Search Results (48)

Unequal-wall-thickness X80-X60 girth welded joints used in pipeline transition sections exhibit strong local mechanical heterogeneity, while the wall-thickness transition also introduces a separate structural geometry effect. Conventional homogeneous or weld-metal-only descriptions may therefore be insufficient for interpreting the fracture response of single-edge-notched tension (SENT) specimens sampled from such joints. In this study, a constraint-driven fracture toughness assessment was conducted for single-edge-notched tension (SENT) specimens representing the X80 base metal, X80 heat-affected zone (HAZ), weld metal, X60 HAZ, and X60 base metal as five distinct material regions. Miniature tensile tests provided the local constitutive input for these regions. The numerical procedure was verified against published SENT force versus crack-mouth-opening-displacement (CMOD) data; because direct SENT fracture-toughness tests for the present X80-X60 target joint are not yet available, the literature comparison is treated as verification of the modeling procedure rather than direct validation of the target joint. Parametric analyses were then performed for an initial crack ratio of a₀/W = 0.10–0.30, a thickness-to-width ratio of B/W = 0.50–1.50, weld width = 10–30 mm, HAZ width = 1–9 mm, and different weld/HAZ strength combinations. The simulations show asymmetric crack-tip plastic deformation toward the lower-strength X60 side. Increasing the crack length increases crack-tip opening displacement (CTOD), whereas increasing B/W, weld width, or HAZ width generally reduces CTOD by increasing local constraint. The influence of weld strength is strongly coupled with the strength level of the adjacent HAZs. The results are therefore interpreted as numerical CTOD-response trends for a heterogeneous SENT specimen; direct experimental fracture-toughness measurements of the target X80-X60 joint remain an important subject of follow-up work. Full article

This study comparatively investigates the coarse-grained heat-affected zone (CGHAZ) responses of two industrial X65 seamless pipeline steels with distinct composition–microstructure characteristics under simulated girth-welding thermal cycles. One steel exhibits a predominantly bainitic initial microstructure, whereas the other consists of a ferrite–bainite dual-phase microstructure. Low-temperature Charpy impact testing, microhardness measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), prior-austenite grain reconstruction, and JMatPro 13.0-based continuous cooling transformation (CCT) calculations were employed to evaluate their heat-input sensitivity and microstructural evolution. At heat inputs of 7–10 kJ/cm, both steels maintained high impact toughness at −20 °C, with average absorbed energies of approximately 250 J. A pronounced difference emerged at 15 kJ/cm the bainite-dominated steel retained relatively high impact toughness and higher crack-initiation and -propagation energies, whereas the ferrite–bainite steel exhibited a marked toughness reduction. At higher heat inputs of 20–30 kJ/cm, both steels showed substantial toughness deterioration associated with severe prior-austenite grain growth and coarsening of the bainitic transformation products. Microstructural and crystallographic analyses showed that the bainite-dominated steel generally retained finer prior-austenite grains and more refined crystallographic features under the investigated thermal cycles. Detailed characterization at 15 kJ/cm further revealed finer prior-austenite grain, packet, and block structures, together with more tortuous crack-propagation paths. JMatPro calculations predicted a lower bainitic transformation temperature for this steel, which is consistent with the experimentally observed tendency toward finer bainitic transformation products. The superior CGHAZ toughness retained by the bainite-dominated steel is therefore associated with the combined effects of alloy composition, initial metallurgical state, transformation behavior, and hierarchical crystallographic refinement rather than the initial microstructure alone. The results highlight the importance of coupled composition–transformation–microstructure effects in determining the welding heat-input tolerance of industrial X65 seamless pipeline steels. Full article

