The problem of stainless steel brazing is still the focus of scientific research. In this work, the Mn-based brazing filler was used to braze 1Cr18Ni9Ti and 1Cr21Ni5Ti stainless steel. The typical microstructure of the 1Cr18Ni9Ti/1Cr21Ni5Ti joint was analyzed in detail, and the interface structure of the joint was determined to be 1Cr18Ni9Ti/Mn(s, s)/1Cr21Ni5Ti. The brazing temperature and holding time were shown to have a great influence on the microstructure of the brazed joint. The tensile strength of brazed joints first increased and then decreased with the rising of the brazing temperature and the holding time. The maximum tensile strength was 566 MPa when the joints were brazed at 1125 °C for 15 min. The diffusion of Mn and Cr was an important factor affecting the quality of the joints. The diffusion distances of Mn and Cr at different brazing temperatures and holding times were measured, and the diffusion activation energy and diffusion coefficient were achieved by the Arrhenius equation.

The wetting properties of the brazing filler on the substrates play an important role in achieving a reliable joint. In this work, the wetting behaviour of the Mn-based brazing alloy on the surface of 1Cr18Ni9Ti stainless steel is investigated. First, surface treatment was adopted to prepare four different surface states on the stainless steel, including polished, acid treatment, nickel coating, and sandblasting. The surface morphology with nickel coating shows micro-and nano-scale protrusions, forming a uniform 3D Ni layer. The surface roughness of substrates increases after sandblasting treatment. It is found that the wetting angle of brazing alloy on the Ni coated substrates decreases noticeably. After sandblasting, the high roughness speeds up the wetting spread of the brazing alloy at the initial stage. The effect of heating temperature and holding time is also studied systematically. With the increase in holding time, the height of the melting brazing alloy decreases slightly on the sample surfaces. Based on the characterization of the joint section, the Ni layer dissolves into the melting brazing alloy. The diffusion and dissolving are indicated between the brazing alloy and substrates, forming a reliable bond.