The Study Of Joint Brazed With BNI-7and Its Diffusing Behaviour For Pur Fe, Ni, Mo Cr

Because of excellent performance of resistance to intergranular corrosion due to the low carbon content,316L stainless steel is widely used in the brazing technology for heat exchangers, coolers and other corrosion-resistant parts. When brazing stainless steel using common nickel-based brazing filler, such as BNi-2and BNi-5with B, Si or other melting point depressants, there always are erosion, grain boundary embrittlement due to infiltration and other negative phenomena. With low melting point and good fluidity, BNi-7filler is suitable for brazing of stainless steel thin-walled parts, but there is less study on the dissolution and diffusion behavior between the base metal and BNi-7filler components.316L stainless steel contains mainly Fe, Ni, Mo, Cr, and other elements. So four kinds of pure metal, Fe, Ni, Mo and Cr, were vacuum brazed with BNi-7nickel-based filler in this paper. The dissolution and diffusion behavior of brazed joints and the influence of brazing process parameters on the organization and performance of brazed joints were studied by optical microscopy (OM), scanning electron microscopy (SEM) and its accompanying spectrum analyzer (EDS) and digital microhardness tester. Finally, the formation of phase in the joints was predicted using Thermo-Calc diagrams software, and the diffusion behavior of elements in the joints was simulated using DICTRA software. Then the feasibility and reliability of Thermo-Calc and DICTRA software for brazing technique were tested, which is used for theoretical basis and technical reference of brazing for316L stainless steel.The results showed that, the joints of Fe/Ni/Mo/Cr vacuum brazed with BNi-7filler possessed three parts, namely, the middle zone in the joints formed by non-isothermal solidification with the structure of Ni-base solid solution rich in Cr and Fe and Ni+Ni3P eutectic and Ni (Cr, Fe)-P intermetallic compounds; the interfacial reaction zone formed by isothermal solidification with the structure of Cr-rich Ni-based solid solution with a small amount of Ni+Ni3P eutectic; the diffusion-affected zone formed by diffusion of element. For the joints at the same brazing process parameters, the amount of solid solution reduced and the amount of brittle compounds increased, along the direction of the head to the tail. And there fore, the head of brazed joints possessed uniformity in structure and more excellent performance. With the increase of brazing temperature or brazing holding time and the decrease of joint clearance, the amount of solid solution increased and the amount of brittle compounds decreased, with stronger metallurgical chemical reaction between the filler and the base metals. According to the microhardness curve of the joints, the microhardness of brittle compounds was highest, followed by the solid solution and the diffusion-affected zone. The peak microhardness decreased and the brittlement of joints has been effectively controlled with the optimization of brazing process parameter. The best value of brazing temperature, holding time and joint clearance were980℃,30min and30μm, respectively, for three kinds of pure metal of Fe/Ni/Mo. And the difference for Cr metal was holding time at5min.The calculations using Thermo-Calc software showed that, there were phases of LIQUID, FCC_A1, BCC_A2and M3P_TETR in the joints of pure Fe. FCC_A1was rich in Ni and Fe, and BCC_A2was rich in Cr and P, which was consistent with the experimental results. The diffusing modeling of elements in joints of pure Fe by DICTRA software showed that, the diffusing simulation of Ni and Cr was consistent with the experimental results, but there were some less errors for Fe and P which was due to the different conditions of non-equilibrium solidification and equilibrium solidification. In conclusion, the Thermo-Calc and DICTRA software had some scientific guidance, feasibility, reliability and efficiency on brazing process according to the simulation results.