Hydrogen trapping is a key mechanism to mitigate hydrogen embrittlement (HE) in pipeline steels, yet how trapping behavior evolves during plastic deformation and which microstructural features are responsible remain poorly understood. In this work, interrupted slow strain rate tensile tests were performed on pipeline girth weld metal (WM) under 6.3 MPa H₂ combined with thermal desorption spectroscopy (TDS), hydrogen microprint technique (HMT), and electron backscatter diffraction (EBSD). The results reveal that hydrogen trapping is highly strain-dependent. In the undeformed stage, only weak traps (Peak A at ~250 °C) and moderate traps (Peak B at ~500 °C) are present, as identified by TDS. Upon tensile straining to 15%, a new strong trap (Peak C at ~700 °C) emerges and intensifies, which is attributed to deformation-induced defects within acicular ferrite (AF). HMT shows that hydrogen distribution becomes homogeneous inside AF grains with increasing strain, while remaining localized at grain boundaries in proeutectoid ferrite (PF). EBSD indicates that AF undergoes more uniform plastic deformation and dislocation accumulation than PF. Most importantly, the WM with a higher AF content (74.5%) exhibits a significantly lower HE index (13.4%) than the WM with a lower AF content (53.9%, HE index 23.5%), confirming the beneficial role of AF. These findings demonstrate that AF is not merely a static hydrogen trap but a dynamic source of strain-induced strong traps, which effectively immobilize hydrogen and reduce embrittlement susceptibility under service-relevant deformation. Full article

This study focused on burn-through leakage at girth welds of mechanically lined pipe (MLP) during field service. Field failure analysis, experimental tests, and numerical simulation were combined to investigate the process parameters of seal welding and multi-pass girth butt welding. Macroscopic metallography and energy dispersive spectroscopy (EDS) of failed specimens showed that excessive welding heat input (high current) caused severe expansion of the heat-affected zone (HAZ) and significant element dilution. The results indicated that the HAZ width of the solid-wire girth weld increased markedly from 1.312 mm to 2.247 mm under high-current conditions. Meanwhile, the Fe mass fraction in the root pass sharply increased to 33.66%, while key corrosion-resistant elements such as Cr and Ni were greatly reduced, which directly led to local pitting corrosion and perforation leakage. In addition, a moving heat source model was established in Abaqus 2024 to simulate the multi-pass welding process. The results showed that strong stress concentration developed at the groove root and the interface between the backing steel pipe and corrosion-resistant liner during repeated thermal cycles. The maximum von Mises stress reached 686.56 MPa during the second butt welding pass. After final cooling, the residual hoop tensile stress and axial tensile stress at the center of the inner surface reached 500–550 MPa and 480–510 MPa, respectively. By correlating microscopic compositional evolution with the macroscopic residual stress field, this study revealed the weld failure mechanism of MLP joints. The proposed finite element method can also be used as an efficient tool to predict the effects of welding speed, current, and voltage on residual stress, providing guidance for field welding procedure optimization and pipeline structural integrity assessment. Full article

This study investigates pulsed gas metal arc welding (pGMAW) of API 5L X65 pipeline steel for sour service applications where H₂S exposure is anticipated. Mechanized pGMAW in the 5G downhill position was employed to fabricate girth welds using ER70S-6 filler wire with Ar-20%CO₂ shielding. Comprehensive characterization, including optical microscopy, tensile testing, fractography, EBSD, and fracture toughness evaluation via SENT specimens, was conducted on specimens tested in both air and H₂S-precharged sour conditions. Microstructural analysis revealed ferritic–pearlitic base metal, weld metal with acicular ferrite and bainitic constituents, and a transformed HAZ gradient. Tensile testing demonstrated severe hydrogen embrittlement in sour conditions, with elongation dropping from 22% in air to 4% after H₂S exposure, accompanied by a transition from ductile cup–cone fracture to quasi-cleavage morphology. EBSD showed texture sharpening toward ⟨101⟩ fiber post-deformation, with a broader orientation spread under sour conditions, indicating heterogeneous strain localization. Fracture toughness testing revealed approximately a 50% reduction in CTOD values under sour exposure, with the weld centerline exhibiting greater degradation (0.50 mm to 0.27 mm) compared to the HAZ (0.92 mm to 0.47 mm). Fractography confirmed hydrogen-assisted cracking features, including shallow dimples, cleavage facets, and secondary cracking. These findings establish critical baseline data for engineering a critical assessment of pGMAW-welded X65 pipelines in sour service. Full article

