In the current study, post-weld heat treatment (PWHT 580 °C) was used for an X80 pipeline steel-welded joint, and the fracture toughness of the welded joint was investigated using a crack tip opening displacement (CTOD) test. The relationship between microstructure evolution and fracture toughness is also discussed in this study. The results showed that the weld center mainly consisted of acicular ferrite (AF). The subcritical heat-affected zone (SCHAZ) consisted of a large amount of fine polygonal ferrite and some AF, and it maintained the rolling state of the base metal. The microstructure of the coarse-grained heat-affected zone (CGHAZ) was composed of granular bainite (GB) and M/A constituents, the latter of which decreased after the PWHT. The CTOD values of the weld center were in the range of 0.18–0.27 mm, while those of the CGHAZ were in the range of 0.02–0.65 mm. A brittle fracture occurred in the CGHAZ for both the as-welded and PWHT samples; the CTOD values were 0.042 mm and 0.026 mm, respectively. The CTOD values of the SCHAZ’s location were in the range of 0.8–0.9 mm. The PWHT did not deteriorate the microstructure of the CGHAZ and had little influence on the fracture toughness of the X80 pipeline steel-welded joint; it ensured the fracture toughness of the welded joints and reduced the welding residual stress.

This study established a machine learning framework to enhance the accuracy of very-high-cycle fatigue (VHCF) life prediction in selective laser melted Inconel 718 alloy by systematically comparing the use of generative adversarial networks (GANs) and variational auto-encoders (VAEs) for data augmentation. We quantified the influence of critical defect parameters (dimensions and stress amplitudes) extracted from fracture analyses on fatigue life and compared the performance of GANs versus VAEs in generating synthetic training data for three regression models (ANN, Random Forest, and SVR). The experimental fatigue data were augmented using both generative models, followed by hyperparameter optimization and rigorous validation against independent test sets. The results demonstrated that the GAN-generated data significantly improved the prediction metrics, with GAN-enhanced models achieving superior R² scores (0.91–0.97 vs. 0.86 ± 0.87) and lower MAEs (1.13–1.62% vs. 2.00–2.64%) compared to the VAE-based approaches. This work not only establishes GANs as a breakthrough tool for AM fatigue prediction but also provides a transferable methodology for data-driven modeling of defect-dominated failure mechanisms in advanced materials.

To analyze the causes and mechanisms affecting the fracture toughness of X80 pipeline steel welded joints against H₂S, the fracture toughness of different zones of X80 pipeline steel welded joints in both air and saturated H₂S solution was investigated. The fracture toughness of welded joints degraded significantly in the saturated H₂S solution, where the crack tip opening displacement (CTOD) characteristic value in the coarse grain heat-affected zone (CGHAZ) and weld metal (WM) was only 8% and 12% of that in air, respectively. However, the sub-critical grain heat-affected zone (SCHAZ) showed better resistance to H₂S corrosion, with the CTOD characteristic value reaching 42% of that in air. The resistance of the welded joint to H₂S corrosion was sensitive to microstructures. The grain boundary ferrite (GBF) presented in WM, and the angle of grain boundary orientation in CGHAZ was not conducive to hindering crack propagation. Moreover, the formation of the resultant hydrogen cracks owing to the H₂S corrosion also reduced the fracture toughness of the welded joint.

Misalignment has a significant impact on the fatigue performance of circumferential weld joints in pipelines, which can significantly reduce the fatigue life. Misalignment generates a structural stress concentration on the pipeline, which proportionally reduces its fatigue strength. Moreover, due to the misalignment, the reinforcement of the root and the transition angle of the pipeline inwall are significantly reduced, increasing its notch stress concentration factor and further reducing its fatigue performance. This work investigates the effect of misalignment on stress concentration in the circumferential welds of pipelines, and it is used to predict the fatigue life. The structural stress method is proposed in the present work, and finite element analysis technology with Abaqus is used to calculate the structural stress concentration factor k_j at the root-pass toe of misaligned circumferential weld joints, and a formula for the relationship between the structural stress concentration factor k_j and the misalignment is established. The total stress concentration factor k of weld joints with different misalignments under several welding processes are calculated, and are compared with the structural stress concentration factor k_j. The fatigue test data of weld joints with different misalignments are studied, and it is shown that the fatigue performance could be predicted by the fitting result.

In the design and integrity assessment of long-distance natural gas pipelines, it is critical to assess how geometrical features and material properties influence the stress and strain states of girth welds. This study conducted an extensive three-dimensional finite element analysis for X80 pipeline girth welds with varying geometric features and material properties to examine stress triaxiality and strain distribution phenomena. The effect of the wall thickness ratio, misalignment, strength matching coefficient, and yield-to-tensile ratio of the weld metal was analyzed under both unloaded and internal pressure-loaded conditions. The results demonstrate that stress triaxiality and local strain increase with the wall thickness ratio, amplified by increasing misalignment, yield-to-tensile ratio, and internal pressure. Enhancing weld metal strength correspondingly improves root stress and strain states; nevertheless, greater wall thickness ratios and misalignment diminish this improvement. Based on these findings, design recommendations were proposed, including suitable wall thickness ratio design, strict misalignment control and the optimization of weld material properties. The findings of this study offer valuable insights into the design and assessment of pipeline girth welds exhibiting intricate geometric features.