Effect of laser power on metallurgical and tensile properties of CO₂ laser welded AISI 409 ferritic stainless steel tubes

One of the high-temperature materials, ferritic stainless steel (FSS), was joined using laser beam welding (LBW), which reduced intermetallic phases and grain growth. The CO₂ laser was effectively applied to join AISI 409 FSS tubes of wall thickness 4 mm. The power of the laser beam, which influences mechanical and metallurgical properties, varied from 2.6 kW to 3.4 kW. No defects were observed at the weld zone (WZ). However, the lowest laser power produced an undercut at the bottom of WZ. Coarsening of grains was not observed in the microstructure at HAZ. The dendritic microstructure was found in the WZ, which consists of both axial and columnar grains. A coarse dendritic structure was formed with high laser power. The δ ferrite phase was partially transformed into martensite phase at the WZ. The dislocation fields along with substructure formation were seen in TEM micrographs. The WZ was strengthened due to the phase transformation. The tensile strength of all the joints was close to the base metal (BM). More than 20 % elongation was observed except for the joint at 2.6 kW. The fracture mode was observed to be ductile in all the joints.