Comparative study of microstructure and toughness of automatic welded joints of X70 pipeline steel with no slope and 25° slope

X70 pipeline steel is the most widely used for oil and gas pipeline transportation. When it is used in mountainous areas, the probability of pipeline failure increases dramatically due to the lack of toughness of welded joints. In order to ensure the welding quality, fully automatic welding has become the trend of pipeline welding. In this paper, X70 pipeline steel is butt welded by fully automatic welding under the condition of both no slope and 25° slope, and the microstructure and the toughness of two welded joints were observed and measured. The results show that the slope affects grain size and impact toughness. The grain size of the coarse-grained heat-affected zone (CGHAZ) in the root weld at the lower groove is larger than that at the upper groove. The impact energy of the heat-affected zone (HAZ) at the upper groove is significantly higher than that at the lower groove. For no slope welded joint, the weld CTOD value in the NQ direction is the lowest. For the 25° slope welded joint, the weld CTOD values are lower than HAZ CTOD values. Meanwhile, the HAZ CTOD values in the NP direction are significantly lower than those in the NQ direction. The fracture morphology of impact and CTOD of two welded joints is similar. The root-weld CGHAZ at the lower groove is the weakest area in the welded joint. For welding under slope conditions, the effect of slope on the performance of welded joints must be taken seriously. •X70 steel is butt welded by fully automatic welding under the condition of no slope and 25° slope.•The grain size of root-weld CGHAZ at lower groove is larger than that at upper groove.•The impact energy of the HAZ at upper groove is significantly higher than that at lower groove.•For 25° slope welded joint, the weld CTOD values are lower than HAZ CTOD values.•The HAZ CTOD values in the NP direction are significantly lower than that in the NQ direction.