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. Full article

In the present work, eleven self-shielded flux-cored wires with nickel (Ni) contents ranging from 1.42 wt.% to 4.02 wt.% were designed for the semi-automatic welding of X80 pipeline steel. The effects of Ni on the microstructural evolution and mechanical properties of the weld metal were investigated. The results indicate that when the Ni content is below 2.06 wt.%, the microstructures of both the solidification zone and the inter-pass reheating zone are dominated by coarse granular bainite and martensite/austenite (M/A) constituents. As the Ni content increases from 2.06 wt.% to 3.73 wt.%, the microstructure transforms to fine lath bainite with M/A constituents characterized by low content, small size, and uniform distribution. When the Ni content reaches 3.73 wt.%, the microstructure becomes almost fully bainite. Furthermore, with increasing the Ni content, both the yield strength and tensile strength of the weld metal increase from ~600 MPa to ~700 MPa and from ~660 MPa to ~730 MPa, respectively. However, the impact energy at −20 °C of the weld metal initially increases and then decreases, reaching a peak of ~110 J with the lowest degree of dispersion at a Ni content of approximately 3.73 wt.%. When the Ni content exceeds 3.73 wt.%, the ductility decreases slightly. Further analyses indicate that the synergistic effects of Ni in refining the microstructure and reducing the activity coefficient and solubility of nitrogen (N) jointly contribute to the impact toughness of the weld metal. Full article

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. Full article

Current studies on fatigue crack initiation in pipelines remain relatively limited. Existing frameworks are confronted with issues including difficulties in crack monitoring and limited consideration of intragranular short-crack propagation. To address these issues, a framework was proposed for predicting fatigue crack initiation life in pipelines with girth welds. The proposed framework incorporates full-scale testing, temperature field simulation and microstructural evolution analysis to overcome limitations in crack measurement and microstructural characterization. In addition, intragranular short-crack propagation has been taken into account in the proposed framework. The proposed framework predicts the fatigue crack initiation life through multiscale coupling. Agreement between the prediction and experimental results supports the validity of the proposed framework. The framework provides reliable predictions of fatigue crack initiation life for pipelines with girth welds under high-cycle fatigue (HCF) conditions. Full article

Accurately predicting the ultimate tensile strain of full-scale pipelines with unequal wall thickness containing cracked girth weld joints is essential for strain-based design, structural integrity assessment, and safe operation. However, many existing limit state prediction methods for full-scale girth welds are developed for equal wall thickness configurations or idealized geometries, and their applicability to unequal wall thickness conditions remains limited. To address this gap, this paper develops a limit state prediction model for the ultimate tensile strain of cracked girth welded joints in full-scale pipelines with unequal wall thickness. The model is established using a numerical database generated from finite element simulations, incorporating realistic pipe geometry, material properties, wall thickness mismatch, and representative crack defect characteristics. By considering the stress and strain concentration effects induced by geometric non-uniformity in the weld region, the proposed model provides a practical and efficient tool for limit state evaluation. During pipeline construction, it supports the formulation of quantitative requirements for key design and fabrication parameters, such as the strength matching level. During stable operation, it enables reliable prediction of the strain capacity of existing girth welds in pipelines with unequal wall thickness, thereby supporting integrity management and decision making for safe service. Full article

The structural integrity of pipeline girth welds is critical, especially when the welds involve heterogeneous materials and non-uniform wall thicknesses. Current evaluation methods that compare the strength of the weld to that of the base metal (BM) are inadequate for such complex welds. This paper addresses this gap by applying micro-tensile specimen testing to characteristic zones within heterogeneous girth welds that exhibit non-uniform wall thicknesses. We conducted tensile performance tests on X80 and X60 steel pipes featuring unequal wall thickness butt joints. The analysis focused on the wall thickness direction of the girth weld as well as the transverse direction, examining differences and patterns in performance across various regions. The findings provide an improved understanding of property gradients in heterogeneous girth welds and offer practical guidance for more reliable safety evaluation of pipelines with unequal wall thickness joints. Full article

Softening in the heat-affected zone (HAZ) of high-strength pipeline welds compromises its service safety but the corresponding softening mechanism is not well-understood. Softening behavior in the HAZ of two X80 pipeline girth welds with different base metal microstructures, i.e., acicular ferrite (AF)-dominated (X80-AF) and granular bainite (GB)-dominated (X80-GB), were investigated through microhardness tests and detailed microstructure characterization. The results showed that softening in the HAZ of two girth welds primarily occurred in the fine-grained (FG) HAZ, while hardening was found in the coarse-grained (CG) HAZ. X80-AF showed higher softening resistance than X80-GB, with softening ratios of 3.44% vs. 12.46%, and softened zone widths of 2.1 mm vs. 3.9 mm, respectively. Due to its high dislocation density and refined interlocking structure, AF could effectively inhibit phase transformation and grain coarsening during reheating, which resulted in smaller grains and a lower fraction of polygonal ferrite (PF) in the FGHAZ (28%). In contrast, coarse GB was more prone to grain coarsening and hence engendered higher PF proportion (68%). Therefore, for the microstructural design of high-strength pipeline steels, increasing the proportion of refined AF is beneficial to the softening resistance and thereby elevates the service safety of pipelines. Full article

