Effect of welding layers on crack tip opening displacement in combination automatic welded joints of long-distance pipelines

Objective With the promotion and application of fully automatic welding technology in the construction of high-steel-grade long-distance pipelines, this technology has been popularized in welding of main line. Combination automatic welding operations has also been used in welded joints of main line, that is root welding (using manual argon arc welding or other manual root welding process) + filling/cover welding (gas-shielded flux-cored wire upward automatic welding process). Because groove form is V-shaped, heat input is large during welding. In the case that CTOD value is regarded as the acceptance value in current pipeline industry, number of layers is the key factor affecting CTOD value. Therefore, in this paper, for the automatic welded butt joints of X80M straight pipes and L555M hot simmering bends with different numbers of welding layers, effect of number of welding layers on CTOD value is studies. Methods CTOD test is conducted with one-sided notched three-point bending samples. CTOD value is calculated according to ISO 15653: 2018, and microstructure and fracture of welded joints are observed by optical microscope. Results Under the condition of number 8 and 9 of welding layers, CTOD values of welded joints are more discrete and lower than standard values, which are 0.113 mm, 0.229 mm and 0.179 mm, respectively. Under the condition of number 10 of welding layers, CTOD values of all samples calculated under ISO 15653: 2018 meet the standard’s requirements, and fracture toughness of welded joints is better. For welded joints under the same number of welding layers, CTOD values of fusion zone are higher than those of weld, and the maximum CTOD value all appears in fusion zone of samples. Conclusion Fracture toughness of fusion zone is better than that of weld, but CTOD value is more dispersed and distribution of fracture toughness is uneven. For welded joints under different welding layers, CTOD values of weld and fusion zone increase with the increase number of welding layers. When CTOD values meet evaluation standard, crack growth zone is relatively gentle.