The effect of hydrogen content on the microstructure, mechanical properties, and fracture mechanisms of low-carbon lath martensitic steel was investigated using both experimental methods and atomistic modeling. Tensile testing revealed a transition in the fracture behavior with increases in hydrogen concentration. Specifically, at a hydrogen content of 0.44 wppm, a shift from transgranular to intergranular fractures was observed. The most probable cause of hydrogen embrittlement was identified to be HELP-mediated HEDE. As the hydrogen concentration increased, the dislocation density in close-packed planes, such as (111) and (100), was found to rise. The key differences between the hydrogen-free and hydrogen-charged specimens were the localization and density of dislocations, as well as the change in the distribution of slip bands. Atomistic modeling supported these experimental findings, showing that “quasi-cleavage” cracks predominantly initiate at block boundaries with higher local hydrogen accumulation. These results underscore the significant role of hydrogen in modifying both the microstructural characteristics and fracture behavior of low-carbon martensitic steel, with important implications for its performance in hydrogen-rich environments.

This study investigates the effect of hydrogen charging time on the mechanical properties and microstructural evolution of low-carbon ferrite–pearlite steel that has been in service for over 30 years in natural gas transmission. Specimens were subjected to in-situ electrochemical hydrogen charging for varying durations, followed by tensile testing. Detailed microstructural analysis was performed using scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Despite negligible changes in the overall hydrogen content ($C_H$≈ 4.0 wppm), significant alterations in fracture morphology were observed. Fractographic and TEM analyses revealed a clear transition from ductile fracture in uncharged specimens to a predominance of brittle fracture modes (quasi-cleavage, intergranular, and transgranular) in hydrogen-charged samples. The results show time-dependent microstructural changes, including increased dislocation density and the formation of prismatic loop debris, particularly within the ferrite phase. Prolonged charging leads to localized embrittlement, which is explained by enhanced hydrogen trapping at ferrite-cementite boundaries, grain boundaries, and dislocation cores. TEM investigations further indicated a sequential activation of hydrogen embrittlement mechanisms: initially, Hydrogen-Enhanced Localized Plasticity (HELP) dominates within ferrite grains, followed by Hydrogen-Enhanced Decohesion (HEDE), particularly at ferrite-cementite interfaces in pearlite colonies. These findings demonstrate that extended hydrogen charging promotes defect localization, dislocation pinning, and interface decohesion, ultimately accelerating fracture propagation. The study provides valuable insight into the degradation mechanisms of ferrite-pearlite steels exposed to hydrogen, highlighting the importance of charging time. The results are essential for assessing the reliability of legacy pipeline steels and guiding their safe use in future hydrogen transport infrastructure.

The utilization of hydrogen as a clean energy carrier requires an assessment of existing natural gas pipelines with respect to hydrogen embrittlement (HE). In this study, the structural integrity and hydrogen sensitivity of X52 (L360) pipeline steel from the Bulgarian gas transmission network after 31 years of service were investigated, focusing on production (longitudinal) and girth (circumferential) welded joints. Hydrogen content was measured in the base metal, production weld and girth weld before and after electrochemical charging, while in situ hydrogen charging during tensile testing was applied to simulate service conditions. Mechanical behavior was evaluated by tensile tests, and microstructural and fracture characteristics were analyzed by SEM and TEM. The results show significant spatial variations in hydrogen concentration, related to local microstructural heterogeneity and hydrogen trapping. In the as-operated state, fracture was localized mainly in the heat-affected zone. Hydrogen charging led to a pronounced reduction in ductility (approximately twofold), whereas yield and tensile strengths were only slightly affected. Failure analyses indicate a transition toward more brittle fracture mechanisms, dominated by quasi-cleavage and intergranular cracking in the as-charged state, with hydrogen embrittlement susceptibility indices demonstrating higher hydrogen sensitivity of the girth-welded joints.

