Fracture behavior of spiral welded API X70 pipeline steels under monotonic and cyclic loading

Spiral welded API 5L X70 pipeline steels are used in energy infrastructure, yet their mechanical behavior across different weld zones remains insufficiently understood. This study presents a detailed investigation of their fracture behavior under monotonic and cyclic loading, with an emphasis on zone-specific responses across the base metal (BM), heat-affected zone (HAZ), weld metal (WM), and full welded joint. Microstructural features were characterized using electron backscatter diffraction (EBSD), while hardness mapping and mechanical testing provided insight into strength, ductility, and fatigue resistance across the weldment. Microstructural characterization using EBSD revealed notable variations in grain morphology, grain boundary angles, and texture across regions, while hardness mapping identified a softened fine-grained HAZ. Tensile testing showed that the WM had the highest strength but lowest ductility, whereas the welded joint exhibited early strain localization and the poorest overall ductility compared to the zone-specific specimens. Under cyclic loading, the welded joint displayed the shortest life, with fracture location shifting from the HAZ to BM depending on stress amplitude. Digital image correlation (DIC) analysis enabled fatigue life partitioning, and a sigmoidal model was developed to relate fatigue initiation life to stress level. Fatigue crack growth tests showed the HAZ as the zone with the lowest fatigue crack growth resistance, with higher growth rates and lower crack tip opening displacement (CTOD) thresholds, particularly under higher stress amplitude cyclic loading. The outcomes of this work support the development of more reliable fatigue assessment frameworks and can inform the design and maintenance strategies for spiral welded steel pipelines. • Welded joint exhibits lowest ductility and fatigue life due to strain localization. • Welded joint’s fatigue fracture site shifts from HAZ to BM at ∼ 405 MPa maximum stress. • Fatigue life partitioning shows sigmoidal N i /N f –stress dependence across weld zones. • Fatigue crack growth rate and CTOD– Δ a confirm HAZ as the most fatigue-critical zone. • Zone-resolved fatigue data fall below design curves such as API 579 and BS 7910.