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• Assessment of hydrogen-induced crack propagation along the grain boundaries orientated to {0 0 1} planes. • Influence of hydrogen on loading at low strain rate test. • Evaluation of the chemical composition of two different pearlitic steels related to hydrogen embrittlement. Understanding hydrogen embrittlement (HE) in pearlitic steels is crucial for manufacturing tensile reinforcements used in pipes for the oil and gas industry. This study aimed to elucidate the susceptibility of pearlitic steels to HE, focusing on microstructural influences. Two types of steel (Fe-0.3 %C and Fe-0.7 %C wt.) used in the tensile armor of flexible ducts, differing in chemical compositions and thermomechanical treatments, were analyzed, both exhibiting HE in a hydrogen-rich environment. Samples were investigated using microstructural examination, mechanical testing at low strain rates, and crystallographic texture analysis via Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD), and X-ray Diffraction (XRD). Slow strain rate tests (SSRT) at 1.0 × 10–⁶ s–1 were conducted in air and in a 3.5 % NaCl solution with cathodic protection. Sample A showed a decrease in yield strength from 810 MPa in air to 687 MPa after hydrogen charging, with total elongation reduced from 11.15 % to 7.38 %. Sample B, with higher carbon content, showed the highest YS and UTS overall, with YS slightly increased from 1216 MPa in air to 1246 MPa after hydrogen charging. UTS increased from 1311 MPa in air to 1336 MPa after hydrogen charging, while elongation significantly dropped from 8.52 % in air to 2.82 % after hydrogen charging. Fracture surfaces were analyzed by SEM, revealing that cracks propagated through ferrite-cementite interfaces and preferential intergranular pathways. The Hollomon model illustrated reduced plasticity due to hydrogen interaction, highlighting embrittlement risks. EBSD analysis revealed that cracks caused by HE propagates more easily along the {0 0 1} planes due to their lower planar atomic density. Taylor factor analysis indicated that {1 1 1} grains harden under load, preventing crack propagation and causing the crack to change direction at {1 1 0} and {1 1 1} grain interfaces. The kernel misorientation map showed that regions of high plastic deformation deflect the crack path, emphasizing the influence of crystallographic orientation on crack propagation dynamics. [ABSTRACT FROM AUTHOR]

• HIC API 5L X65 steel was investigated. • Stress relieving was carried out at different temperatures. • HIC resistance of was investigated in NACE solution A. • HIC susceptibility and inclusion relationship was investigated. • Improved HIC resisatnce was linked with the reduction in residual stresses. This study investigates the hydrogen-induced cracking (HIC) of a thermomechanical controlled processed (TMCP) API 5L X65 steel pipe fabricated using the JCOE forming process. Despite passing HIC testing as a plate, the final pipe exhibited unexpected failure, highlighting the critical influence of pipe fabrication on material performance. In this study, failed pipes were subjected to heat treatment, and HIC testing following NACE TM 0284 standard, SEM, and EDX examinations. The effect of heat treatment on the grain size, residual stress and susceptibility to HIC was investigated. The findings revealed that welding played a detrimental role, as all the HIC failures occurred at the welded area (0° orientation) due to welding-induced stresses and potential microstructural changes. It is believed that the welding process imposed reversible hydrogen traps in the HIC resistance plate ultimately reducing the HIC resistance. The heat treatment proved an effective method for relieving the applied micro-residual stresses that resulted in enhanced HIC resistance of the material. [ABSTRACT FROM AUTHOR]

