PSI - Issue 68

H. Nykyforchyn et al. / Procedia Structural Integrity 68 (2025) 861–867 H. Nykyforchyn et al. / Structural Integrity Procedia 00 (2025) 000–000

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Lines 1 and 2 in Fig. 2 illustrate the effect of ex-situ hydrogen charging on the appearance of the J – R curves up to crack extension close to 0.5 mm, obtained under displacement rate v = 0.5 mm/min. Hydrogenation led to a reduction in fracture toughness J cr of both steel states; this is already significant after moderate hydrogenation ( i cat = 0.05 mА/сm 2 , lines 1 in Fig. 2). It should be noted that the steel in the as-delivered steel didn’t reveal hydrogen susceptibility under SSRT testing (Zvirko et al., 2024) In general, the hydrogenation effect on fracture toughness is more pronounced after hydrogen charging of higher intensity. However, the increase in cathodic current density by 20 times (from 0.05 to 1.0 mА/сm 2 causes only a slight decrease in J values (lines 1 and 2 in Fig. 2). The inclination of the J – R curves to the Δ а axis, the parameter β, is an important indicator of the energy capacity of crack extension. These dependencies are linear in the considered Δ a range (0.0 – 0.5mm), which can be used for obtaining the value J 0 characterising the crack start, i.e. for Δ a = 0. The parameter β thus can be calculated as ( J cr – J 0 ) / 0.2. The crack growth resistance parameters evaluated under the displacement rate of 0.5 mm/min are presented in Table 1.

Table 1 – Crack growth resistance parameters for the tested steel states depending on ex-situ hydrogen charging intensity Pipe steel state i cat [mА/cm 2 ] J cr [N/mm] J 0 [N/mm] β [N/mm 2 ]

0

215.0 117.2

90.2 85.4 62.0 75.0 53.1 37.9

624 159 131 165 113

As-delivered

0.5 1.0

88.2

0

108.0

Operated for 38 years

0.05

75.6 53.8

1.0

80

It is obvious that hydrogenation less affected J 0 than J cr for the as-delivered steel. At the same time, distinctions in the hydrogen effect on these two parameters are less significant for the operated steel. It might indicate a facilitated crack start in the operated metal. 3.2. The effect of the displacement rate on crack growth resistance parameters Figure 3 contains the results obtained at basic (0.5 mm/min) and lower displacement rates (0.05 mm/min and 0.005 mm/min). It is accepted that crack growth resistance in air is independent of displacement rate. This allows us to consider the hydrogen impact as a factor of the changes in fracture toughness and other related fracture mechanics parameters with the displacement rate.

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a

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c

Figure 3. Dependencies of J cr (a) , J 0 (b) and β (c) on the displacement rate for steel in the as-delivered state ( 1 , 2 ) and after operation for 38 years ( 3 , 4 ) tested after hydrogen charging under i cat = 0.05 mА/сm 2 ( 1, 3 ) and 1.0 mА/сm 2 ( 2, 4 ).