Currently, the non-uniformity of girth weld positions makes their limit state a crucial determinant of pipeline safety. The design method based on the limit state is pivotal in ensuring the integrity and reliability of the pipeline system. Challenges often emerge when determining the limit states of girth welds using semi-empirical formula methods, primarily due to difficulties in accurately identifying influential factors. The quantitative impact of each influence parameter on the crack driving force and the results determined by the semi-empirical formula remain unclear. This study utilizes numerical simulation methods to systematically analyze the quantitative sensitivity laws of critical factors such as crack depth on the crack driving force to address this challenge. The findings revealed that the strength matching coefficient, crack depth, and misalignment are the most significant factors influencing the crack driving force, followed by crack length, softening rate, yield-to-strength ratio, internal pressure, and wall thickness. The effects of tensile strength and outer diameter are relatively minor. A comprehensive database of crack driving forces is constructed using a parameter matrix approach. Combined with the LightGBM machine learning algorithm, a full-scale prediction model for the strain capacity of pipeline girth welds is developed. Predictions for 18 sets of wide-plate test results from the literature confirm the high accuracy of the prediction model, with a prediction accuracy of 6.48%. This research provides a robust reference for accurately determining the limit state of pipeline girth welds and effectively meets the demands of rapidly advancing welding technologies and increasingly complex service environments. Full article

The Failure Assessment Diagram (FAD), as a significant method for evaluating the suitability of defective metallic structures, has been subject to considerable debate regarding its applicability in assessing ring welded joints for high-grade steel and large-diameter pipelines. To address this issue, this study first designed and conducted two sets of full-scale pressure-tension tests on large-diameter X80 pipeline ring welded joints, considering factors such as different welding processes, joint configurations, defect dimensions, and locations. Subsequently, three widely adopted failure assessment diagram methodologies—BS 7910, API 579, and API 1104—were selected. Corresponding assessment curves were established based on material performance parameters obtained from the ring weld tests. Finally, predictive outcomes from each assessment method were compared against experimental data to investigate the applicability of failure assessment diagrams for evaluating high-strength, large-diameter, thick-walled ring welds. The research findings indicate that, under the specific material and defect assessment conditions employed in this study, the API 1104 assessment results exhibited significant conservatism (two sets matched). Conversely, the BS 7910 and API 579 assessment results showed a high degree of agreement with the experimental data (eight sets matched), with the BS 7910 assessment providing a relatively higher safety margin compared to API 579. The data from this study provides valuable experimental reference for selecting assessment methods under specific conditions, such as similar materials, defects, and loading patterns. Full article

High-strength oil and gas pipeline girth welds exhibit significant material and geometric discontinuities with high susceptibility to defects, making them a critical weak link in oil and gas pipelines. Researching the fracture assessment technology pipeline’s girth welds is essential for enhancing the pipeline’s inherent safety and protection levels. Key issues and research progress related to fracture assessment technology are systematically addressed from the perspectives of pipeline fracture behavior and fracture assessment methods in this paper. The core focus of fracture behavior research is determining the crack driving force at the girth weld and the material’s fracture toughness. Fracture assessment methods include failure assessment diagrams and limited tensile strain capacity models. The development of single-parameter and multi-parameter fracture mechanics theories in establishing the relationship between in-plane and out-of-plane constraints and material fracture toughness is reviewed. Four commonly used methods for calculating crack driving forces in pipelines are presented. Moreover, the usage scenarios of various failure assessment diagrams in pipeline fracture assessment are analyzed. A comparison of the parameter ranges and applicability of commonly used international tensile strain capacity models is also provided. The paper highlights existing issues in current research on the fracture assessment of high-strength pipelines and outlines directions for further study. Lastly, this paper aims to provide theoretical and technical support for improving the inherent safety level of high-strength pipeline girth welds. Full article