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.

In this paper, we investigate how two different microstructures in pure iron affect the dislocation mobility in hydrogen-charged and non-charged samples by conducting stress-relaxation tests. The effective activation volume of the pure iron for both types of microstructures (cold-rolled and annealed samples) has been determined for both H-charged and uncharged material. Information about the dislocation structures formed during stress relaxation is provided by conducting TEM analysis. We employ a self-consistent kinetic Monte-Carlo (SCkMC) model of the ½ [111] screw dislocation in Fe to investigate how hydrogen affects the mobility and behavior of the dominant mobile dislocation in Fe at different stresses and H concentrations. The results from our simulations show the following: (i) at low stresses the deviation from the primary slip plane in the presence of H is lower than the deviation in the uncharged Fe. The deviation angle decreases with increasing H concentration; (ii) at higher shear stresses, the higher probability for kink-pair formation in the secondary (110) planes in the presence of H, leads to an enhanced deviation from the primary slip plane, which increases with increasing H concentration. We use the results of stress-relaxation tests and SCkMC simulations to propose an explanation for the formation of dislocation cell structures in pure and hydrogen charged Fe in the cold-rolled and annealed samples.

Aluminum-based metal matrix composites reinforced with B₄C are advanced materials recognized for their exceptional combination of lightweight properties, high hardness, and superior wear resistance. These characteristics make them perfectly suited for applications demanding exceptional performance in extreme mechanical and tribological environments. This study investigates the wear behavior, microstructural characteristics, and predictive modeling of Al10Cu-B₄C composites fabricated via powder metallurgy with varying B₄C contents (0, 2.5, 5, and 7.5 wt.%). The addition of B₄C microparticles to Al10Cu composites significantly influenced their tribological properties with 2.5 wt.% B₄C achieving a 21.74% reduction in the coefficient of friction (COF) and 7.5 wt.% B₄C providing a remarkable 65.00% improvement in wear resistance. Microstructural analysis using SEM and EDS was conducted on the unreinforced materials and the reinforced composites both before and after the wear tests. To further analyze and predict the tribological performance, a Gated Recurrent Unit neural network was developed to predict COF values. The need for this model arises from its potential to cost-effectively facilitate the prediction of COF, diminishing the need for extensive experimental testing while being noted for its simplicity and ease of implementation in practical applications. The model achieved excellent accuracy with an R² of 0.9965 for the test set and 0.9917 for the validation set. Additionally, feature importance analysis using Random Forest models identified reinforcement-related features as the dominant predictors for both COF and mass wear. These findings demonstrate the potential of Al10Cu-B₄C composites for emerging industrial applications, where enhanced wear resistance and controlled friction are critical for improving efficiency and durability under rigorous operating conditions. Furthermore, this study highlights the efficacy of neural network models in accurately predicting COF, providing a powerful tool for optimizing the performance of advanced composite materials.

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.

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.