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

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Fig. 1. Scheme showing changes of material properties depending on time of service for materials with different initial properties and degradation rate.

However, the method does not take into account hydrogen influence on metal during long-term service. Therefore hydrogen assisted degradation of pipelines steels under operation calls for more effective methods for in laboratory accelerated degradation. Namely, the studies carried out by Zvirko et al. (2016), Bolzon et al. (2017a, 2017b, 2018) and Tsyrul’nyk et al. (2018) showed that in-service degradation of pipeline steels can be induced in laboratory by subjecting the hydrogenated steel to strain ageing and it enables to predict susceptibility of operated pipeline steels to stress corrosion cracking. The present study is devoted to analysis of the role of hydrogen in degradation of gas pipeline steels and the development of the procedure of laboratory simulation of in-service hydrogen assisted degradation of pipeline steels, developing research work performed by Tsyrul’nyk et al. (2018). 2. Objects, materials and methods The research objects were low-carbon ferrite-pearlite pipeline steels: 17H1S (0.17C-Mn- Si, API 5L Х52 strength grade) and API 5L X52 low-alloyed pipeline steels in different states – as-received and after long-term operation. Sections of pipes being investigated were cut from gas transit pipelines after different time of operation: 17H1S – 28 (pipe thickness t = 20 mm) and 31 ( t = 12 mm) years, and X52 – 30 years ( t = 10 and 12 mm, marked as X52-10 and X52-12, respectively). The 17H1S and X52 steel in the as-received state ( t = 12 mm) were used as reference material. Taking into account a possibility of aggressive influence of the condensed water, being accumulated on the pipe bottom, on the pipeline steel degradation, the top and bottom sections of the pipe, as well as, outer and inner pipe sections, were also distinguished in some cases. A series of specimens were tested: 1) in as-received state; 2) in aged state after in-laboratory artificial deformation aging; 3) in degraded state after in-laboratory accelerated degradation. Sets of the samples of the investigated pipeline steels with size of 6 × 10 × 220 mm were cut out in the longitudinal direction from pipe spools and subjected to artificial deformation aging or accelerated degradation procedures. Artificial deformation aging procedure was performed according to GOST 7268-82. The steel samples were subjected to an axial loading up to the axial strain of 5% and 10% (two series of steel samples) followed by heating at 250ºС for 1 hour. Accelerated degradation procedure was based on influence of combination of hydrogen and deformation aging on steel. The following procedure was used for simulating of degradation. The steel samples were electrolytically hydrogen pre-charged in an aqueous solution of sodium hydroxide (0.1 N) at current density of 5 0 mА/сm 2 for 100 hours at a temperature of 70°С before the specimens were subjected to an axial loading up to a value of the axial