• Modifying microstructure influences hydrogen diffusion and cracking of steels. • HIC susceptibility of steels is dependent on their diffusion characteristics. • Phase composition was seen to play the most significant role in influencing HIC. • Low local misorientaions in steels facilitates the elimination of HIC. This work examines the effect of stress-relief (SR) and quenching and tempering (QT) heat treatments on the hydrogen-induced cracking (HIC) susceptibility of pipeline steels. Two trials of a similar X65 pipeline steel were used in this study. They had distinct differences in microstructural characteristics, which significantly affected their cracking behaviour. The SR treatment reduced local misorientations, without significantly altering the microstructural appearance of the steels. Hence, this treatment resulted in a hydrogen permeation transient and cracking response similar to the as-received steels. However, the QT treatments significantly modified the microstructure of the steels, with some treatments resulting in equiaxed polygonal ferrite grains and lower local misorientations, depending on the quenchant used. These microstructural changes contributed to increasing hydrogen diffusion and permeation through the steels, as well as alleviating or eliminating HIC after hydrogen charging. Comparing the permeation transients and diffusion characteristics of the steels without cracks to those with significant cracking, a dependence of HIC susceptibility on the diffusion and trapping behavior within the steels was observed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The HE indexes of tensile properties of BM are more sensitive than that of the WJ. • The FAC coupling with HE shrinks from the outside to the inside in the FAD. • The reduction trend of safety factors with HE for crack depth differs from pressure. • The impact fracture morphology with HE for WJ presents brittle characteristics. Currently, no systematic approach exists for the safety assessment method of hydrogen transmission low-strength pipeline, it is necessary to study the effect of hydrogen embrittlement (HE) on the method of failure assessment diagram (FAD) for the defective pipe. The hydrogen charging experiments of tensile and Charpy impact specimens for the L245 pipe steel were carried out with charging time of 0, 30, 90, and 120 h, and then the tensile and Charpy impact tests were immediately conducted. The HE indexes of tensile and Charpy impact properties were presented to quantitatively analyze the effect of HE under tensile and impact deformation processes. Subsequently, the FAD method coupled with the effect of HE was investigated to analyze the difference in the safety assessment method between hydrogen and non-hydrogen environments. Finally, the fracture morphology of tensile and Charpy impact specimens was observed to analyze the responses of microscopic fracture modes between hydrogen and non-hydrogen environments. The results show that the effect of HE on the tensile properties of the base material is more sensitive than that of the weld joint. The FAC with the effect of HE for safety assessment of defective pipes shrinks by reducing the safe zone. The decrease ratio of safety factors affected by HE presents an increasing tendency as the increase of crack depth while showing a decreasing tendency as the increase of internal pressure. [ABSTRACT FROM AUTHOR]

• Statistical study on non-metallic inclusion characteristics. • HIC cracks nucleate in cubical and spinal-shaped inclusions. • High hydrogen trapping capacity and misfit strain around sharp edge inclusions. • The inclusion shape is dominant over the size for HIC nucleation. This study consists of a statistical study to identify spatial distribution parameters of non-metallic inclusions (NMIs) at hydrogen-induced cracking (HIC) nucleation sites in both low-strength and high-strength steel pipes. The electrochemical cathodic charging method was used to induce HIC in pipeline steel plates, and the nucleation of the HIC was monitored using straight beam ultrasonic testing. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to evaluate the shape, size, area fraction, and separation distance of NMIs. The hydrogen microprint technique (HMT), electron backscattered diffraction (EBSD) analysis, and finite element (FE) stress analysis were performed to characterize HIC nucleation sites. The findings showed that cubical and spinal NMIs, characterized by strong hydrogen trapping capacity due to high misfit strain and von Mises stress, are favored sites for HIC nucleation. The main finding of this study is that the shape and sharpness of NMIs are the factors that determine when NMIs will be a HIC nucleation site, rather than size, as generally accepted. [ABSTRACT FROM AUTHOR]

