Influence of different root passages on weldability and mechanical properties of API steels in submerged arc welding

Submerged arc welding is widely used during the production of pipes used in oil and gas pipelines. However, due to the demand for high operating pressures, the expected quality of welds is increasing every day. The type of steels used, welding method, welding parameters, and additional welding materials are the factors affecting the performance of welds. These factors are important to emphasize the importance of weld design. In this study, the effect of submerged arc welding method and different root pass applications on low alloy, fine grain, high strength X65 and X70 API 5L steels were investigated in terms of weldability and its effect on mechanical properties. In the experimental studies, X65 and X70 plate pieces of different thicknesses were fixed to each other by spot welding by opening V type welding mouth. After the assembly process of plates, ceramic backing were put on the bottom of the plates and root welds were welded with different welding methods. After root welding, the remaining parts were welded according to submerged arc welding method to complete the welding zone. The test samples were checked by magnetic particle, ultrasonic and radioscopy methods which are non-destructive testing methods. In order to determine the strength values of the welding zone, tensile, notch-impact and bending tests, which are one of the destructive inspection methods, were subjected to the tests. Structural changes that occurred during welding were analyzed by electron microscope on macro sample. The fractured surfaces were examined by Scanning Electron Microscopy (SEM) over notch impact samples. In the results of the experiment, it was observed that the root welds of X65 and X70 grade API 5L steels of different thicknesses can be welded with different welding methods, and compared with each other, the tensile and notch impact values of the root welds using TIG welding method are higher than the other welding methods.