A comprehensive study on the microstructure and mechanical properties of arc girth welded joints of spiral welded high strength API X70 steel pipe

In the paper, the effect of welding technology on the microstructure and mechanical properties of girth welded joints was presented. Metallographic examinations based on light microscopy and SEM were conducted on girth welded joints of API X70 steel pipe. Research has shown that microstructure of the heat-affected zone (HAZ) of MMA girth welded joints is not homogeneous and depends on the thermal history of each area during the welding process. Near the fusion line the zone is coarse, and further away there is a fine-grained zone. In the area of root passes the microstructure consists of recrystallized ferrite grains unlike to cap passes where the fine bainitic microstructure can be observed. In the case of MAG girth welded joints, the weld microstructure consists of primary austenite grains. The primary austenite boundaries serve as nucleation sites of ferrite. The microstructure of the HAZ varies continuously from a coarse—to fine-grained microstructure of the base material. The results of mechanical properties of girth welded joints are also presented. The hardness and strength of arc welded joints depend on welding filler materials as well as welding technology. The results of hardness distribution of MMA and MAG girth welded joints confirmed the results of microstructural evaluation.