PSI - Issue 68

Olha Zvirko et al. / Procedia Structural Integrity 68 (2025) 868–873 Olha Zvirko / Structural Integrity Procedia 00 (2025) 000–000

870

3

degradation of strength, plasticity, and resistance to HE. The smooth cylindrical tensile specimens were cut in different orientations, longitudinally and transversely relative to the pipe rolling direction. The susceptibility to HE has been evaluated by examining the impact of preliminary hydrogen charging on the plasticity of tensile specimens during fracture in air. The reduction in area (RA) is utilized as the primary indicator of plasticity, as it is more suitable for this purpose than elongation, which does not always correctly reflect the material's deformation capability due to the formation and opening of numerous microcracks during the mechanical loading of specimens (Zvirko (2021), Nykyforchyn et al. (2023)). The specimens were hydrogen pre-charged in a solution of H 2 SO 4 (рН 1) with 10 g/l thiourea (NH 2 ) 2 CS as a hydrogen evolution inhibitor. Two hydrogen charging modes were employed: (i) moderate (cathodic current density i = 0.05 mA/cm 2 ) and intensive ( i = 1.0 mA/cm 2 ). The duration of cathodic polarization for 120 hours was intended to achieve a uniform distribution of hydrogen throughout the specimen's cross-section. After hydrogen charging, specimens were held for 30 minutes in air and then subjected to the mechanical testing of the tensile specimens. The strain rate was 3 × 10 –3 s –1 . Each test was conducted three times to ensure consistent results with a reasonable standard deviation. The susceptibility to HE was estimated by using an index given by HES :

• • •

•

••••

•

!"#

(1)

= ! "

• •

where RA H – reduction in area, determined in air after preliminary hydrogenation and RA – reduction in area, determined in air without preliminary hydrogenation). 3. Results and discussion Table 1 presents the average values of strength and plasticity characteristics as well as the impact strength of the investigated steel in both states, considering the cutting direction of the specimens relative to the pipe axis.

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

KCV, J/cm 2

Elongation, %

Reduction in area RA, %

568 589 570 566

393 441 390 422

71 68 66 65

28.1 25.8 25.0 24.0

129

Reserve pipe

Longitudinal Transversal Longitudinal Transversal

71

103

Operated for 38 years

65

The anisotropy of mechanical properties in rolled pipeline steels is a well-documented phenomenon resulting from the manufacturing process. The steel's ultimate tensile strength (σ UTS ) is within a narrow range of 566– 589 MPa for all steel specimens (Figure 1). The yield strength for the operated steel is slightly increased as a result of the operation. The transverse specimens exhibit a significantly higher yield strength for both steel states, indicating an anisotropy of the property. Typically, higher tensile and yield strengths are observed in the transverse direction compared to the longitudinal direction. This mechanical behavior can be attributed to microstructural features such as ferrite and pearlite banding, crystallographic texture, and the non-uniform inclusion size and shape distribution. Consequently, the ferrite/pearlite bands are more compressed in the transverse direction, leading to greater strength in that orientation. The plasticity characteristics, reduction in area and elongation, and impact toughness are remarkably lower for the operated steel than those of the reserved one (Figure 1). These features indicate a higher embrittlement of the operated steel. Therefore, the lower plasticity of the steel after operation compared with the reserved one and the same strength in both steel states are caused by operational changes in the metal.