Effect of beam offset on the microstructure and mechanical properties of Nb and 304 stainless steel electron beam welded joints

Electron beam welding with varying beam offsets was employed to investigate the evolution of microstructure and mechanical properties in Nb/304 stainless steel joints. The weld zone microstructure primarily consisted of a eutectic matrix of ε(Fe₂Nb) and α-Fe. As the electron beam was deflected from the Nb side to the 304 stainless steel side, the amount of molten Nb decreased, reducing the formation of brittle intermetallic compounds. The proeutectic phase in the weld zone transitioned from ε(Fe₂Nb) to α-Fe, and the thickness of the ε(Fe₂Nb) and μ(Fe₇Nb₆) intermetallic compounds reaction layer on the Nb side decreased by 48 %. With the beam offset toward the 304 stainless steel side, the microhardness of the weld zone decreased by 22 %, while the tensile strength of the joint increased by 51 %. The reaction layer exhibited the highest hardness, reaching 1147 HV. The joint with +0.6 mm beam offset achieved the maximum tensile strength of 156 MPa, 61 % of Nb base metal. All joints failed in a brittle manner at the reaction layer, characterized by cleavage fracture. The mechanism underlying joint weakening was revealed.