Microstructure and Mechanical Properties of X70 Pipeline Steel with High Deformability

Three kinds of welding processes were adopted in the welding test for X70 pipeline steels with high deformability, and the microstructure and mechanical properties of the welded joints were analyzed by microstructure observation, tensile test and impact test. The results show that the weld metal of the welded joints by GMAW was composed of acicular ferrites and little quasi-polygonal ferrites, while the HAZ consisted of coarse granular bainites and quasi-polygonal ferrites. And the welded joints had good strength and toughness but low ductility. The weld metal of welded joints by GMAW+FCAW was mainly comprised of acicular ferrites and little quasi-polygonal ferrites, the HAZ was composed of coarse granular bainites and lath bainites, and the welded joint had a very high yield ratio. Coarse polygonal ferrites and little bainites composed the weld metal of the welded joints by SMAW+FCAW, and quasi-polygonal ferrites and bainites composed the HAZ, which led to the hardness of weld metal lower than those of the HAZ and the base metal. Furthermore, the welded joint by SMAW+FCAW had good strength, toughness and ductility and high resistance to deformation ability.