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

6

573

Fig. 2. The ratio σ Y /σ UTS for the Х52 steel in the different states: in the as-received state (1), after deformation aging with applied strain of 5% (2) and 10% (3), after 30 years of operation (4), after in-laboratory accelerated degradation with applied strain of 5% (5) and 10% (6).

Testing of the aged steel revealed that artificial deformation aging caused strong strain hardening, which results in exceeding the maximum allowable level of the ratio of σ Y /σ UTS (Fig. 2), the important parameter characterised a reserve of metal plasticity. At the same time, a significant decrease in relative elongation was observed (Fig. 3a), which does not correspond to changes in the parameter revealed for the operated metal in the previous research by Gabetta et al. (2008), Nykyforchyn et al. (2010), Meshkov et al. (2015) and Zvirko et al. (2018), where dissipated damaging as one of the main mechanism of in-service degradation was considered. Therefore reduction in area was defined to be preferable as assessment of operated metal plasticity. Moreover, artificial deformation aging did not have a significant effect on reduction in area (Fig 3b), which was in disagreement with the results of the referenced papers.

a

b

Fig. 3. Changes in elongation δ (а) and reduction in area RA (b) for the Х52 steel in the different states relative to the as-received state: after deformation aging with applied strain of 5% (1) and 10% (2), after 30 years of operation (3), after in-laboratory accelerated degradation with applied strain of 5% (4) and 10% (5). Comparison of the results obtained by impact toughness testing of the X52 steel specimens in different states showed that the impact toughness value for the operated steel is about only 40% of that for as-received steel (Fig. 4), which corresponds to the data concerning a significant decrease in impact toughness of operated pipeline steels compared with as-received state reported by Kotrechko et al. (2004), Tsyrul'nyk et al. (2004) and others. It is due to intensive in-service embrittlement of the post-operated metal. The influence of deformation aging on impact toughness of the studied steel is markedly reduced in comparison to the effect of operation, even in the case of