Microstructure and Properties of SIMP Steel Joint by Three Step Heating Transient Liquid Phase Diffusion Welding with Different Process Parameters

With BNi₂ and Fe₇₈Si₉B₁₃ amorphous alloy foils as intermediate layer, SIMP steel tubes were joined by three step heating transient liquid phase (TLP) diffusion welding. Microstructure and mechanical properties of the joint welded with different isothermal solidification process parameters (temperature of 1 230, 1 240℃; time of 180, 240 s; pressure of 8, 9 MPa) were studied. The results show that increasing isothermal solidification temperature or time was beneficial to the diffusion of melting elements and reducing formation of brittle hard phases in the weld microstructure, thereby improving the tensile strength of the joint and reducing the microhardness of the weld. The relatively optimal isothermal solidification parameters were listed as follows:temperature of 1 240℃, time of 240 s, and pressure of 9 MPa. After welding with these parameters, the microstructure of the joint was composed of uniform martensite, and the difference between the weld microstructure and the base metal microstructure was small; the tensile strength of the joint reached the largest value of 794 MPa, and the tensile fracture mode was ductile fracture.