Aluminum–graphene nanoplatelet (Al/GNP) composites have attracted significant attention as lightweight structural materials, yet their mechanical performance is strongly influenced by interfacial reactions and the formation of carbides. In this study, Al/GNP composites containing 0.1–1.1 wt.% graphene were produced via powder metallurgy and hot extrusion at 400 °C and 500 °C. Hot extrusion at the higher temperature enables the controlled in situ formation of aluminum carbide (Al₄C₃). A comprehensive microstructural characterization using SEM and HRTEM was combined with tensile testing to elucidate the influence of carbide size on mechanical behavior. Hot extrusion at 500 °C promotes the formation of uniformly distributed, nanoscale Al₄C₃ carbides whose size, morphology, and aspect ratio depend on graphene content. Composites containing nano-sized carbides exhibit a markedly improved strength–ductility balance compared to carbide-free counterparts, with optimal performance achieved at 0.3 and 0.7 wt.% GNPs. The enhancement is attributed to synergistic strengthening mechanisms involving improved interfacial bonding, efficient load transfer, nanoscale dispersion strengthening, and carbide–dislocation interactions. The results indicate that the controlled formation of nanoscale Al₄C₃ is not detrimental; rather, it contributes to the optimization of the mechanical properties of Al/GNP composites. Unlike most previous studies that treat carbide formation as a detrimental effect, this work demonstrates that its controlled nanoscale evolution can be used as a deliberate strengthening strategy through its influence on microstructural mechanisms.

This study investigates hydrogen embrittlement in welded joints of X52 (L360) pipeline steel obtained from an operating natural gas transmission network after 31 years of service, with particular emphasis on production (longitudinal) and girth (circumferential) welds. The aim is to elucidate the influence of microstructural heterogeneity across the pipe wall and within different welded joint types on hydrogen transport, trapping behavior, and fracture mechanisms. The investigation combines X-ray diffraction, electrochemical hydrogen permeation testing, fractographic analysis, and transmission electron microscopy. X-ray diffraction results show that the base metal and girth weld consist predominantly of body-centered cubic ferrite, whereas the production weld additionally contains retained austenite associated with an elevated manganese content. These phase-related differences are consistent with transmission electron microscopy observations of martensite–austenite constituents within the weld microstructure. Electrochemical hydrogen permeation measurements reveal pronounced microstructure-dependent hydrogen transport behavior. The production weld exhibits a significantly lower apparent diffusion coefficient and a markedly higher hydrogen trap density, approximately five times greater than those of the base metal and girth weld, providing a mechanistic explanation for the observed differences in hydrogen uptake behavior. Fractographic analysis demonstrates a transition from ductile microvoid coalescence in the uncharged condition to predominantly brittle fracture following hydrogen charging. This transition is accompanied by a substantial increase in the fraction of brittle fracture zones, reaching approximately 53% in hydrogen-charged specimens. A pronounced gradient in hydrogen embrittlement susceptibility is observed across the pipe wall thickness, with outer-wall specimens consistently exhibiting greater susceptibility than inner-wall specimens. This behavior reflects the combined influence of long-term soil corrosion and hydrogen-assisted degradation. Transmission electron microscopy reveals that plastic deformation governs dislocation generation, while hydrogen significantly modifies dislocation behavior by promoting dislocation pile-ups near martensite–austenite constituents and non-metallic inclusions. These observations indicate strong interactions between hydrogen, dislocations, and microstructural heterogeneities. A clear size-dependent role of non-metallic inclusions is identified. Sub-micron inclusions act primarily as irreversible hydrogen trapping sites that contribute to hydrogen redistribution within the microstructure, whereas larger inclusions serve as preferential crack initiation sites under hydrogen charging conditions. Overall, the results demonstrate that hydrogen embrittlement behavior is governed by the combined effects of microstructural state, welded joint type, and long-term service-induced degradation, resulting in distinct hydrogen transport characteristics and fracture responses across the pipe wall.