Investigating the impact of welding thermal cycles on crack behavior in API X60 pipeline joints

Pipelines are essential for the safe transportation of hydrocarbons, where reliable welds are critical to ensuring structural integrity. This study investigates the influence of the welding thermal cycle on API 5L X60 steel, a widely used material in pipeline construction. Experimental tests revealed significant variations in crack resistance among the base metal, molten metal, and heat-affected zone (HAZ). The HAZ exhibited a fatigue life corresponding to about 43.9% of the base metal, while the molten metal reached approximately 81.3%, with the base metal enduring around 5 × 10⁵ cycles (reference value). Metallurgical and microstructural differences, particularly grain size, were identified as key factors influencing crack behavior. The results indicate that precise control of the welding thermal cycle can enhance pipeline durability by delaying crack propagation. Specifically, the HAZ exhibited a crack growth rate between 9.79 × 10⁻⁵ and 2.14 × 10⁻³ m/cycle, whereas the base metal showed a slower rate of 1.63 × 10⁻⁵ to 4.53 × 10⁻⁴ m/cycle, both measured under a stress intensity factor range of ΔK = 10.77–29.35 MPa√m. These findings highlight the importance of optimizing welding parameters, such as current intensity and voltage, to improve the durability and crack resistance of welded joints in pipelines.