PSI - Issue 28

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 28 (2020) 896–902 H. Nykyforchyn, O. Tsyrulnyk, O. Zvirko, M. Hredil / Structural Integrity Procedia 00 (2019) 000–000

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Stage II is the so-called stage of dissipated damaging development, which is more dangerous with respect to the loss of structural integrity. Presenting a high degree of operational degradation, it is accompanied by untypical mechanical behaviour of the metal, mentioned by Gredil (2008): a) a decrease in strength and hardness, as well as a worsening of resistance to brittle fracture; b) an increase in elongation at a decrease of reduction in area; c) a premature deviation from the straight line at the stress-strain curve (not at the beginning of plastic deformation but due to the compliance increase caused by the development multiple microdamages during active loading). If metal accumulates hydrogen, then the development of these microdamages should be facilitated and, respectively, a probability of the above mentioned untypical mechanical effects rises. Sensitivity of mechanical characteristics of steels to in-service degradation is important for an assessment of degradation degree. The most significant result of in-service material degradation is a reduction of resistance to brittle fracture. Therefore, test conditions promoting embrittlement provide a more proper comparison of corresponding mechanical properties of steels in as-received state vs. after service state. These include the tests of specimens with notches or cracks, under impact loading and/or low temperatures, in corrosion environments either after preliminary hydrogenation or during the hydrogenation. The change in brittle fracture resistance of pipeline steels in a course of operation is illustrated in Fig. 2.

Fig. 2. Effect of operation time on the characteristics KCV , J I c and J 0,2 for a number of oil and gas pipeline steels (API 5L X52 and X60 strength grade): A – KCV value (total fracture energy); B – component of KCV indicating crack propagation energy.

It should be noted from Fig. 2 that parameters of fracture mechanics ( J -integral) J I c and J 0,2 , are more sensitive to material changes than impact toughness KCV ; moreover, the parameter J 0,2 defined as 0,2 mm crack increment is more sensitive in comparison with J I c parameter corresponded to crack start. Taking into account such peculiarity, Gabetta et al. (2008) and Hredil (2011) showed that it is possible to increase the sensitivity of Charpy tests using crack propagation energy instead of total energy of fracture. On the other hand, a susceptibility of operated steels to hydrogen embrittlement also increases, therefore, test conditions involving steel hydrogenation are preferable for an evaluation of in-service degradation of steels, the best thing is the analysis of crack growth under an action of hydrogenating media. 3. Assessment of the steel condition basing on hydrogen behaviour studies The statement about microdamaging as the main factor of degradation of long-term operated steels is consistent with the research results on the hydrogen behaviour in metal, obtained by Gabetta et al. (2008) and Nykyforchyn 1 et al. (2019) using methods of hydrogen permeation and extraction under different temperatures. Hydrogen diffusion coefficient and its concentration in metal are usually determined for prediction of its effect on structural integrity since hydrogen can be accumulated in the pre-fracture zone. However, last time the well-known research methods of