Investigation of Microstructure and Mechanical Properties of X70M Steels Welded by Submerged Arc Welding

In this study, X70M PSL 2 (API 5L) steel used in oil pipelines was welded by submerged arc welding method. Weld seam and HAZ were characterized by optical microscopy, scanning electron microscopy (SEM), tensile test, hardness test and Charpy impact test methods. As a result of the optical inspection, it was observed that no welding defects had occurred within the structure. The weld zone, base metal, and HAZ are distinct, and columnar grain growth has occurred in the weld metal. In the SEM study, it was observed that the base material consists of ferrite grains (angular ferrite, polygonal ferrite), the HAZ consists of coarser grains, and the weld metal consists of fine columnar ferrite. As a result of the tensile test, it was determined that the tensile strength of the base material is approximately 648 MPa, while the tensile strength of the welded joint is approximately 695 MPa. The obtained results indicate that the fracture in the welded joint occurred in the base material. The highest average hardness value was measured as 229 HV in the weld metal. In contrast, the average hardness values in the base material and HAZ are 218 HV and 221 HV, respectively. The average impact energy value obtained in the base material is 428 J. The lowest impact energy was obtained in the weld metal. The average impact energy value obtained in the weld metal is approximately 94 J. As one moves from the weld metal towards the base material, the increase in impact energy is remarkable.