PSI - Issue 77

Olha Zvirko et al. / Procedia Structural Integrity 77 (2026) 484–489 Olha Zvirko / Structural Integrity Procedia 00 (2026) 000–000

486

3

one reserve pipe (as-received state) and one that had been in service for 34 years at the Ukrainian natural gas transmission pipeline (post-operated state). The investigated steels had a microstructure which consisted predominantly of ferrite and pearlite. 2.2. Experimental Testing The mechanical behavior of the steel was studied through tensile, impact, and fracture toughness tests. Additionally, the material's resistance to hydrogen embrittlement was assessed. Specimens were extracted from the pipe sections oriented transversely to the pipe axis. Basic mechanical properties were determined through tensile tests in accordance with standard ISO 6892-1. Cylindrical tensile specimens with a diameter of 5 mm and a gauge length of 25 mm were used. The strain rate during testing was 3·10 − 3 s − 1 . Impact testing of V-notch Charpy specimens (10 mm × 10 mm × 55 mm, working cross-section 10 mm × 8 mm) was carried out according to the standard ISO 148-1. Notches were oriented along the pipe wall thickness. Slow strain rate tensile tests were performed at a strain rate of 3.3×10⁻ 6 s⁻¹ at room temperature to evaluate the sensitivity of the steels to hydrogen embrittlement. Cylindrical tensile specimens were hydrogen pre-charged electrochemically in H 2 SO 4 solution (pH1) with an addition of 10 g/l of thiourea (NH 2 ) 2 CS at a current density of 0.05 mA/cm 2 for 100 h at room temperature. Susceptibility to hydrogen embrittlement was assessed using the HES index, calculated by the following formula: The fracture toughness of the steel as the critical value of J -integral J cr under the crack increment Δ a = 0.2 mm was determined by the J -integral method following the ASTM E 1820 standard. A multiple-specimen method was employed for J -integral evaluation, testing a series of 5–7 specimens. Single-edge notched bending (SENB) specimens with dimensions of 4×15×100 mm for the X52 steel and 10×18×160 mm for the X67 steel were used. Specimens were hydrogen pre-charged electrolytically in a solution of H 2 SO 4 (pH1) + 10 g/l of thiourea at a cathode current density of i = 0.05 mA/cm 2 for 120 hours for the X52 steel and 100 hours for the X67 steel. A feature of the experiments was applying different displacement rates of specimens: the standard 0.5 mm/min and reduced to 0.005 mm/min. It was expected that a decrease in the displacement rate should contribute to the diffusion of hydrogen into the zone in the vicinity of the crack tip and enhance its effect. 3. Test Results and Discussion 3.1. The Influence of Operational Degradation of Pipeline Steels on Tensile Mechanical Properties and Impact Strength The basic mechanical properties and impact strength values of the investigated steels are summarized in Table 1. HES H RA 1 RA 100% = − ⋅ , (1) where RA H – reduction in area, determined in air for hydrogen pre-charged specimen and RA – reduction in area, determined in air.

Table 1. Mechanical properties experimentally observed for the pipeline steels Steel strength grade Steel state Ultimate strength σ UTS , MPa Yield strength σ Y , MPa

Reduction in area RA, %

Elongation at break, %

KCV, J/cm 2

API 5L X52

As-received

589 566 577 576

441 422 482 510

68 65 73 71

25.8 24.0 23.1 22.0

71 65

Operated for 38 years

API 5L X67

As-received

196 154

Operated for 34 years