Microstructure and tensile behavior of CO₂ laser beam welded AISI 409 ferritic stainless steel tubes

Laser beam welding (LBW) is a suitable process to join ferritic stainless steels (FSS). LBW process helps to suppress grain growth and intermetallic phases. CO₂-based laser source was used to join AISI 409 FSS tubes having a thickness of 4 mm. The laser scanning speed, which determines the production rate, was varied from 1.3 m/min to 3.3 m/min. There were no defects at the joint region except an undercut at the highest scanning speed. The micrographs showed no coarsening of grains at HAZ. The fusion zone showed a dendritic structure composed of columnar grains and axial grains. The increase in scanning speed refined the dendritic structure. The fusion zone experienced phase transformation from δ ferrite to martensite. TEM micrographs revealed the formation of substructures and dislocation fields. The microstructural changes caused the strengthening of the fusion zone. The joint strength in most of the weldment reached the strength of the base metal. All the joints failed in a ductile manner and the elongation was more than 10%.