figures 3 and 4.