PSI - Issue 59

Roman Hrabovskyy et al. / Procedia Structural Integrity 59 (2024) 112–119 Roman Hrabovskyy et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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Table 4. Critical values of failure pressure for pipes of main pipelines with crack-like defect.

Type of defect Corrosion pit ( a / c ) cr =1/2.5

Operation time, years

Steel

Corrosion furrows ( a / c ) cr =1/25

( a / t ) cr 0.725 0.670 0.645 0.692 0.632 0.595

2 c cr , mm

p f , MPa

( a / t ) cr 0.547 0.494 0.480 0.514 0.473 0.442

2 c cr , mm

p f , MPa

0

43.5 39.5 37.7 54.3 49.0 45.8

9.45 9.24 8.56

328.2 291.5 280.8 403.5 366.6 340.3

6.45 6.84 6.46 9.49 9.81 9.87

17H1S

30 45

0

13.38 13.07 12.61

28 42

10H2BТ

In general, the obtained results of computational and experimental studies prove that the criterial evaluation of the fracture conditions using fracture mechanics approaches is more conservative than the alternative approach, which is based on the evaluation of the failure pressure of the defective pipe and, depending on the operation time, provides an additional safety margin of strength in the 20 – 33% interval. It should be noted separately that the analysis of mechanical characteristics changes given in Tables 2 and 3 indicates a non-monotonic change over time in the plasticity characteristics and the parameter σ YS /σ UTS which characterizes the ratio of the strength characteristics. This trend does not reflect the real plasticity reserve of the investigated pipe steels. Such features, associated with an ambiguous change in indicators, are explained by the development in steel of gas main pipelines under long-time operation of isolated damage in the pipe wall metal (Bolzon et al. (2021)). It is grounded (Capelle et al. (2009); Dmytrakh (2011); Dmytrakh et al. (2018); Syrotyuk and Dmytrakh (2014, 2015)) that hydrogen absorbed by the metal during long-term operation is the main factor in the intensification of this damage. Moreover, the mechanical stresses which correspond to the gas working pressure in the pipe of the main gas pipeline significantly intensify the hydrogen charging (Dmytrakh et al. (2021)). In connection with the above the change in hydrogen concentration depending on the long-term operation time of gas pipelines was studied. The volume concentration of hydrogen in samples weighing approx. 5.0 g, cut out from pipe fragments in the axial direction, was determined using a LECO DH603 analyzer of the diffusion-mobile and residual hydrogen (Dmytrakh et al. (2021). The results of determining the concentration of the residual hydrogen are shown in Table 5. Table 5. Change of hydrogen concentration in 17H1S and 10H2BТ steels for pipes of main pipelines under long-time operation (without preliminary hydrogen charging). Steel Operation time, years C H , ppm Steel Operation time, years C H , ppm The paper presents the results of researches of hydrogen charging the electrochemical method in the environment of a deoxidized special solution NS4 Capelle et al. (2009) of low- alloyed pipeline steels of the АРI grade: Х52 (σ UTS = 528 MPa, σ YS = 410 MPa) and Х70 (σ UTS = 712 MPa, σ YS = 590 MPa), which are similar in structure and mechanical characteristics to steels 17H1S and 10H2BТ . At the same time, using the methodology presented in (Dmytrakh et al. (2021), the critical concentration of hydrogen cr H C was determined for pipeline steels X52 ( cr H 4.3ppm C  ) and X70 ( cr H 2.3ppm C  ). The analysis of the obtained results of experimental studies and literary data allows to draw the following conclusions:  The maximum concentration of hydrogen in 17H1S and 10H2BТ steels decreases with the growth of strength and plasticity limits; a similar trend is observed for X52 and X70 steels;  The residual hydrogen concentration in 17H1S and 10H2BТ steels is 14.9 and 9.6 times lower than the hydrogen critical concentration for similar pipeline X52 and X70 steels; 17H1S 0 0.072 0.203 0.288 10H2BТ 0 0.060 0.191 0.239 30 45 28 42