• HIC nucleation and propagation in API pipeline steels were investigated. • Elemental segregation and inclusion contents determined HIC nucleation and growth sites. • HIC nucleated at cuboidal and spinal inclusions and propagated along pearlite bands. • The hydrogen microprint technique visualized the preferable HIC nucleation sites. The microstructural features that favor hydrogen-induced cracking (HIC) in internal, middle, and external layers of pipeline steels were investigated. The electrochemical cathodic charging was used to induce HIC, and straight beam ultrasonic inspection was identified in which steel layers HIC nucleate and grow. The cracked plates were examined by optical microscope and scanning electron microscope (SEM). The hydrogen microprint technique (HMT) was used to visualize the site of high hydrogen concentrations. Electron backscatter diffraction (EBSD) was used to analyze the role of crystallographic texture in HIC pathways. The results revealed that HIC nucleation and growth are influenced by the segregation degree and inclusion contents, which are higher in the internal surface layer of our studied steels but not consistently so in the middle thickness layer. Further, HIC nucleated in cuboidal and spinal inclusions and propagated along ferrite/pearlite bands with high misorientations. The HMT results showed that hydrogen is present in pearlite colonies, grain boundaries, and non-metallic inclusions. The results indicated that HIC is not solely controlled by the crack geometry. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the analysis of a brittle fracture explosion accident of a thick-walled 20 steel pipe, which connects the outlet of an ammonia separator and the inlet of a cold exchanger in an ammonia synthesis unit of a chemical fertilizer plant, is presented in detail. The causes and mechanism of the material degradation are investigated by analyzing chemical compositions (especially the elements of nitrogen, hydrogen), testing mechanical properties, observing microstructures and defects, and strain aging sensitivity, and analyzing annealing behavior of the pipe. Furthermore, the mechanism of crack initiation and propagation are analyzed carefully through macro- and micro-crack observations by means of Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The research results not only support the conclusions given by the accident survey group, but also clarify the uncertainty in the report of the survey at that time. The further research makes it clear that the decrease of material toughness due to strain aging embrittlement (SAB), and the crack initiation and propagation is caused by SAB and its lowering effect on the threshold value of hydrogen-induced cracking (HIC). The research work described in the paper has a significant meaning to recognize or eliminate SAB occurred in quite ductile plain carbon steels very commonly used in industry, such as 20 steel in China. [Copyright &y& Elsevier]

• The ratio of crack depth/length increases with hydrogen concentration. • Variance of internal pressure responsible for the different crack structures. • Crack geometry parameter defined as a useful indicator of internal pressure. Quantitative prediction of the hydrogen-induced crack initiation remains challenging, partially due to the difficulties in directly measuring the hydrogen gas pressure in the crack cavity. Through detailed cross-sectional observations, this study shows that the structure of the internal cracks beneath the surface blisters, i.e., the ratio of crack depth/length, can be significantly influenced by the hydrogen charging conditions. This is attributed to the change of hydrogen gas pressure in the crack cavity at different hydrogen contents, supported by the formation of stepwise cracks near the specimen surface as well as the hydrogen concentration measurements. The post-failure crack geometry is considered a useful indicator of the level of internal gas pressure to quantitatively assess the hydrogen-induced cracking sensitivity. The mechanisms of crack initiation and propagation at different hydrogen concentrations are discussed. [ABSTRACT FROM AUTHOR]

• Stress corrosion cracking of X80 steel under SRB, external potential and stress. • The corrosion mechanism under the action of SRB and external potential was clarified. • The stress corrosion sensitivity of X80 steel under different external potentials. In this study, the effect of applied potential on stress corrosion behavior of X80 pipeline steel in Dagang soil containing sulfate-reducing bacteria was comprehensively analyzed by means of the AC impedance technique and slow strain-rate tensile tests and scanning electron microscope observation of fracture morphology. The results indicate that, with decreasing electrode potential, the corrosion degree of the sample first decreases and then increases, and the stress corrosion cracking sensitivity exhibits a certain rule. Under the Ecorr and a potential of −725 mV(vs.CSE), the stress corrosion cracking mechanism of metal is anodic dissolution. The synergistic effect of sulfate-reducing bacteria and stress results in the initiation and propagation of cracks on the surface of the metal electrode. At −1275 mV(vs.CSE), the mechanism of stress corrosion cracking is hydrogen-induced cracking. The sulfate-reducing bacteria exacerbates the cathodic hydrogen evolution at this potential. Under the combined effects of hydrogen and stress, stress corrosion cracking behavior occurs in metals. [ABSTRACT FROM AUTHOR]