Investigation of mechanical and microstructural properties of Laser, MIG and TIG welded API 5L x60 steel

Since API 5L standard pipes used in the transmission of oil and natural gas are generally joined by welding, the quality of the weld is of great importance. For this reason, the suitability of the welding technologies used is very important. In this experimental study, the effects of using laser welding, which is a relatively new technology, as a root welding were investigated. In this context, API 5L X60 quality steel samples were welded at different welding parameters using MIG, TIG and Laser welding technologies. In order to make the study cost-effective, the welding parameters and number of experiments were determined according to the Taguchi optimization method. Radiography, tensile and hardness tests were carried out to determine the welding quality of the samples welded with MIG, TIG and Laser welding technologies. Additionally, Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) unit were used for both imaging and analysis of the fracture surfaces of the determined welded samples. As a result of radiography tests, it was observed that there were many welding defects in MIG and TIG welded joints. On the other hand, it has been determined that the welding qualities of MIG+Laser and TIG+Laser welded joints are much better due to the positive effect of Laser root welding. As a result of the tensile tests, it was revealed that the maximum tensile forces of the joints made with MIG and TIG welding were lower than the laser root welded MIG+Laser and TIG+Laser connections. In the analyzes performed according to the Taguchi optimization method, the highest SN rate was achieved with TIG + Laser welding and the lowest SN rate was achieved with MIG welding. As a result of ANOVA analysis, it was seen that the highest percentage contribution (the most important factor) was welding technology with 84.36%. When the SEM images taken of the fracture surfaces of MIG, TIG, MIG+Laser and TIG+Laser welded joints were examined, it was understood that there were ductile fractures in all images. When the measured hardness values of MIG, TIG, MIG+Laser and TIG+Laser welded joints were compared according to the welding regions, it was seen that the highest hardness values were generally concentrated in the HAZ region.